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Features

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 interface 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

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 interface 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.

Any side effects (state change, token transfers, etc.) made by a failing protected call is rolled back, just like they would be in the unprotected case.

Contract factories and child contracts

Since the version 6.0.0 Sophia supports deploying contracts by other contracts. This can be done in two ways:

These functions take variable number of arguments that must match the created contract's init function. Beside that they take some additional named arguments – please refer to their documentation for the details.

While Chain.clone requires only a contract interface and a living instance of a given contract on the chain, Chain.create needs a full definition of a to-create contract defined by the standard contract syntax, for example

contract IntHolder =
  type state = int
  entrypoint init(x) = x
  entrypoint get() = state

main contract IntHolderFactory =
  stateful entrypoint new(x : int) : IntHolder =
    let ih = Chain.create(x) : IntHolder
    ih

In case of a presence of child contracts (IntHolder in this case), the main contract must be pointed out with the main keyword as shown in the example.

Contract interfaces and polymorphism

Contracts can implement one or multiple interfaces, the contract has to define every entrypoint from the implemented interface and the entrypoints in both the contract and implemented interface should have compatible types.

contract interface Animal =
  entrypoint sound : () => string

 contract Cat : Animal =
  entrypoint sound() = "Cat sound"

Contract interfaces can extend other interfaces. An extended interface has to declare all entrypoints from every parent interface. All the declarations in the extended interface must have types compatible with the declarations from the parent interface.

contract interface II =
  entrypoint f : () => unit

contract interface I : II =
  entrypoint f : () => unit
  entrypoint g : () => unit

contract C : I =
  entrypoint f() = ()
  entrypoint g() = ()

It is only possible to implement (or extend) an interface that has been already defined earlier in the file (or in an included file). Therefore recursive interface implementation is not allowed in Sophia.

// The following code would show an error

contract interface X : Z =
     entrypoint x : () => int

 contract interface Y : X =
     entrypoint x : () => int
     entrypoint y : () => int

 contract interface Z : Y =
     entrypoint x : () => int
     entrypoint y : () => int
     entrypoint z : () => int

 contract C : Z =
     entrypoint x() = 1
     entrypoint y() = 1
     entrypoint z() = 1

Subtyping and variance

Subtyping in Sophia follows common rules that take type variance into account. As described by Wikipedia,

Variance refers to how subtyping between more complex types relates to subtyping between their components.

This concept plays an important role in complex types such as tuples, datatypes and functions. Depending on the context, it can apply to positions in the structure of a type, or type parameters of generic types. There are four kinds of variances:

  • covariant
  • contravariant
  • invariant
  • bivariant

A type is said to be on a "covariant" position when it describes output or a result of some computation. Analogously, position is "contravariant" when it is an input, or a parameter. Intuitively, when a part of the type is produced by values of it, it is covariant. When it is consumed, it is contravariant. When a type appears to be simultaneously input and output, it is described as invariant. If a type is neither of those (that is, it's unused) it's bivariant. Furthermore, whenever a complex type appears on a contravariant position, all its covariant components become contravariant and vice versa.

Variance influences how subtyping is applied. Types on covariant positions are subtyped normally, while contravariant the opposite way. Invariant types have to be exactly the same in order for subtyping to work. Bivariant types are always compatible.

A good example of where it matters can be pictured by subtyping of function types. Let us assume there is a contract interface Animal and two contracts that implement it: Dog and Cat.

contract interface Animal =
  entrypoint age : () => int

contract Dog : Animal =
  entrypoint age() = // ...
  entrypoint woof() = "woof"

contract Cat : Animal =
  entrypoint age() = // ...
  entrypoint meow() = "meow"

The assumption of this exercise is that cats do not bark (because Cat does not define the woof entrypoint). If subtyping rules were applied naively, that is if we let Dog => Dog be a subtype of Animal => Animal, the following code would break:

let f : (Dog) => string  = d => d.woof()
let g : (Animal) => string  = f
let c : Cat = Chain.create()
g(c)  // Cat barking!

That is because arguments of functions are contravariant, as opposed to return the type which is covariant. Because of that, the assignment of f to g is invalid - while Dog is a subtype of Animal, Dog => string is not a subtype of Animal => string. However, Animal => string is a subtype of Dog => string. More than that, (Dog => Animal) => Dog is a subtype of (Animal => Dog) => Animal.

This has consequences on how user-defined generic types work. A type variable gains its variance from its role in the type definition as shown in the example:

datatype co('a) = Co('a) // co is covariant on 'a
datatype ct('a) = Ct('a => unit) // ct is contravariant on 'a
datatype in('a) = In('a => 'a) // in is invariant on 'a
datatype bi('a) = Bi // bi is bivariant on 'a

The following facts apply here:

  • co('a) is a subtype of co('b) when'ais a subtype of'b`
  • ct('a) is a subtype of ct('b) when'bis a subtype of'a`
  • in('a) is a subtype of in('b) when'ais equal to'b`
  • bi('a) is a subtype of `bi('b) always

That altogether induce the following rules of subtyping in Sophia:

  • A function type (Args1) => Ret1 is a subtype of (Args2) => Ret2 when Ret1 is a subtype of Ret2 and each argument type from Args2 is a subtype of its counterpart in Args1.

