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

As pointed out by the other answers, you cannot add implementation of + to an existing type, because extension members are ignored and standalone let binding hides the default (overloaded) implementation.

If you wanted to use + (which is not really needed because F# library contains operator @), you would have to write wrapper for F# list that supports the operator directly:

open System.Collections
open System.Collections.Generic

/// Wrapper for F# list that exposes '+' operator and 
/// implements 'IEnumerable<_>' in order to work with 'for'
type PlusList<'T>(list : list<'T>) =
  member x.List = list
  static member (+) (first : PlusList<'a>, second : PlusList<'a>) =
    first.List @ second.List
  interface IEnumerable with
    member x.GetEnumerator() = (list :> IEnumerable).GetEnumerator()
  interface IEnumerable<'T> with
    member x.GetEnumerator() = (list :> IEnumerable<_>).GetEnumerator()

// Simple function to wrap list
let pl l = PlusList<_>(l)

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

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