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

Article
04/28/2015

Like List.foldBack, but returns both the intermediate and final results.

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

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

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

// Usage:
List.scanBack folder list state

Parameters

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

The function to update the state given the input elements.
list
Type: 'Tlist

The input list.
state
Type: 'State

The initial state.

Return Value

The list of states.

Remarks

This function is named ScanBack 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 shows how to use List.scanBack, and also contrasts its behavior with List.scan.

// A list of functions that transform  
// integers. (int -> int) 
let ops1 =
 [ (fun x -> x + 1), "add 1"
   (fun x -> x + 2), "add 2"
   (fun x -> x - 5), "subtract 5" ]

let ops2 =
 [ (fun x -> x + 1), "add 1"
   (fun x -> x * 5), "multiply by 5"
   (fun x -> x * x), "square" ]

// Compare scan and scanBack, which apply the 
// operations in the opposite order. 
let compareOpOrder ops x0 =
    let ops, opNames = List.unzip ops
    let xs1 = List.scan (fun x op -> op x) x0 ops
    let xs2 = List.scanBack (fun op x -> op x) ops x0

    printfn "Operations:"
    opNames |> List.iter (fun opName -> printf "%s  " opName)
    printfn "" 

    // Print the intermediate results. 
    let xs = List.zip xs1 (List.rev xs2)
    printfn "List.scan List.scanBack" 
    for (x1, x2) in xs do
        printfn "%10d %10d" x1 x2
    printfn ""

compareOpOrder ops1 10
compareOpOrder ops2 10

Output

Operations:
add 1  add 2  subtract 5  
List.scan List.scanBack
        10         10
        11          5
        13          7
         8          8

Operations:
add 1  multiply by 5  square  
List.scan List.scanBack
        10         10
        11        100
        55        500
      3025        501

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

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