How to do tuple augmentation

June 29, 2022

The following code is from chapter 5 of "F# 4.0 Design Patterns".

let a = 1,"car" 
type System.Tuple<'T1,'T2> with 
  member t.AsString() = 
    sprintf "[[%A]:[%A]]" t.Item1 t.Item2 
(a |> box :?> System.Tuple<int,string>).AsString()

The desired output is [[1]:["car"]]

However, a red squiggly appears under AsString(). "The field, constructor or member ‘AsString’ is not defined. Maybe you want one of the following: ToString"

>Solution :

This is a bit odd code sample – I suspect the point that this is making is that F# tuples are actually .NET tuples represented using System.Tuple – by showing that an extension to System.Tuple can be invoked on ordinary F# tuples.

I suspect the behaviour of F# has changed and it no longer allows this – it may have been that adding extensions was allowed on System.Tuple, but not on ordinary F# tuples, but the two have became more unified in the compiler.

However, you can do a very similar thing using the .NET-style extension methods:

let a = 1,"car" 

[<System.Runtime.CompilerServices.ExtensionAttribute>]
type TupleExtensions =
  [<System.Runtime.CompilerServices.ExtensionAttribute>]
  static member AsString(t:System.Tuple<'T1,'T2>) = 
    sprintf "[[%A]:[%A]]" t.Item1 t.Item2 

let st = (a |> box :?> System.Tuple<int,string>)
st.AsString()

This can actually be also invoked directly on an F# tuple value: