Contracts

The main unit of code in Sophia is the contract.

  • A contract implementation, or simply a contract, is the code for a smart contract and consists of a list of types, entrypoints and local functions. Only the entrypoints can be called from outside the contract.
  • A contract instance is an entity living on the block chain (or in a state channel). Each instance has an address that can be used to call its entrypoints, either from another contract or in a call transaction.
  • A contract may define a type state encapsulating its local state. When creating a new contract the init entrypoint is executed and the state is initialized to its return value.

The language offers some primitive functions to interact with the blockchain and contracts. Please refer to the Chain, Contract and the Call namespaces in the documentation.

Calling other contracts

To call a function in another contract you need the address to an instance of the contract. The type of the address must be a contract type, which consists of a number of type definitions and entrypoint declarations. For instance,

// A contract type
contract VotingType =
  entrypoint vote : string => unit

Now given contract address of type VotingType you can call the vote entrypoint of that contract:

contract VoteTwice =
  entrypoint voteTwice(v : VotingType, alt : string) =
    v.vote(alt)
    v.vote(alt)

Contract calls take two optional named arguments gas : int and value : int that lets you set a gas limit and provide tokens to a contract call. If omitted the defaults are no gas limit and no tokens. Suppose there is a fee for voting:

  entrypoint voteTwice(v : VotingType, fee : int, alt : string) =
    v.vote(value = fee, alt)
    v.vote(value = fee, alt)

Named arguments can be given in any order.

Note that reentrant calls are not permitted. In other words, when calling another contract it cannot call you back (directly or indirectly).

To construct a value of a contract type you can give a contract address literal (for instance ct_2gPXZnZdKU716QBUFKaT4VdBZituK93KLvHJB3n4EnbrHHw4Ay), or convert an account address to a contract address using Address.to_contract. Note that if the contract does not exist, or it doesn't have the entrypoint, or the type of the entrypoint does not match the stated contract type, the call fails.

To recover the underlying address of a contract instance there is a field address : address. For instance, to send tokens to the voting contract (given that it is payable) without calling it you can write

  entrypoint pay(v : VotingType, amount : int) =
    Chain.spend(v.address, amount)

Protected contract calls

Attention

This feature is not part of the current protocol version and could be implemented in a future protocol upgrade.

If a contract call fails for any reason (for instance, the remote contract crashes or runs out of gas, or the entrypoint doesn't exist or has the wrong type) the parent call also fails. To make it possible to recover from failures, contract calls takes a named argument protected : bool (default false). The protected argument must be a literal boolean, and when set to true changes the type of the contract call, wrapping the result in an option type. If the call fails the result is None, otherwise it's Some(r) where r is the return value of the call.

contract VotingType =
  entrypoint : vote : string => unit
contract Voter =
  entrypoint tryVote(v : VotingType, alt : string) =
    switch(v.vote(alt, protected = true) : option(unit))
      None    => "Voting failed"
      Some(_) => "Voting successful"

Any gas that was consumed by the contract call before the failure stays consumed, which means that in order to protect against the remote contract running out of gas it is necessary to set a gas limit using the gas argument. However, note that errors that would normally consume all the gas in the transaction still only uses up the gas spent running the contract.

Mutable state

Sophia does not have arbitrary mutable state, but only a limited form of state associated with each contract instance.

  • Each contract defines a type state encapsulating its mutable state. The type state defaults to the unit.
  • The initial state of a contract is computed by the contract's init function. The init function is pure and returns the initial state as its return value. If the type state is unit, the init function defaults to returning the value (). At contract creation time, the init function is executed and its result is stored as the contract state.
  • The value of the state is accessible from inside the contract through an implicitly bound variable state.
  • State updates are performed by calling a function put : state => unit.
  • Aside from the put function (and similar functions for transactions and events), the language is purely functional.
  • Functions modifying the state need to be annotated with the stateful keyword (see below).

To make it convenient to update parts of a deeply nested state Sophia provides special syntax for map/record updates.

Stateful functions

Top-level functions and entrypoints must be annotated with the stateful keyword to be allowed to affect the state of the running contract. For instance,

  stateful entrypoint set_state(s : state) =
    put(s)

Without the stateful annotation the compiler does not allow the call to put. A stateful annotation is required to

  • Use a stateful primitive function. These are
  • put
  • Chain.spend
  • Oracle.register
  • Oracle.query
  • Oracle.respond
  • Oracle.extend
  • AENS.preclaim
  • AENS.claim
  • AENS.transfer
  • AENS.revoke
  • Call a stateful function in the current contract
  • Call another contract with a non-zero value argument.

