Types

Scalars

Scalar types in Aqua follow the Wasm IT notation.

• Unsigned numbers: u8, u16, u32, u64
• Signed numbers: i8, i16, i32, i64
• Floats: f32, f64
• Boolean: bool
• String: string

Data Types

In Aqua terminology, data types are types that actually represent value at runtime. In other words, it is any type that is an immutable combination of scalars:

• Scalars
• Immutable Collections
• Structures

Collection Types

There are two kinds of collections in Aqua: immutable and appendable. To denote a collection of a given type T, use the quantifier before the type name like so:

• ?T for an immutable option of T, e.g. ?[]i64, ?string
• []T for an immutable array of T, e.g. []u32, []?Result
• *T for an appendable stream of T, e.g. *[]Response, *?string

Immutable Collections

Aqua has two types of immutable collections:

• Arrays containing a fixed number of elements of the same type. The quantifier is [], e.g. []string, []?Report, [][]i64 are all valid array types. To access an element of an array, use:
  • The square brackets notation with any expression of integer type inside, e.g. arr[10], arr[arr.length - 1].
  • The bang notation, optionally followed by a number or a variable, e.g. arr!10, arr!index, arr! (the same as arg!0).
• Options containing none or one element of a given type. The quantifier is ?, e.g. ?string, ?[]Error are all valid option types. To access an element of an option, use the bang notation, e.g. opt! or opt!0. There is a literal that represents an empty option: nil.

Appendable Collections

Aqua has appendable streams. The quantifier is *, e.g. *?string, *Result, *[]i64 are all valid stream types. To access an element of a stream, use:

• The square brackets notation, with any expression of integer type inside, e.g. stream[10], stream[requests.length - 1]
• The bang notation, optionally followed by a number or a variable, e.g. stream!5, stream!index, stream! (the same as stream!0). Note that accessing an element of a stream could trigger join behavior on the stream.

Only data types can be used as elements of any collection, e.g. []u32, ?[]string, *[]?u64 are valid types, but []*u32, ?*string, **u64 are not.

For every collection type functor length is defined, e.g. option.length, arr.length, stream.length. It returns the number of elements in the collection.

To see collections usage examples, see Collections.

Structures

Structure types are product types of data types. They are defined with the data keyword:

aqua
data InnerStruct:
    arr: []string
    num: u32
data SomeStruct:
    str: string
    num: u64
    inner: InnerStruct

aqua
data InnerStruct:
    arr: []string
    num: u32
data SomeStruct:
    str: string
    num: u64
    inner: InnerStruct

Structures could be instantiated with the following syntax:

aqua
func createStruct(i: []u32) -> SomeStruct:
    <- SomeStruct(
        str = "some str",
        num = 4,
        inner = InnerStruct(arr = ["a", "b", "c"], num = i[2])
    )

aqua
func createStruct(i: []u32) -> SomeStruct:
    <- SomeStruct(
        str = "some str",
        num = 4,
        inner = InnerStruct(arr = ["a", "b", "c"], num = i[2])
    )

Note that the order of fields does not matter and it is possible to pass just a variable if it has the same name as a field:

aqua
func createStruct(i: []u32) -> SomeStruct:
    str = "some str"
    inner = InnerStruct(["a", "b", "c"], i[2])
    <- SomeStruct(
      str, -- short for `str = str`
      num = 4,
      inner -- short for `inner = inner`
    )

aqua
func createStruct(i: []u32) -> SomeStruct:
    str = "some str"
    inner = InnerStruct(["a", "b", "c"], i[2])
    <- SomeStruct(
      str, -- short for `str = str`
      num = 4,
      inner -- short for `inner = inner`
    )

To modify a structure value, there's a method called 'copy'. It creates a copy with values of specified fields changed. Note that copy operates in an immutable way: it does not modify original structure value.

aqua
func changeStr(someStruct: SomeStruct) -> SomeStruct:
    <- someStruct.copy(str = "new string")

aqua
func changeStr(someStruct: SomeStruct) -> SomeStruct:
    <- someStruct.copy(str = "new string")

Passing just a variable is also possible with copy:

aqua
func changeStr(someStruct: SomeStruct) -> SomeStruct:
    str = "new string"
    <- someStruct.copy(str) -- short for `str = str`

aqua
func changeStr(someStruct: SomeStruct) -> SomeStruct:
    str = "new string"
    <- someStruct.copy(str) -- short for `str = str`

Fields are accessible with the dot operator . , e.g. product.field.

