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Scilla In Depth

Structure of a Scilla Contract

The general structure of a Scilla contract is given in the code fragment below:

  • The contract starts with the declaration of scilla_version, which indicates which major Scilla version the contract uses.
  • Then follows the declaration of a library that contains purely mathematical functions, e.g., a function to compute the Boolean AND of two bits, or a function computing the factorial of a given natural number.
  • Then follows the actual contract definition declared using the keyword contract.
  • Within a contract, there are then four distinct parts:
  • The first part declares the immutable parameters of the contract.
  • The second part describes the contract's constraint, which must be valid when the contract is deployed.
  • The third part declares the mutable fields.
  • The fourth part contains all transition and procedure definitions.
(* Scilla contract structure *)

(***************************************************)
(*                 Scilla version                  *)
(***************************************************)

scilla_version 1

(***************************************************)
(*               Associated library                *)
(***************************************************)

library MyContractLib


(* Library code block follows *)



(***************************************************)
(*             Contract definition                 *)
(***************************************************)

contract MyContract

(* Immutable contract parameters declaration *)

(vname_1 : vtype_1,
 vname_2 : vtype_2)

(* Contract constraint *)
with
  (* Constraint expression *)
=>

(* Mutable fields declaration *)

field vname_1 : vtype_1 = init_val_1
field vname_2 : vtype_2 = init_val_2

(* Transitions and procedures *)


(* Procedure signature *)
procedure firstProcedure (param_1 : type_1, param_2 : type_2)
  (* Procedure body *)

end

(* Transition signature *)
transition firstTransition (param_1 : type_1, param_2 : type_2)
  (* Transition body *)

end

(* Procedure signature *)
procedure secondProcedure (param_1 : type_1, param_2 : type_2)
  (* Procedure body *)

end

transition secondTransition (param_1: type_1)
  (* Transition body *)

end

Immutable Contract Parameters

[Immutable parameters]{.title-ref} are the contract's initial parameters whose values are defined when the contract is deployed, and cannot be modified afterwards.

Immutable parameters are declared using the following syntax:

(vname_1 : vtype_1,
 vname_2 : vtype_2,
  ...  )

Each declaration consists of a parameter name (an identifier) and followed by its type, separated by :. Multiple parameter declarations are separated by ,. The initialization values for parameters are to be specified when the contract is deployed.

::: {.note} ::: {.title} Note :::

In addition to the explicitly declared immutable parameters, a Scilla contract has the following implicitly declared immutable contract parameters

1. _this_address of type ByStr20, which is initialised to the address of the contract when the contract is deployed.

2. _creation_block of type BNum, which is initialized to the block number at which the contract is / was deployed.

These parameters can be freely read within the implementation without having to dereference it using <- and cannot be modified with :=. :::

Contract Constraints

A [contract constraint]{.title-ref} is a requirement placed on the contract's immutable parameters. A contract constraint provides a way of establishing a contract invariant as soon as the contract is deployed, thus preventing the contract being deployed with nonsensical parameters.

A contract constraint is declared using the following syntax:

with
  ...
=>

The constraint must be an expression of type Bool.

The constraint is checked when the contract is deployed. Contract deployment only succeeds if the constraint evaluates to True. If it evaluates to False, then the deployment fails.

Here is a simple example of using contract constraints to make sure a contract with a limited period of functioning is not deployed [after]{.title-ref} that period:

contract Mortal(end_of_life : BNum)
with
  builtin blt _creation_block end_of_life
=>

The snippet above uses the implicit contract parameter _creation_block described in immutable-contract-parameters.

::: {.note} ::: {.title} Note :::

Declaring a contract constraint is optional. If no constraint is declared, then the constraint is assumed to simply be True. :::

Mutable Fields

[Mutable fields]{.title-ref} represent the mutable state (mutable variables) of the contract. They are declared after the immutable parameters, with each declaration prefixed with the keyword field.

field vname_1 : vtype_1 = expr_1
field vname_2 : vtype_2 = expr_2
...

Each expression here is an initialiser for the field in question. The definitions complete the initial state of the contract, at the time of creation. As the contract executes a transition, the values of these fields get modified.

::: {.note} ::: {.title} Note :::

In addition to the explicitly declared mutable fields, a Scilla contract has an implicitly declared mutable field _balance of type Uint128, which is initialised to 0 when the contract is deployed. The _balance field keeps the amount of funds held by the contract, measured in QA (1 ZIL = 1,000,000,000,000 QA). This field can be freely read within the implementation, but can only be modified by explicitly transferring funds to other accounts (using send), or by accepting money from incoming messages (using accept). :::

::: {.note} ::: {.title} Note :::

Both mutable fields and immutable parameters must be of a storable type:

  • Messages, events and the special Unit type are not storable. All other primitive types like integers and strings are storable.
  • Function types are not storable.
  • Complex types involving uninstantiated type variables are not storable.
  • Maps and ADT are storable if the types of their subvalues are storable. For maps this means that the key type and the value type must both be storable, and for ADTs this means that the type of every constructor argument must be storable. :::

Units

The Zilliqa protocol supports three basic tokens units - ZIL, LI (10^-6 ZIL) and QA (10^-12 ZIL).

The base unit used in Scilla smart contracts is QA. Hence, when using money variables, it is important to attach the trailing zeroes that are needed to represent it in QAs.

(* fee is 1 QA *)
let fee = Uint128 1

(* fee is 1 LI *)
let fee = Uint128 1000000

(* fee is 1 ZIL *)
let fee = Uint128 1000000000000

Transitions

[Transitions]{.title-ref} are a way to define how the state of the contract may change. The transitions of a contract define the public interface for the contract, since transitions may be invoked by sending a message to the contract.

Transitions are defined with the keyword transition followed by the parameters to be passed. The definition ends with the end keyword.

transition foo (vname_1 : vtype_1, vname_2 : vtype_2, ...)
  ...
end

where vname : vtype specifies the name and type of each parameter and multiple parameters are separated by ,.

::: {.note} ::: {.title} Note :::

In addition to the parameters that are explicitly declared in the definition, each transition has the following implicit parameters:

  • _amount : Uint128 : Incoming amount, in QA (see section above on the units), sent by the sender. To transfer the money from the sender to the contract, the transition must explicitly accept the money using the accept instruction. The money transfer does not happen if the transition does not execute an accept.
  • _sender : ByStr20 with end : The account address that triggered this transition. If the transition was called by a contract account instead of a user account, then _sender is the address of the contract that called this transition. In a chain call, this is the contract that sent the message invoking the current transition.
  • _origin : ByStr20 with end : The account address that initiated the current transaction (which can possibly be a chain call). This is always a user address, since contracts can never initiate transactions.

The type ByStr20 with end is an [address type]{.title-ref}. Address types are explained in detail in the Addresses <Addresses> section. :::

::: {.note} ::: {.title} Note :::

Transition parameters must be of a serialisable type.

Serialisable:

  • Byte strings, integers, strings, and block numbers are serialisable.
  • Addresses are serialisable only as ByStr20 values.
  • ADT are serialisable if the types of their subvalues are serialisable. This means that the type of every constructor argument must be serialisable.

Not Serialisable:

  • Messages, events and the special Unit type are not serialisable.
  • Function types and map types are not serialisable.
  • Complex types involving uninstantiated type variables are not serialisable. :::

Procedures

[Procedures]{.title-ref} are another way to define now the state of the contract may change, but in contrast to transitions, procedures are not part of the public interface of the contract, and may not be invoked by sending a message to the contract. The only way to invoke a procedure is to call it from a transition or from another procedure.

Procedures are defined with the keyword procedure followed by the parameters to be passed. The definition ends with the end keyword.

procedure foo (vname_1 : vtype_1, vname_2 : vtype_2, ...)
  ...
end

where vname : vtype specifies the name and type of each parameter and multiple parameters are separated by ,.

Once a procedure is defined it is available to be invoked from transitions and procedures in the rest of the contract file. It is not possible to invoke a procedure from transition or procedure defined earlier in the contract, nor is it possible for a procedure to call itself recursively.

Procedures are invoked using the name of the procedure followed by the actual arguments to the procedure:

v1 = ...;
v2 = ...;
foo v1 v2;

All arguments must be supplied when the procedure is invoked. A procedure does not return a result.

::: {.note} ::: {.title} Note :::

The implicit transition parameters _sender, _origin and _amount are implicitly passed to all the procedures that a transition calls. There is therefore no need to declare those parameters explicitly when defining a procedure. :::

::: {.note} ::: {.title} Note :::

Procedure parameters cannot be (or contain) maps. If a procedure needs to access a map, it is therefore necessary to either make the procedure directly access the contract field containing the map, or use a library function to perform the necessary computations on the map. :::

Expressions

[Expressions]{.title-ref} handle pure operations. Scilla contains the following types of expressions:

  • let x = f : Give f the name x in the contract. The binding of x to f is global and extends to the end of the contract. The following code fragment defines a constant one whose values is 1 of type Int32 throughout the contract.
let one = Int32 1
  • let x = f in expr : Bind f to the name x within expression expr. The binding here is local to expr only. The following example binds the value of one to 1 of type Int32 and two to 2 of type Int32 in the expression builtin add one two, which adds 1 to 2 and hence evaluates to 3 of type Int32.
let sum =
  let one = Int32 1 in
  let two = Int32 2 in
  builtin add one two

  • { <entry>_1 ; <entry>_2 ... }: Message or event expression, where each entry has the following form: b : x. Here b is an identifier and x a variable, whose value is bound to the identifier in the message.

  • fun (x : T) => expr : A function that takes an input x of type T and returns the value to which expression expr evaluates.

  • f x : Apply the function f to the parameter x.