A stateful annotation is not required to

  • Read the contract state.
  • Issue an event using the event function.
  • Call another contract with value = 0, even if the called function is stateful.

Payable

Payable contracts

A concrete contract is by default not payable. Any attempt at spending to such a contract (either a Chain.spend or a normal spend transaction) will fail. If a contract shall be able to receive funds in this way it has to be declared payable:

// A payable contract
payable contract ExampleContract =
  stateful entrypoint do_stuff() = ...

If in doubt, it is possible to check if an address is payable using Address.is_payable(addr).

Payable entrypoints

A contract entrypoint is by default not payable. Any call to such a function (either a Remote call or a contract call transaction) that has a non-zero value will fail. Contract entrypoints that should be called with a non-zero value should be declared payable.

payable stateful entrypoint buy(to : address) =
  if(Call.value > 42)
    transfer_item(to)
  else
    abort("Value too low")

Note: In the Aeternity VM (AEVM) contracts and entrypoints were by default payable until the Lima release.

Namespaces

Code can be split into libraries using the namespace construct. Namespaces can appear at the top-level and can contain type and function definitions, but not entrypoints. Outside the namespace you can refer to the (non-private) names by qualifying them with the namespace (Namespace.name). For example,

namespace Library =
  type number = int
  function inc(x : number) : number = x + 1

contract MyContract =
  entrypoint plus2(x) : Library.number =
    Library.inc(Library.inc(x))

Functions in namespaces have access to the same environment (including the Chain, Call, and Contract, builtin namespaces) as function in a contract, with the exception of state, put and Chain.event since these are dependent on the specific state and event types of the contract.

Splitting code over multiple files

Info

This feature is currently not supported by the AEstudio IDE and shall only be used with the standalone compiler.

Code from another file can be included in a contract using an include statement. These must appear at the top-level (outside the main contract). The included file can contain one or more namespaces and abstract contracts. For example, if the file library.aes contains

namespace Library =
  function inc(x) = x + 1

you can use it from another file using an include:

include "library.aes"
contract MyContract =
  entrypoint plus2(x) = Library.inc(Library.inc(x))

This behaves as if the contents of library.aes was textually inserted into the file, except that error messages will refer to the original source locations. The language will try to include each file at most one time automatically, so even cyclic includes should be working without any special tinkering.

Standard library

Sophia offers standard library which exposes some primitive operations and some higher level utilities. The builtin namespaces like Chain, Contract, Map are included by default and are supported internally by the compiler. Others like List, Frac, Option need to be manually included using the include directive. For example

include "List.aes"
include "Pair.aes"
-- Map is already there!

namespace C =
  entrypoint keys(m : map('a, 'b)) : list('a) =
    List.map(Pair.fst, (Map.to_list(m)))

Types

Sophia has the following types:

Type Description Example
int A 2-complement integer -1
address Aeternity address, 32 bytes Call.origin
bool A Boolean true
bits A bit field Bits.none
bytes(n) A byte array with n bytes #fedcba9876543210
string An array of bytes "Foo"
list A homogeneous immutable singly linked list. [1, 2, 3]
('a, 'b) => 'c A function. Parentheses can be skipped if there is only one argument (x : int, y : int) => x + y
tuple An ordered heterogeneous array (42, "Foo", true)
record An immutable key value store with fixed key names and typed values record balance = { owner: address, value: int }
map An immutable key value store with dynamic mapping of keys of one type to values of one type type accounts = map(string, address)
option('a) An optional value either None or Some('a) Some(42)
state A user defined type holding the contract state record state = { owner: address, magic_key: bytes(4) }
event An append only list of blockchain events (or log entries) datatype event = EventX(indexed int, string)
hash A 32-byte hash - equivalent to bytes(32)
signature A signature - equivalent to bytes(64)
Chain.ttl Time-to-live (fixed height or relative to current block) FixedTTL(1050) RelativeTTL(50)
oracle('a, 'b) And oracle answering questions of type 'a with answers of type 'b Oracle.register(acct, qfee, ttl)
oracle_query('a, 'b) A specific oracle query Oracle.query(o, q, qfee, qttl, rttl)
contract A user defined, typed, contract address function call_remote(r : RemoteContract) = r.fun()

