Question

I keep practicing my F# skills on Project Euler. While doing problem 19 (a drastic anticlimax after the cool problem 18) I found myself in need of performing a modulo 7 operation on a list of numbers. I tried this:

List.map ((%) 7)

And ended up with the wrong list of numbers. That’s because List.map binds the second argument, so instead of calculation n%7 it ended up calculating 7%n . Is there a way to make List.map bind the first argument, or a standard way of flipping the two arguments?

I know I can use a fun n->n%7 instead of (%) 7, but I was hoping for a shorter solution.

Answer 1

You can define flip for this purpose:

namespace Microsoft.FSharp.Core
module Operators =
    let flip f x y = f y x

This is a rather common way of doing this in functional programming languages. For example, Haskell also has a flip function in its Prelude.