Implicit generics come back to bite you

[def-]

type Rational*[T] = tuple[num, den: T]
  ## a rational number, consisting of a numerator and denominator

proc toRational*[T](x: SomeInteger): Rational[T] =
  ## Convert some integer `x` to a rational number.
  result.num = x
  result.den = 1

assert toRational[int](5) == (5,1)
# assert toRational[int, int](5) == (5,1) # this works

Fails with:

Error: cannot instantiate: toRational[int]; got 1 type(s) but expected 2

If you leave away the [T] generics of Rational everywhere, then the [int] of x can be determined automatically. Can this be achieved while still using generic Rationals?

[Araq]

The compiler needs to become smarter and allow for both. That doesn't look too hard to do, implicit type parameters already have some anonymous flag iirc.

[andreaferretti]

Why would toRational[int, int](5) ever work? What are those two ints bound to, given that there is only one generic parameter in toRational?

In any case, I have met a similar issue here. The linked issue is actually about a compiler crash, but when I try to make it generic I probably run into the same issue described here (see the second comment)

[andreaferretti]

Still open, and I still don't understand why toRational[int, int](5) works, given that toRational only has one generic parameter

[zah]

@andreaferretti, the second generic parameter is what gets bound to the SomeInteger type class. Each type class used in the signature introduces such implicit generic parameters. I think the best solution is to allow partial application of the generic parameters, but another reasonable option is support both "all explicit" and "all in total" as Araq has suggested.

[narimiran]

proc toRational*[T](x: SomeInteger): Rational[T] =

Shouldn't this be either proc toRational*[T](x: T): Rational[T] = or if you want to restrict yourself to integers proc toRational*[T: SomeInteger](x: T): Rational[T] =?

The way it was originally written, toRational[float, int](5) would make float -> T, int -> SomeInteger.

Can this be closed?

[krux02]

Everything works intentional, even though it might be a bit confusing at times. I am closing this, as this was a quesiton that got solved, and there isn't any activity left anymore..