  • tfun 'T => expr : A type function that takes 'T as a parametric type and returns the value to which expression expr evaluates. These are typically used to build library functions. See the implementation of fst for an example.

::: {.note} ::: {.title} Note :::

Shadowing of type variables is not currently allowed. E.g. tfun 'T => tfun 'T => expr is not a valid expression. :::

  • @x T: Apply the type function x to the type T. This specialises the type function x by instantiating the first type variable of x to T. Type applications are typically used when a library function is about to be applied. See the example application of fst for an example.

  • builtin f x: Apply the built-in function f on x.

  • match expression: Matches a bound variable with patterns and evaluates the expression in that clause. The match expression is similar to the match expression in OCaml. The pattern to be matched can be an ADT constructor (see ADTs) with subpatterns, a variable, or a wildcard _. An ADT constructor pattern matches values constructed with the same constructor if the subpatterns match the corresponding subvalues. A variable matches anything, and binds the variable to the value it matches in the expression of that clause. A wildcard matches anything, but the value is then ignored.

match x with
| pattern_1 =>
  expression_1 ...
| pattern_2 =>
  expression_2 ...
| _ => (*Wildcard*)
  expression ...
end

::: {.note} ::: {.title} Note :::

A pattern-match must be exhaustive, i.e., every legal (type-safe) value of x must be matched by a pattern. Additionally, every pattern must be reachable, i.e., for each pattern there must be a legal (type-safe) value of x that matches that pattern, and which does not match any pattern preceding it. :::

Statements

Statements in Scilla are operations with effect, and hence not purely mathematical. Scilla contains the following types of statements:

  • x <- f : Fetch the value of the contract field f, and store it into the local variable x.

  • f := x : Update the mutable contract field f with the value of x. x may be a local variable, or another contract field.

  • x <- & BLOCKNUMBER : Fetch the value of the blockchain state variable BLOCKNUMBER, and store it into the local variable x.

  • x <- & CHAINID : Fetch the value of the current chain ID, and store it into the local variable x of type Uint32.

  • x <- & TIMESTAMP(block_num) : If block_num refers to an earlier block than the current one, then fetch the timestamp t of that block, and store the value Some t into the local variable x : Option Uint64. If block_num refers to the current block or later, then store the value None into x.

  • x <- & c.f : Remote fetch. Fetch the value of the contract field f at address c, and store it into the local variable x. Note that the type of c must be an address type containing the field f. See the section on Addresses <Addresses> for details on address types.

  • x <- & c as t: Address type cast. Check whether c satisfies the type t. If it does, then store the value Some v into x, where v is the same value as c, but of type t rather than the type of c. If it does not, then store the value None into x. Note that c must be of type ByStr20 or an address type, and that t must be an address type. See the section on Addresses <Addresses> for details on address types.

  • v = e : Evaluate the expression e, and assign the value to the local variable v.

  • p x y z : Invoke the procedure p with the arguments x, y and z. The number of arguments supplied must correspond to the number of arguments the procedure takes.

  • forall ls p : Invoke procedure p for each element in the list ls. p should be defined to take exactly one argument whose type is equal to an element of the list ls.

  • match : Pattern-matching at statement level:

match x with
| pattern_1 =>
  statement_11;
  statement_12;
  ...
| pattern_2 =>
  statement_21;
  statement_22;
  ...
| _ => (*Wildcard*)
  statement_n1;
  statement_n2;
  ...
end

  • accept : Accept the QA of the message that invoked the transition. The amount is automatically added to the _balance field of the contract. If a message contains QA, but the invoked transition does not accept the money, the money is transferred back to the sender of the message. Not accepting the incoming amount (when it is non-zero) is not an error.

  • send and event : Communication with the blockchain. See the next section for details.

  • In-place map operations : Operations on contract fields of type Map. See the Maps section for details.

A sequence of statements must be separated by semicolons ;:

transition T ()
  statement_1;
  statement_2;
  ...
  statement_n
end

Notice that the final statement does not have a trailing ;, since ; is used to separate statements rather than terminate them.

Communication

A contract can communicate with other contract and user accounts through the send instruction:

  • send msgs : Send a list of messages msgs.

The following code snippet defines a msg with four entries _tag, _recipient, _amount and param.

(*Assume contractAddress is the address of the contract being called and the contract contains the transition setHello*)
msg = { _tag : "setHello"; _recipient : contractAddress; _amount : Uint128 0; param : Uint32 0 };

A message passed to send must contain the mandatory fields _tag, _recipient and _amount.

The _recipient field (of type ByStr20) is the blockchain address that the message is to be sent to, and the _amount field (of type Uint128) is the number of QA to be transferred to that account.

The _tag field (of type String) is only used when the value of the _recipient field is the address of a contract. In this case, the value of the _tag field is the name of the transition that is to be invoked on the recipient contract. If the recipient is a user account, the _tag field is ignored.

::: {.note} ::: {.title} Note :::

To make it possible to transfer funds from a contract to both contracts and user accounts, use a standard transition name as per ZRC-5, i.e. AddFunds. Please make sure to check if a contract to which you intend to send funds is implemented in adherence with ZRC-5 convention. :::

In addition to the compulsory fields the message may contain other fields (of any type), such as param above. However, if the message recipient is a contract, the additional fields must have the same names and types as the parameters of the transition being invoked on the recipient contract.

Here's an example that sends multiple messages.

msg1 = { _tag : "setFoo"; _recipient : contractAddress1; _amount : Uint128 0; foo : Uint32 101 };
msg2 = { _tag : "setBar"; _recipient : contractAddress2; _amount : Uint128 0; bar : Uint32 100 };
msgs =
  let nil = Nil {Message} in
  let m1 = Cons {Message} msg1 nil in
  Cons msg2 m1
  ;
send msgs

::: {.note} ::: {.title} Note :::

A transition may execute a send at any point during execution (including during the execution of the procedures it invokes), but the messages will not sent onwards until after the transition has finished. More details can be found in the Chain Calls <Chaincalls> section. :::

::: {.note} ::: {.title} Note :::

Messages sent to a user-defined library are ignored. They do not cause the transaction to fail, but they have no effect. :::

A contract can also communicate to the outside world by emitting events. An event is a signal that gets stored on the blockchain for everyone to see. If a user uses a client application invoke a transition on a contract, the client application can listen for events that the contract may emit, and alert the user.

  • event e: Emit a message e as an event. The following code emits an event with name e_name.
e = { _eventname : "e_name"; <entry>_2 ; <entry>_3 };
event e

An emitted event must contain the compulsory field _eventname (of type String), and may contain other entries as well. The value of the _eventname entry must be a string literal. All events with the same name must have the same entry names and types.

::: {.note} ::: {.title} Note :::

As with the sending of messages, a transition may emit events at any point during execution (including during the execution of the procedures it invokes), but the event will not be visible on the blockchain before the transition has finished. More details can be found in the Chain Calls <Chaincalls> section. :::

Run-time Errors

A transition may encounter a run-time error during execution, such as out-of-gas errors, integer overflows, or deliberately thrown exceptions. A run-time error causes the transition to terminate abruptly, and the entire transaction to be aborted. However, gas is still charged up until the point of the error.

The syntax for throwing an exception is similar to that of events and messages.

e = { _exception : "InvalidInput"; <entry>_2; <entry>_3 };
throw e

Unlike that for event or send, The argument to throw is optional and can be omitted. An empty throw will result in an error that just conveys the location of where the throw happened without more information.

::: {.note} ::: {.title} Note :::

If a run-time error occurs during the execution of a transition, then the entire transaction is aborted, and any state changes in both the current and other contracts are rolled back. (The state of other contracts may have changed due to a chain call).

In particular:

  • All transferred funds are returned to their respective senders, even if an accept was executed before the error.
  • The message queue is cleared, so that as yet unprocessed messages will no longer be sent onwards even if a send was executed before the error.
  • The event list is cleared, so that no events are emitted even if an event was executed before the error.

Gas is still charged for the transaction up until the point the run-time error occurs. :::

::: {.note} ::: {.title} Note :::

Scilla does not have exception handlers. Throwing an exception always aborts the entire transaction. :::

Gas consumption in Scilla

Deploying contracts and executing transitions in them cost gas. The detailed cost mechanism is explained here.

The Nucleus Wallet page can be used to estimate gas costs for some transactions .

Primitive Data Types & Operations

Integer Types

Scilla defines signed and unsigned integer types of 32, 64, 128, and 256 bits. These integer types can be specified with the keywords IntX and UintX where X can be 32, 64, 128, or 256. For example, the type of an unsigned integer of 32 bits is Uint32.

The following code snippet declares a variable of type Uint32:

let x = Uint32 43

Scilla supports the following built-in operations on integers. Each operation takes two integers IntX / UintX (of the same type) as arguments. Exceptions are pow whose second argument is always Uint32 and isqrt which takes in a single UintX argument.

  • builtin eq i1 i2 : Is i1 equal to i2? Returns a Bool.

  • builtin add i1 i2: Add integer values i1 and i2. Returns an integer of the same type.

  • builtin sub i1 i2: Subtract i2 from i1. Returns an integer of the same type.

  • builtin mul i1 i2: Integer product of i1 and i2. Returns an integer of the same type.

  • builtin div i1 i2: Integer division of i1 by i2. Returns an integer of the same type.

  • builtin rem i1 i2: The remainder of integer division of i1 by i2. Returns an integer of the same type.

  • builtin lt i1 i2: Is i1 less than i2? Returns a Bool.

  • builtin pow i1 i2: i1 raised to the power of i2. Returns an integer of the same type as i1.

