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List.foldBack<'T,'State> Function (F#)

Applies a function to each element of the collection, threading an accumulator argument through the computation. If the input function is f and the elements are i0...iN, then this function computes f i0 (...(f iN s)).

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

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

// Signature:
List.foldBack : ('T -> 'State -> 'State) -> 'T list -> 'State -> 'State

// Usage:
List.foldBack folder list state


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

The function to update the state given the input elements.


Type: 'T list

The input list.


Type: 'State

The initial state.

The final state value.

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

The following code example illustrates the use of List.foldBack.

let sumListBack list = List.foldBack (fun acc elem -> acc + elem) list 0
printfn "%d" (sumListBack [1; 2; 3])

// For a calculation in which the order of traversal is important, fold and foldBack have different 
// results. For example, replacing foldBack with fold in the copyList function 
// produces a function that reverses the list, rather than copying it. 
let copyList list = List.foldBack (fun elem acc -> elem::acc) list []
printfn "%A" (copyList [1 .. 10])


[1; 2; 3; 4; 5; 6; 7; 8; 9; 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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