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List.choose<'T,'U> Function (F#)

Applies the given function f to each element x of the list. Returns the list comprised of the results for each element where the function returns Some(f(x)).

Namespace/Module Path: Microsoft.FSharp.Collections.List

Assembly: FSharp.Core (in FSharp.Core.dll)

// Signature:
List.choose : ('T -> 'U option) -> 'T list -> 'U list

// Usage:
List.choose chooser list

chooser

Type: 'T -> 'U option

The function to generate options from the elements.

list

Type: 'T list

The input list.

The list comprising the values selected from the chooser function.

This function is named Choose in compiled assemblies. If you are accessing the function from a language other than F#, or through reflection, use this name.

The following code demonstrates the use of List.choose to select capitalized words out of a list of words.


let listWords = [ "and"; "Rome"; "Bob"; "apple"; "zebra" ]
let isCapitalized (string1:string) = System.Char.IsUpper string1.[0]
let results = List.choose (fun elem ->
    match elem with
    | elem when isCapitalized elem -> Some(elem + "'s")
    | _ -> None) listWords
printfn "%A" results


Output

["Rome's"; "Bob's"]

Windows 7, Windows Vista SP2, Windows XP SP3, Windows XP x64 SP2, Windows Server 2008 R2, Windows Server 2008 SP2, Windows Server 2003 SP2

F# Runtime

Supported in: 2.0, 4.0

Silverlight

Supported in: 3

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