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

Evaluates the equivalent of List.fold for an option.

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

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

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

// Usage:
fold folder state option

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: 'T option

The input option.

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

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: 
10
New list: [10]
Enter a number: 
1
New list: [1; 10]
Enter a number: 
abc
New list: [1; 10]
Enter a number: 
9
New list: [9; 1; 10]

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

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

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