Literals

Type Constant/Literal example(s)
int -1, 2425, 4598275923475723498573485768
address ak_2gx9MEFxKvY9vMG5YnqnXWv1hCsX7rgnfvBLJS4aQurustR1rt
bool true, false
bits Bits.none, Bits.all
bytes(8) #fedcba9876543210
string "This is a string"
list [1, 2, 3], [(true, 24), (false, 19), (false, -42)]
tuple (42, "Foo", true)
record { owner = Call.origin, value = 100000000 }
map {["foo"] = 19, ["bar"] = 42}, {}
option(int) Some(42), None
state state{ owner = Call.origin, magic_key = #a298105f }
event EventX(0, "Hello")
hash #000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f
signature #000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f
Chain.ttl FixedTTL(1050), RelativeTTL(50)
oracle('a, 'b) ok_2YNyxd6TRJPNrTcEDCe9ra59SVUdp9FR9qWC5msKZWYD9bP9z5
oracle_query('a, 'b) oq_2oRvyowJuJnEkxy58Ckkw77XfWJrmRgmGaLzhdqb67SKEL1gPY
contract ct_Ez6MyeTMm17YnTnDdHTSrzMEBKmy7Uz2sXu347bTDPgVH2ifJ

Arithmetic

Sophia integers (int) are represented by 256-bit (AEVM) or arbitrary-sized (FATE) signed words and supports the following arithmetic operations: - addition (x + y) - subtraction (x - y) - multiplication (x * y) - division (x / y), truncated towards zero - remainder (x mod y), satisfying y * (x / y) + x mod y == x for non-zero y - exponentiation (x ^ y)

All operations are safe with respect to overflow and underflow. On AEVM they behave as the corresponding operations on arbitrary-size integers and fail with arithmetic_error if the result cannot be represented by a 256-bit signed word. For example, 2 ^ 255 fails rather than wrapping around to -2²⁵⁵.

The division and modulo operations also throw an arithmetic error if the second argument is zero.

Bit fields

Sophia integers do not support bit arithmetic. Instead there is a separate type bits. See the standard library documentation.

On the AEVM a bit field is represented by a 256-bit word and reading or writing a bit outside the 0..255 range fails with an arithmetic_error. On FATE a bit field can be of arbitrary size (but it is still represented by the corresponding integer, so setting very high bits can be expensive).

Type aliases

Type aliases can be introduced with the type keyword and can be parameterized. For instance

type number = int
type string_map('a) = map(string, 'a)

A type alias and its definition can be used interchangeably. Sophia does not support higher-kinded types, meaning that following type alias is invalid: type wrap('f, 'a) = 'f('a)

Algebraic data types

Sophia supports algebraic data types (variant types) and pattern matching. Data types are declared by giving a list of constructors with their respective arguments. For instance,

datatype one_or_both('a, 'b) = Left('a) | Right('b) | Both('a, 'b)

Elements of data types can be pattern matched against, using the switch construct:

function get_left(x : one_or_both('a, 'b)) : option('a) =
  switch(x)
    Left(x)    => Some(x)
    Right(_)   => None
    Both(x, _) => Some(x)

or directly in the left-hand side:

function
  get_left : one_or_both('a, 'b) => option('a)
  get_left(Left(x))    = Some(x)
  get_left(Right(_))   = None
  get_left(Both(x, _)) = Some(x)

NOTE: Data types cannot currently be recursive.

Lists

A Sophia list is a dynamically sized, homogenous, immutable, singly linked list. A list is constructed with the syntax [1, 2, 3]. The elements of a list can be any of datatype but they must have the same type. The type of lists with elements of type 'e is written list('e). For example we can have the following lists:

[1, 33, 2, 666]                                                   : list(int)
[(1, "aaa"), (10, "jjj"), (666, "the beast")]                     : list(int * string)
[{[1] = "aaa", [10] = "jjj"}, {[5] = "eee", [666] = "the beast"}] : list(map(int, string))

New elements can be prepended to the front of a list with the :: operator. So 42 :: [1, 2, 3] returns the list [42, 1, 2, 3]. The concatenation operator ++ appends its second argument to its first and returns the resulting list. So concatenating two lists [1, 22, 33] ++ [10, 18, 55] returns the list [1, 22, 33, 10, 18, 55].

Sophia supports list comprehensions known from languages like Python, Haskell or Erlang. Example syntax:

[x + y | x <- [1,2,3,4,5], let k = x*x, if (k > 5), y <- [k, k+1, k+2]]
// yields [12,13,14,20,21,22,30,31,32]

Lists can be constructed using the range syntax using special .. operator:

[1..4] == [1,2,3,4] 

The ranges are always ascending and have step equal to 1.

Please refer to the standard library for the predefined functionalities.

Maps and records

A Sophia record type is given by a fixed set of fields with associated, possibly different, types. For instance

  record account = { name    : string,
                     balance : int,
                     history : list(transaction) }

Maps, on the other hand, can contain an arbitrary number of key-value bindings, but of a fixed type. The type of maps with keys of type 'k and values of type 'v is written map('k, 'v). The key type can be any type that does not contain a map or a function type.

