Type Inference: Unexpected implicit instantiation at `None`

[crusso]

Christoph says:

Oh, I don't like that omitting a type parameter defaults it to None... chasing that one down was a pain Eg.

  public func toArray<A>(xs : Iter<A>) : [A] {
    let buffer = Buffer.Buf<A>(8);
    apply(xs, func(x : A, ix : Nat) { buffer.add(x) });
    return buffer.toArray()
  };

If you omit the <A> on the buffer constructor you get a failure about mismatch between A and None at the buffer.add call.

[crusso]

Claudio: I'm not really sure I can better using just local inference though...

Christoph Hegemann : Could this be worth special casing like "applying a non-function type"? Hinting the user that omitting type parameters on constructors defaults them to None?

[rossberg]

Maybe the simple solution is to just warn if inference defaults a type argument to None? In the rare case where it's actually intended, devs can shut up the warning by making the type explicit (which probably helps readability as well).

[crusso]

Actually we might be able to do better. I think the problem here I that A in Buf invariant so any underdetermined choice is not minimal/principal thus bad. Will need to look at the algorithm again.

Is this still a problem? Care to commit a test case that exhibits the bad error message, so that it’s clear whether/when this is fixed?

[kritzcreek]

Does https://github.com/dfinity/motoko/pull/2149 look good?

[nomeata]

Perfect, thanks