Operators.( << )<'T2,'T3,'T1> Function (F#)

Article
02/04/2013

Composes two functions, the function on the right being applied first.

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

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

// Signature:
( << ) : ('T2 -> 'T3) -> ('T1 -> 'T2) -> 'T1 -> 'T3

// Usage:
func2 << func1

Parameters

func2
Type: 'T2 -> 'T3

The second function to apply.
func1
Type: 'T1 -> 'T2

The first function to apply.

Return Value

The composition of the input functions.

Remarks

This operator is referred to as the backward or reverse composition operator.

Example

The following example demonstrates the use of the reverse composition operator (<<).

let append1 string1 = string1 + ".append1"
let append2 string1 = string1 + ".append2"

// Reverse composition of two functions.
let appendBothReverse = append1 << append2
printfn "%s" (appendBothReverse "abc")

// Reverse composition of three functions.
let append3 string1 = string1 + ".append3"
printfn "%s" ((append1 << append2 << append3) "abc")

// Reverse composition of functions with more than one parameter.
let appendString (string1:string) (string2:string) = string1 + string2

let appendFileExtension extension =
    appendString extension << appendString "." 
printfn "%s" (appendFileExtension "myfile" "txt")

abc.append2.append1
abc.append3.append2.append1
myfile.txt

Platforms

Windows 7, Windows Vista SP2, Windows XP SP3, Windows XP x64 SP2, Windows Server 2008 R2, Windows Server 2008 SP2, Windows Server 2003 SP2

Version Information

F# Runtime

Supported in: 2.0, 4.0

Silverlight

Supported in: 3