Arrow Types

Every function has an arrow type that maps a list of input types to an optional output type.

It can be denoted as: Type1, Type2 -> Result

In the type definition, the absence of a result is denoted with (), e.g., string -> ()

The absence of arguments is denoted -> ().That is, this mapping takes no argument and has no return type.

Note that there's no Unit type in Aqua: you cannot assign a non-existing result to a value.

aqua
-- Assume that arrow has type: -> ()
-- This is possible:
arrow()
-- This will lead to error:
x <- arrow()

aqua
-- Assume that arrow has type: -> ()
-- This is possible:
arrow()
-- This will lead to error:
x <- arrow()

Type Alias

It is possible to alias a type to a name. It may help with self-documented code and refactoring.

aqua
alias PeerId: string
alias MyDomain: DomainType

aqua
alias PeerId: string
alias MyDomain: DomainType

Type Variance

Aqua is made for composing data on the open network. That means that you want to compose things if they do compose, even if you don't control its source code.

Therefore Aqua follows the structural typing paradigm: if a type contains all the expected data, then it fits. For example, you can pass u8 in place of u16 or i16, ?bool in place of []bool (or vice-versa), *string instead of ?string or []string. The same holds for products.

But note that immutable collections could not be used in place of appendable streams, e.g. []string is not a subtype of *string.

For arrow types, Aqua checks the variance on arguments and contravariance on the return type.

aqua
-- We expect u32
xs: *u32
-- u16 is less then u32
foo1: -> u16
-- works
xs <- foo1()
-- i32 has sign, so cannot fit into u32
foo2: -> i32
-- will fail
xs <- foo2()
-- Function takes an arrow as an argument
func bar(callback: u32 -> u32): ...
foo3: u16 -> u16
-- Will not work
bar(foo3)  
foo4: u64 -> u16
-- Works
bar(foo4)

aqua
-- We expect u32
xs: *u32
-- u16 is less then u32
foo1: -> u16
-- works
xs <- foo1()
-- i32 has sign, so cannot fit into u32
foo2: -> i32
-- will fail
xs <- foo2()
-- Function takes an arrow as an argument
func bar(callback: u32 -> u32): ...
foo3: u16 -> u16
-- Will not work
bar(foo3)  
foo4: u64 -> u16
-- Works
bar(foo4)

Arrow type A: D -> C is a subtype of A1: D1 -> C1, if D1 is a subtype of D and C is a subtype of C1.

Service type

A service type is a product of arrows.

aqua
service MyService:
  foo(arg: string) -> bool

aqua
service MyService:
  foo(arg: string) -> bool

See services

Ability type

An ability type is a product of arrows, scalars, structures and other abilities.

aqua
ability Ability:
    arrow(x: i32) -> i32
    field: string
    struct: SomeStruct
    ab: AnotherAbility

aqua
ability Ability:
    arrow(x: i32) -> i32
    field: string
    struct: SomeStruct
    ab: AnotherAbility

See abilities

Type of a file

A file is a product of all defined constants and functions (treated as arrows). Type definitions in the file do not go to the file type.

aqua
-- MyFile.aqua
func foo(arg: string) -> bool:
  ...
const FLAG ?= true

aqua
-- MyFile.aqua
func foo(arg: string) -> bool:
  ...
const FLAG ?= true

See Imports and Exports for module declarations.

See the types system implementation