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

Den Delimarsky|Last Updated: 8/18/2017
1 Contributor

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 (... (f s i0)...) iN.

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

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


// Signature:
Seq.fold : ('State -> 'T -> 'State) -> 'State -> seq<'T> -> 'State

// Usage:
Seq.fold folder state source


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

A function that updates the state with each element from the sequence.

state Type: 'State

The initial state.

source Type: seq<'T>

The input sequence.


ArgumentNullExceptionThrown when the input sequence is null.

Return Value

The final result of the computation.


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 shows how to use Seq.fold to implement a function that computes the sum of the elements of a sequence.

let sumSeq sequence1 = Seq.fold (fun acc elem -> acc + elem) 0 sequence1
Seq.init 10 (fun index -> index * index)
|> sumSeq
|> printfn "The sum of the elements is %d."


The sum of the elements is 285.


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

Collections.Seq Module (F#)

Microsoft.FSharp.Collections Namespace (F#)

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