  • builtin isqrt i: Computes the integer square root of i, i.e. the largest integer j such that j * j <= i. Returns an integer of the same type as i.

  • builtin to_nat i1: Convert a value of type Uint32 to the equivalent value of type Nat.

  • builtin to_(u)int32/64/128/256: Convert a UintX / IntX or a String (that represents a decimal number) value to the result of Option UintX or Option IntX type. Returns Some res if the conversion succeeded and None otherwise. The conversion may fail when

  • there is not enough bits to represent the result;

  • when converting a negative integer (or a string representing a negative integer) into a value of an unsigned type;
  • the input string cannot be parsed as an integer.

Here is the list of concrete conversion builtins for better discoverability: to_int32, to_int64, to_int128, to_int256, to_uint32, to_uint64, to_uint128, to_uint256.

Addition, subtraction, multiplication, pow, division and remainder operations may raise integer overflow, underflow and division_by_zero errors. This aborts the execution of the current transition and unrolls all the state changes made so far.

::: {.note} ::: {.title} Note :::

Variables related to blockchain money, such as the _amount entry of a message or the _balance field of a contract, are of type Uint128. :::

Strings

String literals in Scilla are expressed using a sequence of characters enclosed in double quotes. Variables can be declared by specifying using keyword String.

The following code snippet declares a variable of type String:

let x = "Hello"

Scilla supports the following built-in operations on strings:

  • builtin eq s1 s2 : Is s1 equal to s2? Returns a Bool. s1 and s2 must be of type String.
  • builtin concat s1 s2 : Concatenate string s1 with string s2. Returns a String.
  • builtin substr s idx len : Extract the substring of s of length len starting from position idx. idx and len must be of type Uint32. Character indices in strings start from 0. Returns a String or fails with a runtime error if the combination of the input parameters results in an invalid substring.
  • builtin to_string x: Convert x to a string literal. Valid types of x are IntX, UintX, ByStrX and ByStr. Returns a String. Byte strings are converted to textual hexadecimal representation.
  • builtin strlen s : Calculate the length of s (of type String). Returns a Uint32.
  • builtin strrev s : Returns the reverse of the string s.
  • builtin to_ascii h : Reinterprets a byte string (ByStr or ByStrX) as a printable ASCII string and returns an equivalent String value. If the byte string contains any non-printable characters, a runtime error is raised.

Byte strings

Byte strings in Scilla are represented using the types ByStr and ByStrX, where X is a number. ByStr refers to a byte string of arbitrary length, whereas for any X, ByStrX refers to a byte string of fixed length X. For instance, ByStr20 is the type of byte strings of length 20, ByStr32 is the type of byte strings of length 32, and so on.

Byte strings literals in Scilla are written using hexadecimal characters prefixed with 0x. Note that it takes 2 hexadecimal characters to specify 1 byte, so a ByStrX literal requires 2 * X hexadecimal characters. The following code snippet declares a variable of type ByStr32:

let x = 0x123456789012345678901234567890123456789012345678901234567890abff

Scilla supports the following built-in operations for computing on and converting between byte string types:

  • builtin to_bystr h : Convert a value h of type ByStrX (for some known X) to one of arbitrary length of type ByStr.
  • builtin to_bystrX h : (note that X is a numerical parameter here and not a part of the builtin name, see the examples below)
  • if the argument h is of type ByStr: Convert an arbitrary size byte string value h (of type ByStr) to a fixed sized byte string of type ByStrX, with length X. The result is of type Option ByStrX in this case: the builtin returns Some res if the length of the argument is equal to X and None otherwise. E.g. builtin to_bystr42 bs returns Some bs' if the length of bs is 42.
  • if the argument h is of type Uint(32/64/128/256): Convert unsigned integers to their big endian byte representation, returning a ByStr(4/8/16/32) value (notice it's not an optional type in this case). For instance, builtin to_bystr4 x (this only typechecks if x has type Uint32) or builtin to_bystr16 x (this only typechecks if x is of type Uint128).
  • builtin to_uint(32/64/128/256) h : Convert a fixed sized byte string value h to an equivalent value of type Uint(32/64/128/256). h must be of type ByStrX for some known X less than or equal to (4/8/16/32). A big-endian representation is assumed.
  • builtin concat h1 h2: Concatenate byte strings h1 and h2.
  • If h1 has type ByStrX and h2 has type ByStrY, then the result will have type ByStr(X+Y).
  • If the arguments are of type ByStr, the result is also of type ByStr.
  • builtin strlen h: The length of byte string (ByStr) h. Returns Uint32.
  • eq a1 a2: Is a1 equal to a2? Returns a Bool.

Addresses

Addresses on the Zilliqa network are strings of 20 bytes, and raw addresses are therefore represented by values of type ByStr20.

Additionally, Scilla supports structured address types, i.e., types that are equivalent to ByStr20, but which, when interpreted as an address on the network, provide additional information about the contents of that address. Address types are written using the form ByStr20 with <address contents> end, where <address contents> refers to what the address contains.

The hierarchy of address types is as follows:

  • ByStr20: A raw byte string of length 20. The type does not provide any guarantee as to what is located at the address. (Generally, ByStr20 is not regarded as an address type, because it can refer to any byte string of length 20, whether it is meant to represent an address or not.)
  • ByStr20 with end: A ByStr20 which, when interpreted as a network address, refers to an address that is in use. An address is in use if it either contains a contract or a library, or if the balance or the nonce of the address is greater than 0. (The balance of an address is the number of Qa held by the address account. The nonce of an address is the number of transactions that have been initiated from that address).
  • ByStr20 with contract end: A ByStr20 which, when interpreted as a network address, refers to the address of a contract.
  • ByStr20 with contract field f1 : t1, field f2 : t2, ... end: A ByStr20 which, when interpreted as a network address, refers to the address of a contract containing the mutable fields f1 of type t1, f2 of type t2, and so on. The contract in question may define more fields than the ones specified in the type, but the fields specified in the type must be defined in the contract.

::: {.note} ::: {.title} Note :::

All addresses in use, and therefore by extension all contract addresses, implicitly define a mutable field _balance : Uint128. For user accounts the _balance field refers to the account balance. :::

::: {.note} ::: {.title} Note :::

Address types specifying immutable parameters or transitions of a contract are not supported. :::

Address subtyping

The hierarchy of address types defines a subtype relation:

  • Any address type ByStr20 with ... end is as subtype of ByStr20. This means that any address type can be used in place of a ByStr20, e.g., for comparing equality using builtin eq, or as the _recipient value of a message.
  • Any contract address type ByStr20 with contract ... end is a subtype of ByStr20 with end.
  • Any contract address type specifying explicit fields ByStr20 with contract field f1 : t11, field f2 : t12, ... end is a subtype of a contract address type specifying a subset of those fields ByStr20 with contract field f1 : t21, field f2 : t22, ... end, provided that t11 is a subtype of t21, t12 is a subtype of t22, and so on for each field specified in both contract types.
  • For ADTs with type parameters such as List or Option, an ADT T t1 t2 ... is a subtype of S s1 s2 ... if T is the same as S, and t1 is a subtype of s1, t2 is a subtype of s2, and so on.
  • A map with key type kt1 and value type vt1 is a subtype of another map with key type kt2 and value type vt2 if kt1 is a subtype of kt2 and vt1 is a subtype of vt2.

Dynamic typecheck of addresses

In general, address types cannot be fully typechecked statically by the Scilla checker. This can happen, e.g., because a byte string is a transition parameter and thus not known statically, or because a byte string refers to an address that does not currently contain a contract, but which might contain a contract in the future.

For this reason immutable parameters (i.e., contract parameters supplied when the contract is deployed) and transition parameters of address types are typechecked dynamically, when the actual byte string is known.

For example, a contract might specify an immutable field init_owner as follows:

contract MyContract (init_owner : ByStr20 with end)

When the contract is deployed, the byte string supplied as init_owner is looked up as an address on the blockchain, and if the contents of that address matches the address type (in this case that the address is in use either by a user or by a contract), then deployment continues, and init_owner can be treated as a ByStr20 with end throughout the contract.

Similarly, a transition might specify a parameter token_contract as follows:

transition Transfer (
    token_contract : ByStr20 with contract
                                    field balances : Map ByStr20 Uint128
                             end
    )

When the transition is invoked, the byte string supplied as the token_contract parameter is looked up as an address on the blockchain, and if the contents of that address matches the address type (in this case that the address contains a contract with a field balances of a type that is assignable to Map ByStr20 Uint128), then the transition parameter is initialised successfully, and token_contract can be treated as a ByStr20 with contract field balances : Map ByStr20 Uint128 end throughout this transition invocation.

In either case, if the contents of the address does not match the specified type, then the dynamic typecheck is unsuccessful, causing deployment (for failed immutable parameters) or transition invocation (for transition parameters) to fail. A failed dynamic typecheck is considered a run-time error, causing the current transaction to abort. (For the purposes of dynamic typechecks of immutable fields the deployment of a contract is considered a transaction).

It is also possible to perform a dynamic typecheck as a statement, using the address type cast construct:

x <- & c as ByStr20 with ... end

If c satisfies the address type, then x is bound to Some v, where v is the same value as c but is treated as having the address type specified in cast. On the other hand, if c does not satisfy the address type, then x is bound to None.

The advantage of using an address type cast instead of a dynamic typecheck of transition parameters is that a type cast does not cause a run-time failure in case of a failed cast - a failed cast just results in a None value.

The disadvantage of address type casts is that they cost more gas. The type cast itself costs the same as a typecheck of a transition parameter, but one then needs to pattern-match on the result of the type cast, which costs additional gas.

