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 (+) (First : List<'a>) (Second : List<'a>) =
        First.Concat(Second)

let a = [1; 2; 3; 4; 54; 9]
let b = [3; 5; 6; 4; 54]


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

and I want to convert the last line into

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

but doing so gives me a

The type 'int list' does not support any operands named '+'

The documentation and examples on the web are flakey, and despite my google-fu i have been unable to get it to work. Basically, coming from a python background, I want to get my list manipulation syntax as terse as I am used to: it should not need more than 1 character in infix notation.

Answer 1

Note that @ is already a 1-char infix operator to concat lists.

Answer 2

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.