Native Types

This page describes the built-in data shapes that are part of the language itself rather than stdlib modules.

Tasks

Type form: 'a task

Form	Signature	Description
Task.spawn (fun () -> expr)	(unit -> 'a) -> 'a task	Starts concurrent work and returns a task handle.
Task.await taskValue	'a task -> 'a	Waits for completion and returns the task result.

let pending = Task.spawn (fun () -> 40 + 2)
let answer = Task.await pending

Tasks are opaque values. They do not expose indexers or fields. The 'a task type is native to the language, while Task.spawn and Task.await come from the default runtime extern registry and may be omitted by an embedding host.

Lists

Type form: 'a list

Form	Signature	Description
[]	'a list	Empty list literal.
head :: tail	'a -> 'a list -> 'a list	Prepends one item to a list.
left @ right	'a list -> 'a list -> 'a list	Concatenates two lists.
values[index]	'a list -> int -> 'a option	Zero-based optional indexer. Negative or out-of-range indices return None.

let xs = [10; 20; 30]
let maybeSecond = xs[1]

Maps

Type form: 'v map

Map keys are always string.

Form	Signature	Description
{}	'v map	Empty map literal.
{ ["key"] = value }	string -> 'v -> 'v map	Map literal entry syntax.
values[key]	'v map -> string -> 'v option	Optional key lookup. Missing keys return None.
{ ..tail }	'v map -> 'v map	Spread syntax used when building or updating a map.

let scores = { ["math"] = 18; ["english"] = 16 }
let maybeMath = scores["math"]

Options

Type form: 'a option

Form	Signature	Description
Some value	'a -> 'a option	Wraps a present value.
None	'a option	Represents the absence of a value.

Use pattern matching or Option.* helpers to consume options.

Tuples

Type form: (t1 * t2 * ...)

Form	Signature	Description
(a, b)	'a * 'b	Tuple literal.
let (a, b) = value	('a * 'b) -> unit	Tuple destructuring in bindings.
tuple pattern matching	('a * 'b) -> 'r	Use match value with (a, b) -> ... to decompose tuples.

Tuples do not have a native indexer. Destructure them instead.

Records

Type form: { Field: t; ... }

Form	Signature	Description
{ Name = value }	field-set dependent	Record literal.
record.Field	{ Field: 'a; ... } -> 'a	Field access by name.
{ record with Field = value }	same record type -> same record type	Record update syntax.

Unions

Type form: type Shape = | Point | Circle of int

Form	Signature	Description
Case	case dependent	Union constructor without payload.
Case value	payload type -> union type	Union constructor with payload.
union pattern matching	union type -> 'r	Use match value with Case x -> ... to branch on union cases.

Scalars

Primitive built-in types:

• int
• float
• bool
• string
• unit

These do not use indexers. For parsing and formatting helpers, see Int, Float, Bool Modules and String Module.