Table of contents
TOC
Collapse the table of content
Expand the table of content

Option.fold<'T,'State> Function (F#)

Den Delimarsky|Last Updated: 7/8/2016
|
2 Contributors

Evaluates the equivalent of List.fold for an option.

Namespace/Module Path: Microsoft.FSharp.Core.Option

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

Syntax

// Signature:
fold : ('State -> 'T -> 'State) -> 'State -> 'T option -> 'State

// Usage:
fold folder state option

Parameters

folder Type: 'State -> 'T -> 'State

A function to update the state data when given a value from an option.

state Type: 'State

The initial state.

option Type: 'Toption

The input option.

The original state if the option is None, otherwise it returns the updated state with the folder and the option value.

Remarks

The expression fold f s inp evaluates to match inp with None -> s | Some x -> f s x.

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

The following code illustrates the use of Option.fold.


    let consOption list opt =
        Option.fold (fun state value -> value :: state) list opt
    printfn "%A" <| consOption [1 .. 10] None
    printfn "%A" <| consOption [1 .. 10] (Some(0))

    // Read input from the console, and if the input parses as
    // an integer, cons to the list.
    let readNumber () =
        let line = System.Console.ReadLine()
        let (success, value) = System.Int32.TryParse(line)
        if success then Some(value) else None
    let mutable list1 = []
    let mutable count = 0
    while count < 5 do
        printfn "Enter a number: "
        list1 <- consOption list1 (readNumber())
        printfn "New list: %A" <| list1
        count <- count + 1

Output[1; 2; 3; 4; 5; 6; 7; 8; 9; 10][0; 1; 2; 3; 4; 5; 6; 7; 8; 9; 10]Enter a number: New list: []Enter a number: 10New list: [10]Enter a number: 1New list: [1; 10]Enter a number: abcNew list: [1; 10]Enter a number: 9New list: [9; 1; 10]

Platforms

Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

Version Information

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

See Also

Core.Option Module (F#)

Microsoft.FSharp.Core Namespace (F#)

© 2016 Microsoft