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Event.pairwise<'Del,'T> Function (F#)

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

Returns a new event that triggers on the second and subsequent triggerings of the input event. The Nth triggering of the input event passes the arguments from the N-1th and Nth triggering as a pair. The argument passed to the N-1th triggering is held in hidden internal state until the Nth triggering occurs.

Namespace/Module Path: Microsoft.FSharp.Control.Event

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


// Signature:
Event.pairwise : IEvent<'Del,'T> -> IEvent<'T * 'T> (requires delegate)

// Usage:
Event.pairwise sourceEvent


sourceEvent Type: IEvent<'Del,'T>

The input event.

Return Value

An event that triggers on pairs of consecutive values passed from the source event.


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


In this example, the function makes data available from more than one MouseMove event, and the data is used to draw a line between consecutive mouse positions.

open System.Windows.Forms
open System.Drawing

let form = new Form(Text = "F# Windows Form",
                    Visible = true,
                    TopMost = true)

let graphics = BufferedGraphicsManager.Current.Allocate(form.CreateGraphics(), 
                                new Rectangle( 0, 0, form.Width, form.Height ))
let whitePen = new Pen(Color.White)

    |> Event.pairwise
    |> Event.add ( fun (evArgs1, evArgs2) ->
        graphics.Graphics.DrawLine(whitePen, evArgs1.X, evArgs1.Y, evArgs2.X, evArgs2.Y)

    |> Event.add(fun evArgs -> graphics.Render(evArgs.Graphics))


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

Control.Event Module (F#)

Microsoft.FSharp.Control Namespace (F#)

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