  • A list type list(A) is a subtype of list(B) if A is a subtype of B.

  • An option type option(A) is a subtype of option(B) if A is a subtype of B.

  • A map type map(A1, A2) is a subtype of map(B1, B2) if A1 is a subtype of B1, and A2 is a subtype of B2.

  • An oracle type oracle(A1, A2) is a subtype of oracle(B1, B2) if B1 is a subtype of A1, and A2 is a subtype of B2.

  • An oracle_query type oracle_query(A1, A2) is a subtype of oracle_query(B1, B2) if A1 is a subtype of B1, and A2 is a subtype of B2.

  • A user-defined datatype t(Args1) is a subtype of t(Args2)

  • When a user-defined type t('a) is covariant in 'a, then t(A) is a subtype of t(B) when A is a subtype of B.

  • When a user-defined type t('a) is contravariant in 'a, then t(A) is a subtype of t(B) when B is a subtype of A.

  • When a user-defined type t('a) is binvariant in 'a, then t(A) is a subtype of t(B) when either A is a subtype of B or when B is a subtype of A.

  • When a user-defined type t('a) is invariant in 'a, then t(A) can never be a subtype of t(B).

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
  • AENS.update
  • 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")

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.

To avoid mentioning the namespace every time it is used, Sophia allows including the namespace in the current scope with the using keyword: 

include "Pair.aes"
using Pair
contract C =
  type state = int
  entrypoint init() =
    let p = (1, 2)
    fst(p)  // this is the same as Pair.fst(p)

It is also possible to make an alias for the namespace with the as keyword: 

include "Pair.aes"
contract C =
  using Pair as P
  type state = int
  entrypoint init() =
    let p = (1, 2)
    P.fst(p)  // this is the same as Pair.fst(p)

Having the same alias for multiple namespaces is possible and it allows referening functions that are defined in different namespaces and have different names with the same alias: 

namespace Xa = function f() = 1
namespace Xb = function g() = 2
contract Cntr =
  using Xa as A
  using Xb as A
  type state = int
  entrypoint init() = A.f() + A.g()

Note that using functions with the same name would result in an ambiguous name error: 

namespace Xa = function f() = 1
namespace Xb = function f() = 2
contract Cntr =
  using Xa as A
  using Xb as A
  type state = int

  // the next line has an error because f is defined in both Xa and Xb
  entrypoint init() = A.f()

Importing specific parts of a namespace or hiding these parts can also be done like this: 

using Pair for [fst, snd]       // this will only import fst and snd
using Triple hiding [fst, snd]  // this will import everything except for fst and snd

Note that it is possible to use a namespace in the top level of the file, in the contract level, namespace level, or in the function level.

Splitting code over multiple files

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 æternity 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 arbitrary-sized signed words and support 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. The division and modulo operations throw an arithmetic error if the right-hand operand is zero.

Bit fields

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

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.

Sophia also supports the assignment of patterns to variables: 

function f(x) = switch(x)
  h1::(t = h2::_) => (h1 + h2)::t  // same as `h1::h2::k => (h1 + h2)::h2::k`
  _ => x

function g(p : int * option(int)) : int =
  let (a, (o = Some(b))) = p  // o is equal to Pair.snd(p)
  b

Guards are boolean expressions that can be used on patterns in both switch statements and functions definitions. If a guard expression evaluates to true, then the corresponding body will be used. Otherwise, the next pattern will be checked:

function get_left_if_positive(x : one_or_both(int, 'b)) : option(int) =
  switch(x)
    Left(x)    | x > 0 => Some(x)
    Both(x, _) | x > 0 => Some(x)
    _                  => None

function
  get_left_if_positive : one_or_both(int, 'b) => option(int)
  get_left_if_positive(Left(x))    | x > 0 = Some(x)
  get_left_if_positive(Both(x, _)) | x > 0 = Some(x)
  get_left_if_positive(_)                  = None

Guards cannot be stateful even when used inside a stateful function.

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.

Chars

There is a builtin type char (the underlying representation being an integer), mainly used to manipulate strings via String.to_list/String.from_list.

Characters can also be introduced as character literals (`'x', '+', ...).

Please refer to the Char 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).

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 network id + 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 network id + 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 network id + 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 æternity 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)

Note: From the Iris hardfork more strict rules apply for AENS pointers, when a Sophia contract lookup or update (bad) legacy pointers, the bad keys are automatically removed so they will not appear in the pointers map.

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 æternity 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(Event1(42, 34, "foo"))
    Chain.event(Event2("This is not indexed", Contract.address))

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)

Aside from that, there is an almost equivalent function exit

exit(reason : string) : 'a

Just like abort, it breaks the execution with the given reason. The difference however is in the gas consumption — while abort returns unused gas, a call to exit burns it all.

Delegation signature

Some chain operations (Oracle.<operation> and AENS.<operation>) have an optional delegation signature. This is typically used when a user/accounts would like to allow a contract to act on it's behalf. The exact data to be signed varies for the different operations, but in all cases you should prepend the signature data with the network_id (ae_mainnet for the æternity mainnet, etc.).