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6790 :: Generics constrain with two types

• archnim (orginal) [2020-09-09T17:41:45+02:00] view original

  Hello world. Here is the first line of my generic proc:

  proc myProc[T1, T2](param1: T1, param2: T2) =
  
  Run

  Is there a way to say that one of the two params must be a string ? Or am I forced to create two complementary procs with exactly the same body ?

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• doofenstein (orginal) [2020-09-09T17:55:06+02:00] view original

  this is pretty unusual, but you can do something like this:

  proc myProc[T1, T2](param1: T1, param2: T2) =
    when param1 isnot string and param2 isnot string:
      {.fatal: "atleast one of the parameter must be a string".}
  Run

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• jcosborn (orginal) [2020-09-10T07:13:35+02:00] view original

  This will ensure that at least one is a string, but I'm not sure how reliable it is.

  type
    CondStr[T] = concept x
      (x is string) or (T is string)
  
  proc myProc[T](param1: T, param2: CondStr[T]) =
    echo $T
  
  #myproc(0, 0)  # fails
  myproc("", 0)
  myproc(0, "")
  myproc("", "")
  Run

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• archnim (orginal) [2020-09-10T10:14:37+02:00] view original

  Thanks a lot.

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• archnim (orginal) [2020-09-10T10:17:08+02:00] view original

  Thanks a lot, @doofenstein. Maybe in a near future, a more precise generics syntax will be provided.

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• mratsim (orginal) [2020-09-10T12:04:01+02:00] view original

  Maybe in a near future, a more precise generics syntax will be provided.

  I think this is an acceptable limitation, there is no other language that supports what you propose either and it seems like it would make the proc declaration complex.

  @jcosborn solution is actually quite elegant, otherwise use private "implementation proc or template".

  template myProcImpl(param1, param2: typed) =
    discard "your implementation
  
  proc myProc[T](param1: T, param2: string) =
    myProcImpl(param1, param2)
  
  proc myProc*[T](param1: string, param2: T) =
    myProcImpl(param1, param2)
  
  proc myProc*[T](param1: string, param2: T) =
    myProcImpl(param1, param2)
  Run

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• archnim (orginal) [2020-09-10T12:59:47+02:00] view original

  Please, how does @jcosborn's code work in detail ?

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• mratsim (orginal) [2020-09-10T22:29:48+02:00] view original

  @jcosborn code uses concept which are detailed here: https://nim-lang.org/docs/manual_experimental.html#concepts

  This allows to represent arbitrary requirements as an abstract type.

  The concept definition here

  type
    CondStr[T] = concept x
      (x is string) or (T is string)
  Run

  Does:

  1. For my concept type, which we will call x in the following definition
  2. It also requires knowledge of the type T from CondStr[T]
  3. If x is a string then we're fine, our type is an actual instance of the concept
  4. Otherwise we need to ensure that T is a string
  5. Everything else is not an instantiation of the concept and should be rejected if that concept is required.

  Then his code

  proc myProc[T](param1: T, param2: CondStr[T]) =
    echo $T
  Run

  If param 1 is a string, then param2 will be a CondStr[T]

  If param1 is not a string, then param2 satisfies CondStr[T] only if it's actually a string.

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• archnim (orginal) [2020-09-11T08:43:54+02:00] view original

  You're too nice. :-)

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