Please refer to the standard library for the predefined functionalities.

Constructing maps and records

A value of record type is constructed by giving a value for each of the fields. For the example above,

  function new_account(name) =
    {name = name, balance = 0, history = []}

Maps are constructed similarly, with keys enclosed in square brackets

  function example_map() : map(string, int) =
    {["key1"] = 1, ["key2"] = 2}

The empty map is written {}.

Accessing values

Record fields access is written r.f and map lookup m[k]. For instance,

  function get_balance(a : address, accounts : map(address, account)) =
    accounts[a].balance

Looking up a non-existing key in a map results in contract execution failing. A default value to return for non-existing keys can be provided using the syntax m[k = default]. See also Map.member and Map.lookup below.

Updating a value

Record field updates are written r{f = v}. This creates a new record value which is the same as r, but with the value of the field f replaced by v. Similarly, m{[k] = v} constructs a map with the same values as m except that k maps to v. It makes no difference if m has a mapping for k or not.

It is possible to give a name to the old value of a field or mapping in an update: instead of acc{ balance = acc.balance + 100 } it is possible to write acc{ balance @ b = b + 100 }, binding b to acc.balance. When giving a name to a map value (m{ [k] @ x = v }), the corresponding key must be present in the map or execution fails, but a default value can be provided: m{ [k = default] @ x = v }. In this case x is bound to default if k is not in the map.

Updates can be nested:

function clear_history(a : address, accounts : map(address, account)) : map(address, account) =
  accounts{ [a].history = [] }

This is equivalent to accounts{ [a] @ acc = acc{ history = [] } } and thus requires a to be present in the accounts map. To have clear_history create an account if a is not in the map you can write (given a function empty_account):

  accounts{ [a = empty_account()].history = [] }

Map implementation

Internally in the VM maps are implemented as hash maps and support fast lookup and update. Large maps can be stored in the contract state and the size of the map does not contribute to the gas costs of a contract call reading or updating it.

Strings

There is a builtin type string, which can be seen as an array of bytes. Strings can be compared for equality (==, !=), used as keys in maps and records, and used in builtin functions String.length, String.concat and the hash functions described below.

Please refer to the String library documentation.

Byte arrays

Byte arrays are fixed size arrays of 8-bit integers. They are described in hexadecimal system, for example the literal #cafe creates a two-element array of bytes ca (202) and fe (254) and thus is a value of type bytes(2).

Please refer to the Bytes library documentation.

Cryptographic builtins

Libraries Crypto and String provide functions to hash objects, verify signatures etc. The hash is a type alias for bytes(32).

AEVM note

The hash functions in String hash strings interpreted as byte arrays, and the Crypto hash functions accept an element of any (first-order) type. The result is the hash of the binary encoding of the argument as described below. Note that this means that for s : string, String.sha3(s) and Crypto.sha3(s) will give different results on AEVM.

Authorization interface

When a Generalized account is authorized, the authorization function needs access to the transaction and the transaction hash for the wrapped transaction. (A GAMetaTx wrapping a transaction.) The transaction and the transaction hash is available in the primitive Auth.tx and Auth.tx_hash respectively, they are only available during authentication if invoked by a normal contract call they return None.

Oracle interface

You can attach an oracle to the current contract and you can interact with oracles through the Oracle interface.

For a full description of how Oracle works see Oracles. For a functionality documentation refer to the standard library.

Example

Example for an oracle answering questions of type string with answers of type int:

contract Oracles =

  stateful entrypoint registerOracle(acct : address,
                                     sign : signature,   // Signed oracle address + contract address
                                     qfee : int,
                                     ttl  : Chain.ttl) : oracle(string, int) =
     Oracle.register(acct, signature = sign, qfee, ttl)

  entrypoint queryFee(o : oracle(string, int)) : int =
    Oracle.query_fee(o)

  payable stateful entrypoint createQuery(o    : oracle_query(string, int),
                                          q    : string,
                                          qfee : int,
                                          qttl : Chain.ttl,
                                          rttl : int) : oracle_query(string, int) =
    require(qfee =< Call.value, "insufficient value for qfee")
    Oracle.query(o, q, qfee, qttl, RelativeTTL(rttl))

  stateful entrypoint extendOracle(o   : oracle(string, int),
                                   ttl : Chain.ttl) : unit =
    Oracle.extend(o, ttl)

  stateful entrypoint signExtendOracle(o    : oracle(string, int),
                                     sign : signature,   // Signed oracle address + contract address
                                     ttl  : Chain.ttl) : unit =
    Oracle.extend(o, signature = sign, ttl)

