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

Parameters

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

  The function to update the state given the input elements.

• list
  Type: 'T list

  The input list.

• state
  Type: 'State

  The initial state.

Return Value

The final state value.

Remarks

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.

Example

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])

Output

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

Reference

Collections.List Module (F#)

Microsoft.FSharp.Collections Namespace (F#)