Native Types

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

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.