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3446 :: Problem with default values for a generic parameter

• doofenstein (orginal) [2017-12-30T01:14:12+01:00] view original

  I have a procedure which should accept two units(absolute width and width relative to the parent widget). For this purpose I created two distinct types and a typeclass which contains both:

  type
     UIAbsLength = distinct float32
     UIRelLength = distinct float32
     
     UILength = UIAbsLength|UIRelLength
  Run

  Then I created a procedure which accepts both types and has a default value(this is an Immediate Gui, were widgets are instatiated with a single call, so default values are really handy here):

  proc button*(self: UISystem, buttonLabel = none(string), width, height: UILength = UIAbsLength(0)) =
     when width is UIAbsLength:
        discard
     elif height is UIAbsLength:
        discard
  Run

  Now the problem arrives, that once I call the procedure with UIRelLength the compiler throws an error(Error: type mismatch: got (UIAbsLength) but expected 'UIRelLength = distinct float32')

  Should this work at all? Or could this be done in some other way? Or is it maybe a compiler bug?

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• jibal (orginal) [2017-12-30T05:55:43+01:00] view original

  The error message seems strange, but you really can't expect this to work. At compile time, width and height are UILength, not UIAbsLength, so your when tests will fail. You need a template if you want to generate different code depending on the type at the call site. And if you want the decision to be made at run time (which I would think is necessary, else you can't store or pass around your two kinds of length while remembering which type they are) then you need a discriminated type ... a float32 only contains the 32 value bits, not any type information.

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• Udiknedormin (orginal) [2017-12-30T12:09:44+01:00] view original

  In fact, I consider it a bug in the compiler that the following doesn't work:

  proc button*[W, H: UILength = UIAbsLength](self: UISystem,
                                             buttonLabel = none(string),
                                             width = W(0.0),
                                             height = H(0.0)) =
     
     when width is UIAbsLength:
        echo "width is Abs!", cast[float32](width)
     else:
        echo "height is Abs!", cast[float32](width)
  Run

  And here is why: UILength is a generic type consisting only of finite number of types. It means it's even entirely possible to check every time for compilability of W(0.0) and H(0.0). But still, Nim uses lazy instantiation and compile-time duck typing. Which means it should, as a rule, accept any constructors I give it unless I try to instantiate the proc and break it by trying to use constructors not in existence. And I think so because the following works as expected:

  proc button*[W, H: UILength](self: UISystem,
                               buttonLabel = none(string),
                               width: W, height: H) =
     
     let x = W(0.0)
     let y = H(0.0)
  Run

  So all in all: Nim makes some inconsistencies between declarations and definitions.

  Btw. oh, how much I miss Rust's where... It's a true beautifier of generic signatures...

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• yglukhov (orginal) [2017-12-30T12:51:31+01:00] view original

  @doofenstein, I think your issue is worth reporting. Your code looks pretty valid.

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• doofenstein (orginal) [2017-12-30T15:30:30+01:00] view original

  Thank you for responses, I created an issue on Github: https://github.com/nim-lang/Nim/issues/7000

  Does somebody knows of a workaround to this issue? For now the only thing I can do is probably to just remove the default values.

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• yglukhov (orginal) [2017-12-31T07:58:39+01:00] view original

  Does somebody knows of a workaround to this issue? For now the only thing I can do is probably to just remove the default values.

  You can do that and also provide overloads for respective number of arguments to achieve the same result at the call site.

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