Overloading + operator in F#

Question

So i have this:

open System
open System.Linq
open Microsoft.FSharp.Collections
type Microsoft.FSharp.Collections.List<\'a> with
    static member (+) (         


        

Answer 1

Actually there is a way to 're-wire' existing operators, using static constraints and overloads.

type ListExtension = ListExtension with
    static member        (?<-) (ListExtension, a , b) = a @ b
    static member inline (?<-) (ListExtension, a , b) = a + b

let inline (+) a b = (?<-) ListExtension a b

// test

let lst = [1;2] + [3;4]
// val lst : int list = [1; 2; 3; 4]

let sum = 1 + 2 + 3 + 4
// val sum : int = 10

By using the ternary operator the static constraints will be automatically inferred, another option would be to create a method and write the constraints by hand. The first overload cover the case you want to add (lists), the second covers the existing definition.

So now in your code you can do:

for x in (+) a b do
    Console.WriteLine(x)

And that will not break existing (+) for numeric types.