::: {.note} ::: {.title} Note :::

It is not possible to specify a ByStr20 literal and have it interpreted as an address. In other words, the following code snippet will result in a static type error:

let x : ByStr20 with end = 0x1234567890123456789012345678901234567890

The only ways for a byte string to be validated against an address type is either to pass it as the value of an immutable field or as a transition parameter of the appropriate type, or to perform a cast statement to the appropriate type. :::

Remote fetches

To perform a remote fetch x <- & c.f, the type of c must be some address type declaring the field f. For instance, if c has the type ByStr20 with contract field paused : Bool end, then the value of the field paused at address c can be fetched using the statement x <- & c.paused, whereas it is not possible to fetch the value of an undeclared field (e.g., admin) of c, even if the contract at address c does actually contain a field admin. To be able to fetch the value of the admin field, the type of c must contain the admin field as well, e.g, ByStr20 with contract field paused : Bool, field admin : ByStr20 end

Remote fetches of map fields can be performed using in-place operations in the same way as for locally declared map fields, i.e., x <- & c.m[key], x <- & c.m[key1][key2], x <- & exists m[key], etc. As with remote fetches of map fields, the remote map field must be declared in the type of c, e.g., ByStr20 with contract field m : Map Uint128 (Map Uint32 Bool) end.

Writing to a remote field is not allowed.

Crypto Built-ins

A hash in Scilla is declared using the data type ByStr32. A ByStr32 represents a hexadecimal byte string of 32 bytes (64 hexadecimal characters). A ByStr32 literal is prefixed with 0x.

Scilla supports the following built-in operations on hashes and other cryptographic primitives, including byte sequences.

  • builtin eq h1 h2: Is h1 equal to h2? Both inputs are of the same type ByStrX (or both are of type ByStr). Returns a Bool.
  • builtin sha256hash x : Convert x of any non-closure type to its SHA256 hash. Returns a ByStr32.
  • builtin keccak256hash x: Convert x of any non-closure type to its Keccak256 hash. Returns a ByStr32.
  • builtin ripemd160hash x: Convert x of any non-closure type to its RIPEMD-160 hash. Returns a ByStr20.
  • builtin substr h idx len : Extract the sub-byte-string of h of length len starting from position idx. idx and len must be of type Uint32. Character indices in byte strings start from 0. Returns a ByStr or fails with a runtime error.
  • builtin strrev h : Reverse byte string (either ByStr or ByStrX). Returns a value of the same type as the argument.
  • builtin schnorr_verify pubk data sig : Verify a signature sig of type ByStr64 against a byte string data of type ByStr with the Schnorr public key pubk of type ByStr33.
  • builtin schnorr_get_address pubk: Given a public key of type ByStr33, returns the ByStr20 Zilliqa address that corresponds to that public key.
  • builtin ecdsa_verify pubk data sig : Verify a signature sig of type ByStr64 against a byte string data of type ByStr with the ECDSA public key pubk of type ByStr33.
  • builtin ecdsa_recover_pk data sig recid : Recover data (of type ByStr), having signature sig (of type ByStr64) and a Uint32 recovery integer recid, whose value is restricted to be 0, 1, 2 or 3, the uncompressed public key, returning a ByStr65 value.
  • builtin bech32_to_bystr20 prefix addr. The builtin takes a network specific prefix ("zil" / "tzil") of type String and an input bech32 string (of type String) and if the inputs are valid, converts it to a raw byte address ([ByStr20]{.title-ref}). The return type is Option ByStr20. On success, Some addr is returned and on invalid inputs None is returned.
  • builtin bystr20_to_bech32 prefix addr. The builtin takes a network specific prefix ("zil" / "tzil") of type String and an input ByStr20 address, and if the inputs are valid, converts it to a bech32 address. The return type is Option String. On success, Some addr is returned and on invalid inputs None is returned.
  • builtin alt_bn128_G1_add p1 p2. The builtin takes two points p1, p2 on the alt_bn128 curve and returns the sum of the points in the underlying group G1. The input points are each a Pair {Bystr32 ByStr32}. The return type is Option (Pair (ByStr32) (ByStr32)). On success, Some point is returned and on invalid inputs None is returned. Each scalar component ByStr32 of a point is a big-endian encoded number. Also see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-196.md
  • builtin alt_bn128_G1_mul p s. The builtin takes a point p on the alt_bn128 curve (as described previously), and a scalar ByStr32 value s and returns the sum of the point p taken s times. The return type is Option (Pair (ByStr32) (ByStr32)). On success, Some point is returned and on invalid inputs None is returned.
  • builtin alt_bn128_G1_neg p. The builtin takes a point p on the alt_bn128 curve and returns the negation of the point. The result is Option (Pair (ByStr32) (ByStr32)). On success, Some point is returned and on invalid inputs None is returned.
  • builtin alt_bn128_pairing_product pairs. This builtin takes in a list of pairs pairs of points. Each pair consists of a point in group G1 (Pair {Bystr32 ByStr32}) as the first component and a point in group G2 (Pair {Bystr64 ByStr64}) as the second component. Hence the argument has type List {(Pair (Pair ByStr32 ByStr32) (Pair ByStr64 ByStr64)) }. The function applies a pairing function on each point to check for equality and returns True or False depending on whether the pairing check succeeds or fails. Also see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-197.md

Maps

A value of type Map kt vt provides a key-value store where kt is the type of keys and vt is the type of values (in some other programming languages datatypes like Scilla's Map are called associative arrays, symbol tables, or dictionaries). The type of map keys kt may be any one of the following primitive types: String, IntX, UintX, ByStrX, ByStr or BNum. The type of values vt may be any type except a function type, this includes both builtin and user-defined algebraic datatypes. :::

Since compound types are not supported as map key types, the way to model, e.g. association of pairs of values to another value is by using nested maps. For instance, if one wants to associate with an account and a particular trusted user some money limit the trusted user is allowed to spend on behalf of the account, one can use the following nested map:

field operators: Map ByStr20 (Map ByStr20 Uint128)
  = Emp ByStr20 (Map ByStr20 Unit)

The first and the second key are of type ByStr20 and represent accounts and the trusted users correspondingly. We represent the money limits with the Uint128 type.

Scilla supports a number of operations on map, which can be categorized as

  • in-place operations which modify field maps without making any copies, hence they belong to the imperative fragment of Scilla. These operations are efficient and recommended to use in almost all of the cases;
  • functional map operations are intended to use in pure functions, e.g. when designing a Scilla library, because they never modify the original map they are called on. These operations may incur significant performance overhead as some of them create a new (modified) copy of the input map. Syntactically, the copying operations are all prefixed with builtin keyword (see below). Note that to call the functional builtins on a field map one first needs to make a copy of the field map using a command like so: map_copy <- field_map, which results in gas consumption proportional to the size of field_map.

In-place map operations

  • m[k] := v: In-place insert. Inserts a key k bound to a value v into a map m. If m already contains key k, the old value bound to k gets replaced by v in the map. m must refer to a mutable field in the current contract. Insertion into nested maps is supported with the syntax m[k1][k2][...] := v. If the intermediate keys do not exist in the nested maps, they are freshly created along with the map values they are associated with.
  • x <- m[k]: In-place local fetch. Fetches the value associated with the key k in the map m. m must refer to a mutable field in the current contract. If k has an associated value v in m, then the results of the fetch is Some v (see the Option type below), otherwise the result is None. After the fetch, the result gets bound to the local variable x. Fetching from nested maps is supported with the syntax x <- m[k1][k2][...]. If one or more of the intermediate keys do not exist in the corresponding map, the result of the fetch is None.
  • x <- & c.m[k]: In-place remote fetch. Works in the same way as the local fetch operation, except that m must refer to a mutable field in the contract at address c.
  • x <- exists m[k]: In-place local key existence check. m must refer to a mutable field in the current contract. If k has an associated value in the map m then the result (of type Bool) of the check is True, otherwise the result is False. After the check, the result gets bound to the local variable x. Existence checks through nested maps is supported with the syntax x <- exists m[k1][k2][...]. If one or more of the intermediate keys do not exist in the corresponding map, the result is False.
  • b <- & exists c.m[k]: In-place remote key existence check. Works in the same way as the local key existence check, except that m must refer to a mutable field in the contract at address c.
  • delete m[k]: In-place remove. Removes a key k and its associated value from the map m. The identifier m must refer to a mutable field in the current contract. Removal from nested maps is supported with the syntax delete m[k1][k2][...]. If one or more of the intermediate keys do not exist in the corresponding map, then the operation has no effect. Note that in the case of a nested removal delete m[k1][...][kn-1][kn], only the key-value association of kn is removed. The key-value bindings of k1 to kn-1 will remain in the map.

Functional map operations

  • builtin put m k v: Insert a key k bound to a value v into a map m. Returns a new map which is a copy of the m but with k associated with v. If m already contains key k, the old value bound to k gets replaced by v in the result map. The value of m is unchanged. The put function is typically used in library functions. Note that put makes a copy of m before inserting the key-value pair.
  • builtin get m k: Fetch the value associated with the key k in the map m. Returns an optional value (see the Option type below) -- if k has an associated value v in m, then the result is Some v, otherwise the result is None. The get function is typically used in library functions.
  • builtin contains m k: Is the key k associated with a value in the map m? Returns a Bool. The contains function is typically used in library functions.
  • builtin remove m k: Remove a key k and its associated value from the map m. Returns a new map which is a copy of m but with k being unassociated with a value. The value of m is unchanged. If m does not contain key k the remove function simply returns a copy of m with no indication that k is missing. The remove function is typically used in library functions. Note that remove makes a copy of m before removing the key-value pair.
  • builtin to_list m: Convert a map m to a List (Pair kt vt) where kt and vt are key and value types, respectively (see the List type below).
  • builtin size m: Return the number of bindings in map m. The result type is Uint32. Calling this builtin consumes a small constant amount of gas. But calling it directly on a field map is not supported, meaning that getting the size of field maps while avoiding expensive copying needs some more scaffolding, which you can find out about in the Field map size <field_map_size> section.

