Fix the parameter order of Array.foldBack

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"