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Fix the parameter order of Array.foldBack

The built in foldBack has parameters in an odd order: you can't pipe the array into it, and the folder signature is different from Array.fold's. Here's how to fix it.

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let letters = [|"a"; "b"; "c"; "d"|]

let forward = 
    letters
    |> Array.fold (fun acc elem -> acc + elem) "->"
// "->abcd"

// The built in foldBack has parameters in an odd order:
// you can't pipe the array into it, and the folder 
// signature is different from Array.fold's:
let nasty =
    Array.foldBack (fun elem acc -> acc + elem) letters "->"
// "->dcba"

// Let's fix that:
module Array =

    let foldBack' folder state array =
        let folder' x y = folder y x
        Array.foldBack folder' array state

// Now a foldBack call looks like a fold call but
// still goes backwards through the array:
let backward = 
    letters
    |> Array.foldBack' (fun acc elem -> acc + elem) "->"
// "->dcba"
val letters : string []

Full name: Script.letters
val forward : string

Full name: Script.forward
module Array

from Microsoft.FSharp.Collections
val fold : folder:('State -> 'T -> 'State) -> state:'State -> array:'T [] -> 'State

Full name: Microsoft.FSharp.Collections.Array.fold
val acc : string
val elem : string
val nasty : string

Full name: Script.nasty
val foldBack : folder:('T -> 'State -> 'State) -> array:'T [] -> state:'State -> 'State

Full name: Microsoft.FSharp.Collections.Array.foldBack
val foldBack' : folder:('a -> 'b -> 'a) -> state:'a -> array:'b [] -> 'a

Full name: Script.Array.foldBack'
val folder : ('a -> 'b -> 'a)
val state : 'a
Multiple items
val array : 'b []

--------------------
type 'T array = 'T []

Full name: Microsoft.FSharp.Core.array<_>
val folder' : ('b -> 'a -> 'a)
val x : 'b
val y : 'a
val backward : string

Full name: Script.backward
Multiple items
module Array

from Script

--------------------
module Array

from Microsoft.FSharp.Collections
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More information

Link:http://fssnip.net/7Vz
Posted:1 month ago
Author:Kit Eason
Tags: #fold , #foldback