Question

I tried to use a tuple to create a new instance of a class defined in F#. To duplicate the problem, I tried the following code.

type test(x: int, y:int) =
    let distance =
        x * x + y * y |> float |> sqrt
    new (x: int, y:int, z:int) =
        new test(x, y)
let args = 1, 2
let test2 = new test(args)

It complains that

Error 1 The member or object
constructor ‘test’ does not take 1
argument(s). An overload was found
taking 2 arguments.

If I remove the non-default constructor, things are fine.
I don’t understand why it becomes two/three arguments instead of tuples.

Thank you very much.

Answer 1

This is subtle, but per the spec. Here’s an old email response I dug up where someone asked a similar question:

…

At play, there is also (subtle) difference between “tuples” (in the F# language) and “syntactic tuples” (in the F# specification).

Method application resolution is different when there are overloads. If there are none, the decomposition of the argument (i.e. the “stuff” specified between ( and ) in your method invocation) to a tuple form does not happen, so the compiler is happy and say “oh ok, the ctor for MyClass takes 1 argument (a tuple) and I am seeing 1 argument (“tuple” in your code) so I’ll use that”.

Whereas, when you have 2 overloads, then the rule above does not apply anymore and the compiler will try to decompose the argument to a tuple form (which in your case would resolve in something like: “Oh ok, there is 1 argument which is tuple. But wait, I have 2 overloads. And your list of argument (one element, tuple) does not match either an argument list, hence the error message”

At least, that’s my interpretation of the F# specification, section 14.4.