::: {.note} ::: {.title} Note :::

Builtin functions put and remove return a new map, which is a possibly modified copy of the original map. This may affect performance! :::

::: {.note} ::: {.title} Note :::

Empty maps can be constructed using the Emp keyword, specifying the key and value types as its arguments. This is the way to initialise Map fields to be empty. For example field foomap : Map Uint128 String = Emp Uint128 String declares a Map field with keys of type Uint128 and values of type String, which is initialized to be the empty map. :::

Block Numbers

Block numbers have a dedicated type BNum in Scilla. Variables of this type are specified with the keyword BNum followed by an integer value (for example BNum 101).

Scilla supports the following built-in operations on block numbers:

  • eq b1 b2: Is b1 equal to b2? Returns a Bool.
  • blt b1 b2: Is b1 less than b2? Returns a Bool.
  • badd b1 i1: Add i1 of type UintX to b1 of type BNum. Returns a BNum.
  • bsub b1 b2: Subtract b2 from b1, both of type BNum. Returns an Int256.

Algebraic Datatypes

An [algebraic datatype]{.title-ref} (ADT) is a composite type used commonly in functional programming. Each ADT is defined as a set of constructors. Each constructor takes a set of arguments of certain types. :::

Scilla is equipped with a number of built-in ADTs, which are described below. Additionally, Scilla allows users to define their own ADTs.

Boolean

Boolean values are specified using the type Bool. The Bool ADT has two constructors True and False, neither of which take any arguments. Thus the following code fragment constructs a value of type Bool by using the constructor True:

x = True

Option

Optional values are specified using the type Option t, where t is some type. The Option ADT has two constructors:

  • Some represents the presence of a value. The Some constructor takes one argument (the value, of type t).
  • None represents the absence of a value. The None constructor takes no arguments.

The following code snippet constructs two optional values. The first value is an absent string value, constructed using None. The second value is the Int32 value 10, which, because the value is present, is constructed using Some:

let none_value = None {String}

let some_value =
  let ten = Int32 10 in
  Some {Int32} ten

Optional values are useful for initialising fields where the value is not yet known:

field empty_bool : Option Bool = None {Bool}

Optional values are also useful for functions that might not have a result, such as the get function for maps:

getValue = builtin get m _sender;
match getValue with
| Some v =>
  (* _sender was associated with v in m *)
  v = v + v;
  ...
| None =>
  (* _sender was not associated with a value in m *)
  ...
end

List

Lists of values are specified using the type List t, where t is some type. The List ADT has two constructors:

  • Nil represents an empty list. The Nil constructor takes no arguments.
  • Cons represents a non-empty list. The Cons constructor takes two arguments: The first element of the list (of type t), and another list (of type List t) representing the rest of the list.

All elements in a list must be of the same type t. In other words, two values of different types cannot be added to the same list.

The following example shows how to build a list of Int32 values. First we create an empty list using the Nil constructor. We then add four other values one by one using the Cons constructor. Notice how the list is constructed backwards by adding the last element, then the second-to-last element, and so on, so that the final list is [11; 10; 2; 1]:

let one = Int32 1 in
let two = Int32 2 in
let ten = Int32 10 in
let eleven = Int32 11 in

let nil = Nil {Int32} in
let l1 = Cons {Int32} one nil in
let l2 = Cons {Int32} two l1 in
let l3 = Cons {Int32} ten l2 in
  Cons {Int32} eleven l3

Scilla provides three structural recursion primitives for lists, which can be used to traverse all the elements of any list:

  • list_foldl: ('B -> 'A -> 'B) -> 'B -> (List 'A) -> 'B : Recursively process the elements in a list from front to back, while keeping track of an accumulator (which can be thought of as a running total). list_foldl takes three arguments, which all depend on the two type variables 'A and 'B:

  • The function processing the elements. This function takes two arguments. The first argument is the current value of the accumulator (of type 'B). The second argument is the next list element to be processed (of type 'A). The result of the function is the next value of the accumulator (of type 'B).

  • The initial value of the accumulator (of type 'B).
  • The list of elements to be processed (of type (List 'A)).

The result of applying list_foldl is the value of the accumulator (of type 'B) when all list elements have been processed.

  • list_foldr: ('A -> 'B -> 'B) -> 'B -> (List 'A) -> 'B : Similar to list_foldl, except the list elements are processed from back to front. Notice also that the processing function takes the list element and the accumulator in the opposite order from the order in list_foldl.

  • list_foldk: ('B -> 'A -> ('B -> 'B) -> 'B) -> 'B -> (List 'A) -> 'B : Recursively process the elements in a list according to a folding function, while keeping track of an accumulator. list_foldk is a more general version of the left and right folds, which, by the way, can be both implemented in terms of it. list_foldk takes three arguments, which all depend on the two type variables 'A and 'B:

  • The function describing the fold step. This function takes three arguments. The first argument is the current value of the accumulator (of type 'B). The second argument is the next list element to be processed (of type 'A). The third argument represents the postponed recursive call (of type 'B -> 'B). The result of the function is the next value of the accumulator (of type 'B). The computation terminates if the programmer does not invoke the postponed recursive call. This is a major difference between list_foldk and the left and right folds which process their input lists from the beginning to the end unconditionally.

  • The initial value of the accumulator z (of type 'B).
  • The list of elements to be processed (of type List 'A).

::: {.note} ::: {.title} Note :::

When an ADT takes type arguments (such as List 'A), and occurs inside a bigger type (such as the type of list_foldl), the ADT and its arguments must be grouped using parentheses ( ). This is the case even when the ADT occurs as the only argument to another ADT. For instance, when constructing an empty list of optional values of type Int32, one must instantiate the list type using the syntax Nil {(Option Int32)}. :::

To further illustrate the List type in Scilla, we show a small example using list_foldl to count the number of elements in a list. For an example of list_foldk see list_find.

let list_length : forall 'A. List 'A -> Uint32 =
   tfun 'A =>
   fun (l : List 'A) =>
   let foldl = @list_foldl 'A Uint32 in
   let init = Uint32 0 in
   let one = Uint32 1 in
   let iter =
     fun (z : Uint32) =>
     fun (h : 'A) =>
       builtin add one z
   in
     foldl iter init l

list_length defines a function that takes a type argument 'A, and a normal (value) argument l of type List 'A.

'A is a type variable which must be instantiated by the code that intends to use list_length. The type variable is specified in line 2.

In line 4 we instantiate the types for list_foldl. Since we are traversing a list of values of type 'A, we pass 'A as the first type argument to list_foldl, and since we are calculating the length of the list (a non-negative integer), we pass Uint32 as the accumulator type.

In line 5 we define the initial value of the accumulator. Since an empty list has length 0, the initial value of the accumulator is 0 (of type Uint32, to match the accumulator type).

In lines 6-10 we specify the processing function iter, which takes the current accumulator value z and the current list element h. In this case the processing function ignores the list element, and increments the accumulator by 1. When all elements in the list have been processed, the accumulator will have been incremented as many times as there are elements in the list, and hence the final value of the accumulator will be equal to the length of the list.

In line 12 we apply the type-instantiated version of list_foldl from line 4 to the processing function, the initial accumulator, and the list of values.

Common utilities for the List type (including list_length) are provided in the ListUtils library as part of the standard library distribution for Scilla.

Pair

::: {#Pairs} Pairs of values are specified using the type Pair t1 t2, where t1 and t2 are types. The Pair ADT has one constructor: :::

  • Pair represents a pair of values. The Pair constructor takes two arguments, namely the two values of the pair, of types t1 and t2, respectively.

::: {.note} ::: {.title} Note :::

Pair is both the name of a type and the name of a constructor of that type. An ADT and a constructor typically only share their names when the constructor is the only constructor of the ADT. :::

A Pair value may contain values of different types. In other words, t1 and t2 need not be the same type.

Below is an example where we declare a field pp of type Pair String Uint32, which we then initialise by constructing a pair consisting of a value of type String and a value of type Uint32:

field pp: Pair String Uint32 =
              let s1 = "Hello" in
              let num = Uint32 2 in
              Pair {String Uint32} s1 num

Notice the difference in how we specify the type of the field as Pair A' B', and how we specify the types of values given to the constructor as Pair { A' B' }.

We now illustrate how pattern matching can be used to extract the first element from a Pair. The function fst shown below is defined in the PairUtils library of the Scilla standard library. :::

let fst =
  tfun 'A =>
  tfun 'B =>
  fun (p : Pair ('A) ('B)) =>
    match p with
    | Pair a b => a
    end

To apply fst to one must first instantiate the type variables 'A and 'B, which is done as follows:

p <- pp;
fst_specialised = @fst String Uint32;
p_fst = fst_specialised p

The value associated with the identifier p_fst will be the string "Hello".

::: {.note} ::: {.title} Note :::

Using Pair is generally discouraged. Instead, the programmer should define an ADT which is specialised to the particular type of pairs that is needed in the particular use case. See the section on User-defined ADTs below. :::

Nat

Peano numbers are specified using the type Nat. The Nat ADT has two constructors:

  • Zero represents the number 0. The Zero constructor takes no arguments.
  • Succ represents the successor of another Peano number. The Succ constructor takes one argument (of type Nat) which represents the Peano number that is one less than the current number.

The following code shows how to build the Peano number corresponding to the integer 3:

let three =
  let zero = Zero in
  let one  = Succ zero in
  let two  = Succ one in
  Succ two

Scilla provides two structural recursion primitives for Peano numbers, which can be used to traverse all the Peano numbers from a given Nat down to Zero:

  • nat_fold: ('A -> Nat -> 'A) -> 'A -> Nat -> 'A: Recursively process the succession of numbers from a Nat down to Zero, while keeping track of an accumulator. nat_fold takes three arguments, two of which depend on the type variable 'A:

  • The function processing the numbers. This function takes two arguments. The first argument is the current value of the accumulator (of type 'A). The second argument is the next Peano number to be processed (of type Nat). Incidentally, the next number to be processed is the predecessor of the current number being processed. The result of the function is the next value of the accumulator (of type 'A).

  • The initial value of the accumulator (of type 'A).
  • The first Peano number to be processed (of type Nat).

The result of applying nat_fold is the value of the accumulator (of type 'A) when all Peano numbers down to Zero have been processed.

  • nat_foldk: ('A -> Nat -> ('A -> 'A) -> 'A) -> 'A -> Nat -> 'A: Recursively process the Peano numbers down to zero according to a folding function, while keeping track of an accumulator. nat_foldk is a more general version of the left fold allowing for early termination. It takes three arguments, two depending on the type variable 'A.

  • The function describing the fold step. This function takes three arguments. The first argument is the current value of the accumulator (of type 'A). The second argument is the predecessor of the Peano number being processed (of type Nat). The third argument represents the postponed recursive call (of type 'A -> 'A). The result of the function is the next value of the accumulator (of type 'A). The computation terminates if the programmer does not invoke the postponed recursive call. Left folds inevitably process the whole list whereas nat_foldk can differ in this regard.

  • The initial value of the accumulator z (of type 'A).
  • The Peano number to be processed (of type Nat).

To better understand nat_foldk, we explain how nat_eq works. nat_eq checks to see if two Peano numbers are equivalent. Below is the program, with line numbers and an explanation.

let nat_eq : Nat -> Nat -> Bool =
fun (n : Nat) => fun (m : Nat) =>
  let foldk = @nat_foldk Nat in
  let iter =
    fun (n : Nat) => fun (ignore : Nat) => fun (recurse : Nat -> Nat) =>
      match n with
      | Succ n_pred => recurse n_pred
      | Zero => m   (* m is not zero in this context *)
      end in
  let remaining = foldk iter n m in
  match remaining with
  | Zero => True
  |   _ => False
  end

Line 2 specifies that we take two Peano numbers m and n. Line 3 instantiates the type of nat_foldk, we give it Nat because we will be passing a Nat value as the fold accumulator.

Lines 4 to 8 specify the fold description, this is the first argument that nat_foldk takes usually of type 'A -> Nat -> ('A -> 'A) -> 'A but we have specified that 'A is Nat in this case. Our function takes the accumulator n and ignore : Nat is the predecessor of the number being processed which we don't care about in this particular case.

Essentially, we start accumulating the end result from n and iterate at most m times (see line 10), decrementing both n and m at each recursive step (lines 4 - 9). The m variable gets decremented implicitly because this is how nat_foldk works under the hood. And we explicitly decrement n using pattern matching (lines 6, 7). To continue iteratively decrement both m and n we use recurse on line 7. If the two input numbers are equal, we will get the accumulator (n) equal to zero in the end. We call the final value of the accumulator remaining on line 10. At the end we will be checking to see if our accumulator ended up at Zero to say if the input numbers are equal. The last lines, return True when the result of the fold is Zero and False otherwise as described above.

In the case when accumulator n reaches zero (line 8) while m still has not been fully processed, we stop iteration (hence no recurse on that line) and return a non-zero natural number to indicate inequality. Any number (e.g. Succ Zero) would do, but to make the code concise we return the original input number m because we know iter gets called on m only if it's not zero.

In the symmetrical case when m reaches zero while the accumulator n is still strictly positive, we indicate inequality, because remaining gets this final value of n.

User-defined ADTs

In addition to the built-in ADTs described above, Scilla supports user-defined ADTs.

ADT definitions may only occur in the library parts of a program, either in the library part of the contract, or in an imported library. An ADT definition is in scope in the entire library in which it is defined, except that an ADT definition may only refer to other ADT definitions defined earlier in the same library, or in imported libraries. In particular, an ADT definition may not refer to itself in an inductive/recursive manner.

Each ADT defines a set of constructors. Each constructor specifies a number of types which corresponds to the number and types of arguments that the constructor takes. A constructor may be specified as taking no arguments.

The ADTs of a contract must have distinct names, and the set of all constructors of all ADTs in a contract must also have distinct names. Both the ADT and constructor names must begin with a capital letter ('A' - 'Z'). However, a constructor and an ADT may have the same name, as is the case with the Pair type whose only constructor is also called Pair.

As an example of user-defined ADTs, consider the following type declarations from a contract implementing a chess-like game called Shogi or Japanese Chess (https://en.wikipedia.org/wiki/Shogi). When in turn, a player can choose to either move one of his pieces, place a previously captured piece back onto the board, or resign and award the victory to the opponent.

The pieces of the game can be defined using the following type Piece:

type Piece =
| King
| GoldGeneral
| SilverGeneral
| Knight
| Lance
| Pawn
| Rook
| Bishop

Each of the constructors represents a type of piece in the game. None of the constructors take any arguments.

The board is represented as a set of squares, where each square has two coordinates:

type Square =
| Square of Uint32 Uint32

The type Square is an example of a type where a constructor has the same name as the type. This usually happens when a type has only one constructor. The constructor Square takes two arguments, both of type Uint32, which are the coordinates (the row and the column) of the square on the board.

Similar to the definition of the type Piece, we can define the type of direction of movement using a constructor for each of the legal directions as follows:

type Direction =
| East
| SouthEast
| South
| SouthWest
| West
| NorthWest
| North
| NorthEast

We are now in a position to define the type of possible actions that a user may choose to perform when in turn:

type Action =
| Move of Square Direction Uint32 Bool
| Place of Piece Square
| Resign

If a player chooses to move a piece, she should use the constructor Move, and provide four arguments:

  • An argument of type Square, indicating the current position of the piece she wants to move.
  • An argument of type Direction, indicating the direction of movement.
  • An argument of type Uint32, indicating the distance the piece should move.
  • An argument of type Bool, indicating whether the moved piece should be promoted after being moved.

If instead the player chooses to place a previously captured piece back onto the board, she should use the constructor Place, and provide two arguments:

  • An argument of type Piece, indicating which piece to place on the board.
  • An argument of type Square, indicating the position the piece should be placed in.

Finally, if the player chooses to resign and award the victory to her opponent, she should use the constructor Resign. Since Resign does not take any arguments, no arguments should be provided.

To check which action a player has chosen we use a match statement or a match expression:

transition PlayerAction (action : Action)
  ...
  match action with
  | Resign =>
    ...
  | Place piece square =>
    ...
  | Move square direction distance promote =>
    ...
  end;
  ...
end

Type identity for user-defined ADTs

::: {.note} ::: {.title} Note :::

Due to a bug in the Scilla implementation the information in this section is only valid from Scilla version 0.10.0 and forwards. Contracts written in Scilla versions prior to 0.10.0 and which exploit this bug will have to be rewritten and redeployed, as they will no longer work from version 0.10.0 and onwards. :::

Each type declaration defines a unique type. In particular this means that even if two libraries both define identical types, the types are considered different.

As an example, consider the following two contracts:

library C1Lib

type T =
| C1 of Uint32
| C2 of Bool

contract Contract1()

field contract2_address : ByStr20 = 0x1234567890123456789012345678901234567890

transition Sending ()
  c2 <- contract2_address;
  x = Uint32 0;
  out = C1 x;
  msg = { _tag : "Receiving" ; _recipient : c2 ; _amount : Uint128 0 ;
         param : out };
  no_msg = Nil {Message};
  msgs = Cons {Message} msg no_msg;
  send msgs
end

(* ******************************* *)

(* Contract2 is deployed at address 0x1234567890123456789012345678901234567890 *)
library C2Lib

type T =
| C1 of Uint32
| C2 of Bool

contract Contract2()

transition Receiving (param : T)
  match param with
  | C1 v =>
  | C2 b =>
  end
end

Even though both contracts define identical types T, the two types are considered different in Scilla. In particular this means that the message sent from Contract1 to Contract2 will not trigger the Receiving transition, because the value sent as the param message field has the type T from Contract1, whereas the type T from Contract2 is expected.

In order to pass a value of a user-defined ADT as a parameter to a transition, the type must be defined in a user-defined library, which both the sending and the receiving contract must import:

library MutualLib

type T =
| C1 of Uint32
| C2 of Bool

(* ******************************* *)

import MutualLib

library C1Lib

contract Contract1()

field contract2_address : ByStr20 = 0x1234567890123456789012345678901234567890

transition Sending ()
  c2 <- contract2_address;
  x = Uint32 0;
  out = C1 x;
  msg = { _tag : "Receiving" ; _recipient : c2 ; _amount : Uint128 0 ;
         param : out };
  no_msg = Nil {Message};
  msgs = Cons {Message} msg no_msg;
  send msgs
end

(* ******************************* *)

(* Contract2 is deployed at address 0x1234567890123456789012345678901234567890 *)

scilla_version 0

import MutualLib

library C2Lib

contract Contract2()

transition Receiving (param : T)
  match param with
  | C1 v =>
  | C2 b =>
  end
end

The section user-defined_libraries has more information on how to define and use libraries.

More ADT examples

To further illustrate how ADTs can be used, we provide some more examples and describe them in detail. Versions of both the functions described below can be found in the ListUtils part of the Scilla standard library <stdlib>.

