string.md

id	stdlib-string
title	String Module
slug	/stdlib/string

String.replace : string -> string -> string -> string

Argument order is oldValue -> newValue -> source.

Replaces all non-overlapping occurrences using ordinal, case-sensitive matching. An empty oldValue returns source unchanged; an empty newValue removes matches. This operation always returns a string.

let text = String.replace "-" "_" "a-b-c"

String.indexOf : string -> string -> int option

Argument order is value -> source. Matching is ordinal and case-sensitive; returns the first zero-based index or None when absent.

let maybeIndex = String.indexOf "script" "fscript"

String.toLower : string -> string

Converts a string to lowercase using invariant casing.

let lower = String.toLower "FSCRIPT"

String.toUpper : string -> string

Converts a string to uppercase using invariant casing.

let upper = String.toUpper "fscript"

String.substring : int -> int -> string -> string option

Argument order is start -> length -> source. Returns None for invalid ranges.

let maybeSub = String.substring 0 4 "fscript"

String.concat : string -> string list -> string

Joins a list of strings with a separator.

let joined = String.concat "," ["a"; "b"; "c"]

String.split : string -> string -> string list

Argument order is separator -> source. Splits on the exact separator string and preserves empty entries, including trailing ones.

let parts = String.split "," "a,b,c"

String.endsWith : string -> string -> bool

Argument order is suffix -> source. Checks whether the source ends with the suffix.

let isScript = String.endsWith ".fss" "samples/hello.fss"

Pure support additions

Whitespace helpers use a fixed Unicode White_Space set on every runtime: U+0009–000D, U+0020, U+0085, U+00A0, U+1680, U+2000–200A, U+2028–2029, U+202F, U+205F, and U+3000. U+FEFF is preserved.

Count/size arguments must fit 0..2147483647; chunk sizes must also be positive. Invalid arguments raise script errors. Collection callbacks run in the documented order and may themselves have effects; these helpers are pure when their callbacks are pure.

String.length

String.length : string -> int

Returns the number of UTF-16 code units, not Unicode characters or grapheme clusters.

String.length "abc"
// 3

String.isEmpty

String.isEmpty : string -> bool

Tests whether the string has zero length.

String.isEmpty ""
// true

String.isWhiteSpace

String.isWhiteSpace : string -> bool

Tests whether the string is empty or consists only of Unicode whitespace.

String.isWhiteSpace " \t"
// true

String.trim

String.trim : string -> string

Removes Unicode whitespace from both ends.

String.trim " hi "
// "hi"

String.trimStart

String.trimStart : string -> string

Removes leading Unicode whitespace.

String.trimStart " hi "
// "hi "

String.trimEnd

String.trimEnd : string -> string

Removes trailing Unicode whitespace.

String.trimEnd " hi "
// " hi"

String.splitLines

String.splitLines : string -> string list

Splits CRLF, LF, or CR line endings. Empty and trailing lines are preserved; empty input produces [""].

String.splitLines "a\nb\n"
// ["a";"b";""]

String.startsWith

String.startsWith : string -> string -> bool

Tests an ordinal, case-sensitive prefix.

String.startsWith "ab" "abc"
// true

String.contains

String.contains : string -> string -> bool

Tests for an ordinal, case-sensitive substring. The empty substring matches every string.

String.contains "b" "abc"
// true

String.padLeft

String.padLeft : int -> string -> string

Pads on the left with spaces to the requested UTF-16 width. Strings already at least that wide are unchanged.

String.padLeft 3 "a"
// "  a"

String.padRight

String.padRight : int -> string -> string

Pads on the right with spaces to the requested UTF-16 width. Strings already at least that wide are unchanged.

String.padRight 3 "a"
// "a  "

String.repeat

String.repeat : int -> string -> string

Concatenates the string the requested number of times. Zero produces the empty string.

String.repeat 3 "ab"
// "ababab"

String.compare

String.compare : string -> string -> int

Returns -1, 0, or 1 using ordinal UTF-16 ordering, independently of culture. This is useful for custom comparers over records.

String.compare "a" "b"
// -1