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1937 :: Type parameters dependent on one another

• andrea (orginal) [2016-01-11T16:21:39+01:00] view original

  Say I use a concept to define the notion that a type is convertible to another one. The following works ok:

  type ConvertibleTo[A] = concept x
    convert(x) is A
  
  proc convert(x: int): string = $x
  
  echo 5 is ConvertibleTo[string] # true
  Run

  Now, I want to define a function that takes a ConvertibleTo[A], where A is left generic. I would like to do something like the following, but it does not work

  proc printAs[A, B: ConvertibleTo[A]](x: B) =
    echo $(convert(x))
  
  printAs[string, int](5)
  Run

  The Nim compiler complains that A in the type signature is not a known identifier.

  Similar issues can arise when using static[T]. One may want to have a signature such as

  proc foo[A, X: static[A]](...)
  Run

  Again, the A identifier in static[A] is unbound.

  Does anyone know how to work around this limitation? Is it something that will eventually be valid?

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• filwit (orginal) [2016-01-11T19:28:11+01:00] view original

  Using a , doesn't actually separate A from B's constraint.. so Nim is trying to constrain both by ConvertibleTo[A] which is impossible.. Simply replace the comma with a semicolon like printAs[A; B: ConvertibleTo[A]](x: B) and your code compiles.

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• andrea (orginal) [2016-01-11T23:45:54+01:00] view original

  Wow! It was much simpler than I had hoped! :-)

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