  stateful entrypoint respond(o    : oracle(string, int),
                              q    : oracle_query(string, int),
                              sign : signature,        // Signed oracle query id + contract address
                              r    : int) =
    Oracle.respond(o, q, signature = sign, r)

  entrypoint getQuestion(o : oracle(string, int),
                         q : oracle_query(string, int)) : string =
    Oracle.get_question(o, q)

  entrypoint hasAnswer(o : oracle(string, int),
                       q : oracle_query(string, int)) =
    switch(Oracle.get_answer(o, q))
      None    => false
      Some(_) => true

  entrypoint getAnswer(o : oracle(string, int),
                       q : oracle_query(string, int)) : option(int) =
    Oracle.get_answer(o, q)

Sanity checks

When an Oracle literal is passed to a contract, no deep checks are performed. For extra safety Oracle.check and Oracle.check_query functions are provided.

AENS interface

Contracts can interact with the Aeternity Naming System. For this purpose the AENS library was exposed.

Example

In this example we assume that the name name already exists, and is owned by an account with address addr. In order to allow a contract ct to handle name the account holder needs to create a signature sig of addr | name.hash | ct.address.

Armed with this information we can for example write a function that extends the name if it expires within 1000 blocks:

  stateful entrypoint extend_if_necessary(addr : address, name : string, sig : signature) =
    switch(AENS.lookup(name))
      None => ()
      Some(AENS.Name(_, FixedTTL(expiry), _)) =>
        if(Chain.block_height + 1000 > expiry)
          AENS.update(addr, name, Some(RelativeTTL(50000)), None, None, signature = sig)

And we can write functions that adds and removes keys from the pointers of the name:

  stateful entrypoint add_key(addr : address, name : string, key : string,
                              pt : AENS.pointee, sig : signature) =
    switch(AENS.lookup(name))
      None => ()
      Some(AENS.Name(_, _, ptrs)) =>
        AENS.update(addr, name, None, None, Some(ptrs{[key] = pt}), signature = sig)

  stateful entrypoint delete_key(addr : address, name : string,
                                 key : string, sig : signature) =
    switch(AENS.lookup(name))
      None => ()
      Some(AENS.Name(_, _, ptrs)) =>
        let ptrs = Map.delete(key, ptrs)
        AENS.update(addr, name, None, None, Some(ptrs), signature = sig)

Events

Sophia contracts log structured messages to an event log in the resulting blockchain transaction. The event log is quite similar to Events in Solidity. Events are further discussed in the protocol.

To use events a contract must declare a datatype event, and events are then logged using the Chain.event function:

  datatype event 
    = Event1(int, int, string)
    | Event2(string, address)

  Chain.event(e : event) : unit

The event can have 0-3 indexed fields, and an optional payload field. A field is indexed if it fits in a 32-byte word, i.e. - bool - int - bits - address - oracle(_, _) - oracle_query(_, _) - contract types - bytes(n) for n ≤ 32, in particular hash

The payload field must be either a string or a byte array of more than 32 bytes. The fields can appear in any order.

NOTE: Indexing is not part of the core aeternity node.

Events are emitted by using the Chain.event function. The following function will emit one Event of each kind in the example.

  entrypoint emit_events() : () =
    Chain.event(TheFirstEvent(42))
    Chain.event(AnotherEvent(Contract.address, "This is not indexed"))

Argument order

It is only possible to have one (1) string parameter in the event, but it can be placed in any position (and its value will end up in the data field), i.e.

AnotherEvent(string, indexed address)

...

Chain.event(AnotherEvent("This is not indexed", Contract.address))

would yield exactly the same result in the example above!

Compiler pragmas

To enforce that a contract is only compiled with specific versions of the Sophia compiler, you can give one or more @compiler pragmas at the top-level (typically at the beginning) of a file. For instance, to enforce that a contract is compiled with version 4.3 of the compiler you write

@compiler >= 4.3
@compiler <  4.4

Valid operators in compiler pragmas are <, =<, ==, >=, and >. Version numbers are given as a sequence of non-negative integers separated by dots. Trailing zeros are ignored, so 4.0.0 == 4. If a constraint is violated an error is reported and compilation fails.

Exceptions

Contracts can fail with an (uncatchable) exception using the built-in function

abort(reason : string) : 'a

Calling abort causes the top-level call transaction to return an error result containing the reason string. Only the gas used up to and including the abort call is charged. This is different from termination due to a crash which consumes all available gas.

For convenience the following function is also built-in:

function require(b : bool, err : string) =
    if(!b) abort(err)