stdlib-reference.md

id	stdlib-reference
title	Standard Library Signature Index
slug	/reference/stdlib

This is the complete catalog of core functions/values and default .NET runtime externs. Each name links to argument order, examples, and failure behavior. Type variables are local to each signature; repeated variables in a row must agree.

Core functions are always available. Runtime functions require host registration, are enabled by default in the CLI, and are disabled by --no-default-externs or runtime.default-externs = false. JavaScript hosts provide their own externs. Registered custom providers add functions beyond this standard catalog.

Top-level surface

Name	Type	Availability
ignore	'a -> unit	Core function; evaluates and discards its argument
Env	Environment	Value injected by CLI/REPL, optional in embedding hosts
Environment	{ ScriptName: string option; Arguments: string list }	Built-in record type
FsKind	File of string / Directory of string / Missing	Built-in union type

Core library

Bool

Name	Signature
Bool.toString	bool -> string
Bool.tryParse	string -> bool option

Float

Name	Signature
Float.toString	float -> string
Float.tryParse	string -> float option

Int

Name	Signature
Int.toString	int -> string
Int.tryParse	string -> int option

List

Name	Signature
List.append	'a list -> 'a list -> 'a list
List.choose	('a -> 'b option) -> 'a list -> 'b list
List.collect	('a -> 'b list) -> 'a list -> 'b list
List.contains	'a -> 'a list -> bool
List.distinct	'a list -> 'a list
List.empty	'a list
List.exists	('a -> bool) -> 'a list -> bool
List.filter	('a -> bool) -> 'a list -> 'a list
List.fold	('b -> 'a -> 'b) -> 'b -> 'a list -> 'b
List.iter	('a -> unit) -> 'a list -> unit
List.length	'a list -> int
List.map	('a -> 'b) -> 'a list -> 'b list
List.rev	'a list -> 'a list
List.tail	'a list -> 'a list
List.tryFind	('a -> bool) -> 'a list -> 'a option
List.tryGet	('a -> bool) -> 'a list -> int option
List.tryHead	'a list -> 'a option
List.tryItem	int -> 'a list -> 'a option

Map

Name	Signature
Map.add	'b -> 'a -> 'a map['b] -> 'a map['b]
Map.choose	('b -> 'a -> 'c option) -> 'a map['b] -> 'c map['b]
Map.containsKey	'b -> 'a map['b] -> bool
Map.count	'a map['b] -> int
Map.empty	'a map['b]
Map.filter	('b -> 'a -> bool) -> 'a map['b] -> 'a map['b]
Map.fold	('c -> 'b -> 'a -> 'c) -> 'c -> 'a map['b] -> 'c
Map.iter	('b -> 'a -> unit) -> 'a map['b] -> unit
Map.map	('a -> 'c) -> 'a map['b] -> 'c map['b]
Map.ofList	('b * 'a) list -> 'a map['b]
Map.remove	'b -> 'a map['b] -> 'a map['b]
Map.tryGet	'b -> 'a map['b] -> 'a option

Option

Name	Signature
Option.defaultValue	'a -> 'a option -> 'a
Option.defaultWith	(unit -> 'a) -> 'a option -> 'a
Option.isNone	'a option -> bool
Option.isSome	'a option -> bool
Option.map	('a -> 'b) -> 'a option -> 'b option

String

Name	Signature
String.concat	string -> string list -> string
String.endsWith	string -> string -> bool
String.indexOf	string -> string -> int option
String.replace	string -> string -> string -> string
String.split	string -> string -> string list
String.substring	int -> int -> string -> string option
String.toLower	string -> string
String.toUpper	string -> string

Default .NET runtime

Console

Name	Signature
Console.readLine	unit -> string option
Console.writeLine	string -> unit

Fs

Name	Signature
Fs.combinePath	string -> string -> string
Fs.createDirectory	string -> bool
Fs.enumerateFiles	string -> string -> string list option
Fs.exists	string -> bool
Fs.extension	string -> string option
Fs.fileNameWithoutExtension	string -> string
Fs.glob	string -> string list option
Fs.kind	string -> FsKind
Fs.parentDirectory	string -> string option
Fs.readText	string -> string option
Fs.writeText	string -> string -> bool

Guid

Name	Signature
Guid.new	'a -> string option

Hash

Name	Signature
Hash.md5	string -> string option

Json

Name	Signature
Json.deserialize	type -> string -> 'a option
Json.serialize	'a -> string option

Regex

Name	Signature
Regex.matchGroups	string -> string -> string list option

Task

Name	Signature
Task.await	'a task -> 'a
Task.spawn	(unit -> 'a) -> 'a task

Xml

Name	Signature
Xml.queryValues	string -> string -> string list option

Syntax and data forms

Constructors and syntax are separate from function registration: Some/None, union cases, literals, ranges, field access, list/map indexers, typeof, and nameof are covered in Native Types and Syntax and Operators. Host-provided type descriptors are covered in Host-Provided Types.

Json.deserialize uses a runtime type token; its generic result scheme does not statically encode the relationship between that token and the result. Guid.new accepts an ignored argument; use Guid.new (). List.contains compares scalars only; use List.exists with = for composite data.