Computing the Head of a List

The function list_head returns the first element of a list.

Since a list may be empty, list_head may not always be able to compute a result, and thus should return a value of the Option type. If the list is non-empty, and the first element is h, then list_head should return Some h. Otherwise, if the list is empty, list_head should return None.

The following code snippet shows the implementation of list_head, and how to apply it:

let list_head =
  tfun 'A =>
  fun (l : List 'A) =>
    match l with
    | Cons h t =>
      Some {'A} h
    | Nil =>
      None {'A}
    end

let int_head = @list_head Int32 in

let one = Int32 1 in
let two = Int32 2 in
let three = Int32 3 in
let nil = Nil {Int32} in

let l1 = Cons {Int32} three nil in
let l2 = Cons {Int32} two l1 in
let l3 = Cons {Int32} one l2 in
int_head l3

Line 2 specifies that 'A is a type parameter to the function, while line 3 specifies that l is a (value) parameter of type List 'A. In other words, lines 1-3 specify a function list_head which can be instantiated for any type 'A, and which takes as an argument a value of type List 'A.

The pattern-match in lines 4-9 matches on the value of l. In line 5 we match on the list constructor Cons h t, where h is the first element of the list, and t is the rest of the list. If the list is not empty then the match is successful, and we return the first element as an optional value Some h. In line 7 we match on the list constructor Nil. If the list is empty then the match is successful, and we return the optional value None indicating that there was no head element of the list.

Line 11 instantiates the list_head function for the type Int32, so that list_head can be applied to values of type List Int32. Lines 13-20 build a list of type List Int32, and line 21 invokes the instantiated list_head function on the list that was built.

Computing a Left Fold

The function list_foldl returns the result of a left fold given a function f : 'B -> 'A -> 'B, accumulator z : 'B and list xs : List 'A. This can be implemented as a recursion primitive or a list utility function.

A left fold is a recursive application of an accumulator z and next list element x : 'A with f repetitively until there are no more list elements. For example the left fold on [1,2,3] using subtraction starting with accumulator 0 would be ((0-1)-2)-3 = -6. The left fold is explained in pseudocode below, note that the result is always the accumulator type.

list_foldl _ z [] = z
list_foldl f z (x:xs) = list_foldl f (f z x) xs

The same can be achieved with list_foldk by partially applying a left fold description; this avoids illegal direct recursion. Our fold description left_f : 'B -> 'A -> ('B -> 'B) -> 'B takes arguments accumulator, next list element and recursive call. The recursive call will be supplied by the list_foldk function. An implementation is explained below.

let list_foldl : forall 'A. forall 'B. ( 'B -> 'A -> 'B) -> 'B -> List 'A -> 'B =
tfun 'A => tfun 'B =>
fun (f : 'B -> 'A -> 'B) =>
let left_f = fun (z: 'B) => fun (x: 'A) =>
  fun (recurse : 'B -> 'B) => let res = f z x in
  recurse res in
let folder = @list_foldk 'A 'B in
folder left_f

On line 1, we declare the name and type signature as according to the first paragraph. On the second line, we say that the function takes two types as arguments 'A and 'B. The third line says that we take some function f to process the list element and accumulator, as in paragraph two.

On line 4, we define the fold description using f. The fold description does not take a function but instead it should be implemented in terms of some function, as according to the type signature, left_f : 'B -> 'A -> ('B -> 'B) -> 'B. left_f takes arguments as described in paragraph two. We calculate the new accumulator f z x and call it res. Then we recursively call with the new accumulator.

On line 7, we instantiate an instance of list_foldk that has the right types for the job using a type application.

On line 8, we partially apply folder with the left fold description. . What is significant about list_foldk is that when calling the description, it provides a recursive call to itself, changing to the next element in the list and respective tail each time. This results in a function that just needs the user to provide the updated accumulator in the description.

Computing a Right Fold

The function list_foldr returns the result of a right fold given some function f : 'A -> 'B -> 'B, accumulator z : 'B and list xs : List 'A. Like list_foldl, this can be a recursion primitive or a list utility function.

A right fold is similar to a left fold but is reversed in a way. The right fold applies a function f with an accumulator z starting from the end and then combines with the second last element, third last element, etc... until it reaches the beginning. For example a right fold on the list [1,2,3] with subtraction starting with accumulator 0 would be equal to 1-(2-(3-0)) = 2. It is listed below in pseudocode, note that the result is always the accumulator type.

list_foldr _ z [] = z
list_foldr f z (x:xs) = f x (list_foldr f z xs)

Like before, the same can be achieved with list_foldk by partially applying a right fold description. The fold description takes arguments accumulator z : 'B, next list element x : 'A and recursive call recurse : 'B -> 'B. The recursive call will be supplied by the list_foldk function. An implementation is explained below.

let list_foldr : forall 'A. forall 'B. ('A -> 'B -> 'B) -> 'B -> List 'A -> 'B =
tfun 'A => tfun 'B =>
fun (f : 'A -> 'B -> 'B) =>
let right_f = fun (z: 'B) => fun (x: 'A) =>
  fun (recurse : 'B -> 'B) => let res = recurse z in f x res in
let folder = @list_foldk 'A 'B in
folder right_f

This is very similar to before. On line 1 we declare the name and type signature, according to the first paragraph. On line 2, we take two type arguments 'A and 'B. The third line says that we take some function f to process the list element x : 'A and accumulator z. The argument order is necessarily different to that of a left fold.

Following that we write a fold description like before. list_foldk processes lists from left to right. But we need list_foldr to emulate the right-to-left traversal. By calling recurse z on line 5 as our first action, we postpone actual computation with the combining function f preserving the original accumulator until the very end. Once the recursive call reaches an empty list it returns the original accumulator. Then the function calls f x res (line 5) will evaluate outwards combining from the end to the beginning, see paragraph two.

The recursive call recurse z on line 5 may seem to be the same each time but what is changing is the list element we process.

On line 6, we instantiate list_foldk by applying the types 'A and 'B to make a type-specific function. The last line we partially apply folder with the right fold description. Like before what is special about list_foldk is that it calls this function with a recursive call to itself that each time slightly truncates the list; this provides the recursion.

Checking for Existence in a List

The function list_exists takes a predicate function and a list, and returns a value indicating whether the predicate holds for at least one element in the list.

A predicate function is a function returning a Boolean value, and since we want to apply it to elements in the list, the argument type of the function should be the same as the element type of the list.

list_exists should return either True (if the predicate holds for at least one element) or False (if the predicate does not hold for any element in the list), so the return type of list_exists should be Bool.

The following code snippet shows the implementation of list_exists, and how to apply it:

let list_exists =
  tfun 'A =>
  fun (f : 'A -> Bool) =>
  fun (l : List 'A) =>
    let folder = @list_foldl 'A Bool in
    let init = False in
    let iter =
      fun (z : Bool) =>
      fun (h : 'A) =>
        let res = f h in
        match res with
        | True =>
          True
        | False =>
          z
        end
    in
      folder iter init l

let int_exists = @list_exists Int128 in
let f =
  fun (a : Int128) =>
    let three = Int128 3 in
    builtin lt a three

(* build list l3 similar to the previous example *)
...

(* check if l3 has at least one element satisfying f *)
int_exists f l3

As in the previous example 'A is a type variable to the function. The function takes two arguments:

  • A predicate f, i.e., a function that returns a Bool. In this case, f will be applied to elements of the list, so the argument type of the predicate should be 'A. Hence, f should have the type 'A -> Bool.
  • A list of elements l of type List 'A, so that the type of the elements in the list matches the argument type of f.

To traverse the elements of the input list l we use list_foldl. In line 5 we instantiate list_foldl for lists with elements of type 'A and for the accumulator type Bool. In line 6 we set the initial accumulator value to False to indicate that no element satisfying the predicate has yet been seen.

The processing function iter defined in lines 7-16 tests the predicate on the current list element, and returns an updated accumulator. If an element has been found which satisfies the predicate, the accumulator is set to True and remains so for the rest of the traversal.

The final value of the accumulator is either True, indicating that f returned True for at least one element in the list, or False, indicating that f returned False for all elements in the list.

In line 20 we instantiate list_exists to work on lists of type Int128. In lines 21-24 we define the predicate, which returns True if its argument is less than 3, and returns False otherwise.

Omitted in line 27 is building the same list l3 as in the previous example. In line 30 we apply the instantiated list_exists to the predicate and the list.

Finding the first occurrence satisfying a predicate

The function list_find searches for the first occurrence in a list that satisfies some predicate p : 'A -> Bool. It takes the predicate and the list, returning Some {'A} x :: Option 'A if x is the first element such that p x and None {'A} :: Option 'A otherwise.

Below we have an implementation of list_find that illustrates how to use list_foldk.

let list_find : forall 'A. ('A -> Bool) -> List 'A -> Option 'A =
tfun 'A =>
fun (p : 'A -> Bool) =>
  let foldk = @list_foldk 'A (Option 'A) in
  let init = None {'A} in
  (* continue fold on None, exit fold when Some compare st. p(compare) *)
  let predicate_step =
    fun (ignore : Option 'A) => fun (x : 'A) =>
    fun (recurse: Option 'A -> Option 'A) =>
      let p_x = p x in
      match p_x with
      | True => Some {'A} x
      | False => recurse init
      end in
  foldk predicate_step init

Like before, we take a type variable 'A on line 2 and take the predicate on the next line. We begin by using this type variable to instantiate foldk, by giving it our processing type and return type. The processing type being the list element type and the result type being Option 'A. The next line is our accumulator, we assume that at the start of the search there is no satisfier.

