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7625 :: Using typedesc in function arguments

• vitreo12 (orginal) [2021-03-14T17:59:14+01:00] view original

  Hello people,

  I am hitting a weird problem with using typedesc as arguments to procs. Consider this example:

  proc test(T : typedesc[SomeNumber] = float, a : T = 0.5) : T =
      return a * 2
  
  echo test(a = 0.5)
  Run

  I'd expect T to simply default to float, but it won't compile unless I expliclitly call it with

  echo test(float, 0.5)
  Run

  Is this intendend behaviour? How can I go around it to not having to express the typedesc when it's already defaulted to a value?

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• Stefan_Salewski (orginal) [2021-03-14T18:08:08+01:00] view original

  Your example code is invalid as SomeNumber includes int numbers, and for that the default value 0.5 would be not allowed.

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• Stefan_Salewski (orginal) [2021-03-14T20:43:04+01:00] view original

  But there is indeed some form of an issue maybe...

  proc test(T: typedesc[SomeNumber] = float) =
      discard
  
  test(int)
  
  proc test2(T: typedesc[SomeNumber] = float; i: int = 1) =
      discard
  
  test2(i = 1)
  
  test2(1)
  Run

  The last proc call does not work:

  
  /tmp/hhh/t.nim(11, 6) Error: type mismatch: got <int literal(1)>
  but expected one of:
  proc test2(T: typedesc[SomeNumber] = float; i: int = 1)
    first type mismatch at position: 1
    required type for T: typedesc[SomeNumber]
    but expression '1' is of type: int literal(1)
  
  expression: test2(1)
  Run

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• Stefan_Salewski (orginal) [2021-03-14T21:02:32+01:00] view original

  But it is a general property of Nim default parameters:

  proc t3(x: float = 0.0; s: string) =
    discard
  
  proc t4(s: string; x: float = 0.0) =
    discard
  
  #t3("hallo")
  
  t4("hallo")
  Run

  The commented out t3() call does not compile. So we should use the proc vars with defaults after other vars. Maybe we should read tutorials or the manual again.

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• ynfle (orginal) [2021-03-14T23:43:54+01:00] view original

  I think that the easiest way to do what you are trying to do is to use generics.

  See this playground example for your use case. The type of the argument is infered, but it can also be explicility stated (with [] after the proc name before the argument(s)) to "cast" the type automatically

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• vitreo12 (orginal) [2021-03-15T10:08:10+01:00] view original

  I know I can use generics for an use case that simple, I just wanted to make a clear simple example.

  I am asking about typedescs specifically for a code-generation macro that I am dealing with.

  Anyway, I solved the problem by injecting the default value of the typedesc together with the value of the arguments.

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