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1599 :: Getting the element type from a container

• Henry (orginal) [2015-09-01T17:37:41+02:00] view original

  Is it possible using type or by some other means to get the element type of Nim containers? For example if I have some generic function which takes a container argument which might be a string or a seq or user defined how does one obtain the type of an element of the container?

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• Araq (orginal) [2015-09-01T18:18:48+02:00] view original

  type(container[0])

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• Henry (orginal) [2015-09-01T18:46:21+02:00] view original

  Ahh yes, I should have clarified, I don't necessarily have an instance of the container; I would like to get the element type directly from the container type but type Elem = type(string[0]) doesn't work.

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• yglukhov (orginal) [2015-09-01T19:55:11+02:00] view original

  Developing further Araq's idea you can try this hacky one:

  template IndexedContainerElementType(a: typedesc): typedesc =
      when a is openarray:
          type((proc(v: a): auto = v[0])([]))
      else:
          type(cast[ptr a](nil)[][0])
  
  type
      MyChar = IndexedContainerElementType(string)
      MyInt = IndexedContainerElementType(seq[int])
  
  proc foo(a: openarray[int]) =
      type ElemType = IndexedContainerElementType(type(a))
      var b: ElemType
      b = 123
      echo b
  Run

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• Henry (orginal) [2015-09-01T21:58:06+02:00] view original

  Thanks for the hack. I was hoping there is a standard way to obtain this kind of type information like there is in the C++ STL. If I create a container type is there a way to store in the type the element type like a typedef in a C++ class?

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• yglukhov (orginal) [2015-09-01T22:44:25+02:00] view original

  Not exactly "inside", but in a nim way. E.g.

  type MyContainerTemplate[T] = object
      elements: seq[T]
  
  # Return type of this pseudo-proc will mean the element type.
  proc getElemTypeHelper[T](a: typedesc[MyContainerTemplate[T]]): T = discard
  
  # Public API
  template ElemType(t: typedesc[MyContainerTemplate]): typedesc =
      type(t.getElemTypeHelper)
  
  # Test
  type MyContainer = MyContainerTemplate[int]
  type MyContainerElemType = MyContainer.ElemType
  Run

  I'm not sure, however, if such exercises would be "canonical" Nim. Usually there are ways to do it more straightforward. But if you're just investigating the limits of Nim, you have to seek further ;)

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• Henry (orginal) [2015-09-01T23:29:16+02:00] view original

  Thank for the help, this is an interesting solution. However, it is radically different to the way type information is passed around in complex templated C++; I think I am going to struggle converting my C++ into Nim.

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• yglukhov (orginal) [2015-09-01T23:52:25+02:00] view original

  Well, yes, translating complex metaprogramming stuff from C++ to Nim may require some thought. But imho, Nim's generics, templates and macros allow you to think of the problem from more different perspectives, usually resulting in simpler/cleaner concepts than in C++. Anyways, good luck converting! ;)

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