On line 7, we write a fold description for foldk. This embodies the order of the recursion and conditions for recursion. predicate_step has the type Option 'A -> 'A -> (Option 'A -> Option 'A) -> Option 'A. The first argument is the accumulator, the second x is the next element to process and the third recurse is the recursive call. We do not care what the accumulator ignore is since if it mattered we will have already terminated.

On lines 10 to 12 check for p x and if so return Some {'A} x. In the case that p x does not hold, try again from scratch with the next element and so on via recursion. recurse init is in pseudo-code equal to λk. foldk predicate_step init k xs where xs is the tail of our list of to be processed elements.

With the final line we partially apply foldk so that it just takes a list argument and gives us our final answer. The first argument of foldk gives us the specific fold we want, for example if you wanted a left fold you would replace predicate_step with something else.

Standard Libraries

The Scilla standard library is documented<stdlib>{.interpreted-text role="doc"} separately.

User-defined Libraries

In addition to the standard library provided by Scilla, users are allowed to deploy library code on the blockchain. Library files are allowed to only contain pure Scilla code (which is the same restriction that in-contract library code has). Library files must use the .scillib file extension.

Below is an example of a user-defined library that defines a single function add_if_equal that adds to Uint128 values if they are equal and returns 0 otherwise.

import IntUtils

library ExampleLib

let add_if_equal =
  fun (a : Uint128) => fun (b : Uint128) =>
  let eq = uint128_eq a b in
  match eq with
  | True => builtin add a b
  | False => Uint128 0

The structure of a library file is similar to the structure of the library part of a Scilla contract. A library file contains definitions of variables and pure library functions, but does not contain an actual contract definition with parameters, fields, transitions and so on.

Of particular importance is that a library cannot declare fields. Therefore, all libraries are stateless and can only contain pure code.

Similar to how contracts can import libraries, a library can import other libraries (including user-defined libraries) too. The scope of variables in an imported library is restricted to the immediate importer. So if X imports library Y which in turn imports library Z, then the names in Z are not in scope in [X]{.title-ref}[, but only in ]{.title-ref}[Y]{.title-ref}`. Cyclic dependencies in imports are not allowed and flagged as errors during the checking phase.

Local Development with User-defined Libraries

To use variables and functions declared in an external (user-defined) library module, the command line argument to the Scilla executables must include a -libdir option, along with a list of directories as an argument. If the Scilla file imports a library ALib, then the Scilla executable will search for a library file called ALib.scillib in the directories provided. If more than one directory contains a file with the correct name, then the directories are given priority in the same order as they are provided to the Scilla executable. Alternatively, the environment variable SCILLA_STDLIB_PATH can be set to a list of library directories.

scilla-checker typechecks library modules in the same way as contract modules. Similarly, scilla-runner can deploy libraries. Note that scilla-runner takes a blockhain.json as argument (the way it does for Contract Creation <calling-interface>) to be command line argument compatible with contract creation.

User-defined Libraries on the Blockchain

While the Zilliqa blockchain is designed to provide the standard Scilla libraries to an executing contract, it must be provided with extra information to support user-defined libraries.

The [init.json]{.title-ref} of a library must include a Bool entry named _library, set to True:

[
  // ...,
  {
    vname: "_library",
    type: "Bool",
    value: { constructor: "True", argtypes: [], arguments: [] },
  },
];

For contracts the _library entry must be set to False.

A contract or a library that imports user-defined libraries must include in its [init.json]{.title-ref} an entry named _extlibs, of Scilla type List (Pair String ByStr20). Each entry in the list maps an imported library's name to its address in the blockchain.

Continuing the previous example, a contract or library that imports Examplelib should have the following entry in its init.json:

[
  // ...,
  {
    "vname" : "_library",
    "type" : "Bool",
    "value": { "constructor": "False", "argtypes": [], "arguments": [] }
  }
  {
    "vname" : "_extlibs",
    "type" : "List(Pair String ByStr20)",
    "value" : [
        {
            "constructor" : "Pair",
            "argtypes" : ["String", "ByStr20"],
            "arguments" : ["ExampleLib", "0x986556789012345678901234567890123456abcd"]
        },
        // ...
    ]
  }
]

Namespaces

Import statements can be used to define separate namespaces for imported names. To push the names from a library Foo into the namespace Bar, use the statement import Foo as Bar. Accessing a variable v in Foo must now be done using the qualified name Bar.v. This is useful when importing multiple libraries that define the same name.

The same variable name must not be defined more than once in the same namespace, so if multiple imported libraries define the same name, then at most one of the libraries may reside in the default (unqualified) namespace. All other conflicting libraries must be pushed to separate namespaces.

Extending our previous example, shown below is a contract that imports ExampleLib in namespace Bar, to use the function add_if_equal.

scilla_version 0

import ExampleLib as Bar

library MyContract

let adder = fun (a : Uint128) => fun (b : Uint128) =>
  Bar.add_if_equal a b

contract MyContract ()
...

Scilla versions

Major and Minor versions

Scilla releases have a major version, minor version and a patch number, denoted as X.Y.Z where X is the major version, Y is the minor version, and Z the patch number.

  • Patches are usually bug fixes that do not impact the behaviour of existing contracts. Patches are backward compatible.
  • Minor versions typically include performance improvements and feature additions that do not affect the behaviour of existing contracts. Minor versions are backward compatible until the latest major version.
  • Major versions are not backward compatible. It is expected that miners have access to implementations of each major version of Scilla for running contracts set to that major version.

Within a major version, miners are advised to use the latest minor revision.

The command scilla-runner -version will print major, minor and patch versions of the interpreter being invoked.

Contract Syntax

Every Scilla contract must begin with a major version declaration. The syntax is shown below:

(***************************************************)
(*                 Scilla version                  *)
(***************************************************)

scilla_version 0

(***************************************************)
(*               Associated library                *)
(***************************************************)

library MyContractLib

...

(***************************************************)
(*             Contract definition                 *)
(***************************************************)

contract MyContract

...

When deploying a contract the output of the interpreter contains the field scilla_version : X.Y.Z, to be used by the blockchain code to keep track of the version of the contract. Similarly, scilla-checker also reports the version of the contract on a successful check.

The init.json file

In addition to the version specified in the contract source code, it is also required that the contract's init.json specifies the same version when the contract is deployed and when the contract's transitions are invoked. This eases the process for the blockchain code to decide which interpreter to invoke.

A mismatch in the versions specified in init.json and the source code will lead to a gas-charged error by the interpreter.

An example init.json for a contract:

[
  {
    "vname": "_creation_block",
    "type": "BNum",
    "value": "1"
  },
  {
    "vname": "_scilla_version",
    "type": "Uint32",
    "value": "1"
  },
  {
    "vname": "_library",
    "type": "Bool",
    "value": { "constructor": "False", "argtypes": [], "arguments": [] }
  }
]

Chain Calls

::: {#Chaincalls} When a user invokes a transition on a contract by sending a message with the contract's address as the recipient, then that transition may then send one or more messages onwards, possibly invoking other transitions on other contracts. The resulting collection of messages, fund transfers, transition invocations, and contract state changes are referred to as a transaction. :::

A transition that sends a message invoking another transition (typically on another contract) is referred to as a chain call.

During a transaction a LIFO queue (i.e., a stack) of unprocessed messages is maintained. Initially, the message queue contains only the single message sent by the original user, but additional messages may be added to the queue when a transition performs a chain call.

When a transition finishes, its outgoing messages are added to the message queue. The first message in the queue is then removed from the queue for processing. If there are no messages left to process, then the transaction finishes, and all state changes and fund transfers are committed to the blockchain.

When a transition sends multiple messages, the messages are added to the queue in the following order:

  • If multiple send statements are executed, then the messages of the last send are added first. This means that the messages of the first send get processed first.
  • If a send statement is given a list with multiple messages in it, then the messages of the tail of the list are added to the queue before the head of the list is added. This means that the first message in the list gets processed first.

Any run-time failure during the execution of a transaction causes the entire transaction to be aborted, with no further statements being executed, no further messages being processed, all state changes being rolled back, and all transferred funds returned to their respective senders. However, gas is still charged for the transaction up until the point of the failure.

The total number of messages that can be sent in a single transaction is currently set at 20. The number is subject to revision in the future.

Contracts of different Scilla versions may invoke transitions on each other. The semantics of message passing between contracts is guaranteed to be backward compatible between major versions.

Accounting

For the transfer of native ZIL funds, Scilla follows an [acceptance semantics]{.title-ref}. For a transfer to take place between contracts the funds must explicitly be accepted by the recipient by executing an accept statement - it is not sufficient that the sender executes a send statement.

When a contract executes a send statement the _amount values of the outgoing messages are deducted from the _balance field. However, the funds are not yet added to the recipient's _balance field, because they have not yet been explicitly accepted by the recipient.

When the recipient executes an accept statement the _amount of the incoming message is added to the contract's _balance field.

If the recipient does not execute and accept statement, the _amount is transferred back to the sender before the next message is processed.

While the funds in a message are "in transit", i.e., between the point at which they is sent by the sender and the point at which they are accepted by the recipient (or, in the case of non-acceptance, the point at which the recipient transition finishes executing), the funds are stored away in the message itself, and therefore do not appear in neither the sender's nor the recipient's balances.

::: {.note} ::: {.title} Note :::

A user account (i.e., an address that does not hold a contract) implicitly accepts all incoming funds, but does not do so until the message carrying the funds is processed. :::

If a contract executes a send statement on messages for which the total amount is more than the contract's current balance, then a run-time error occurs, and the entire transaction is aborted. In other words, no account balance may drop below 0 at any point during a transaction.