Collections

FScript has lists, tuples, options, records, maps, and unions.

Lists

let numbers = [1; 2; 3]
let doubled = numbers |> List.map (fun n -> n * 2)

Tuples

let pair = ("api", 3)

Options

let maybeName = Some "Ada"
let name = maybeName |> Option.defaultValue "unknown"

Records

type Person = { Name: string; Age: int }
let p = { Name = "Ada"; Age = 37 }
let older = { p with Age = 38 }

Maps

let scores = { ["math"] = 18; ["science"] = 20 }
let maybeMath = scores |> Map.tryGet "math"

Keys may be string, int, bool, or a discriminated union whose cases all have no payload. A map has one key type and one value type, inferred from its entries.

type Priority = | Low | High
let byId = { [0] = "first"; [1] = "second" }
let byFlag = { [true] = "enabled"; [false] = "disabled" }
let byPriority = { [Low] = "later"; [High] = "now" }
let maybeHigh = byPriority[High]

When an annotation is useful, put the value type first: string map[int], string map[bool], or string map[Priority]. For string keys, use int map; int map[string] is equivalent and is displayed as int map.

Function arguments also participate in inference:

let getZero m = m |> Map.tryGet 0
let maybeFirst = byId |> getZero

getZero is inferred as 'v map[int] -> 'v option, with no argument annotation. An empty map ({} or Map.empty) gets its key and value types from later use.

See the Map module for operations and key restrictions, and Pattern Matching for map patterns.