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1513 :: Check type inside macro

• andrea (orginal) [2015-08-11T11:52:37+02:00] view original

  I have a typed NimNode inside a macro. I am trying to detect whether the value represented by it is of a given type, say string. So far I have been using this technique:

  let tp = getType(typedNode)
  let isString = $(tp) == "string"
  Run

  There are a few issues with this approach.

  • First, $ is not even defined on complex types, hence in those cases I get a compilation error, while I would be happy to go on knowing that typedNode is not a string.
  • Second, I rely on the symbol string being bound to the actual string type, while it would be possible - although certainly uncommon and unadvisable - to shadow the "string" identifier with a different type. This is more relevant when the type is question is one that is less ubiquous than string

  What I would like is something like

  proc isT[T](n: NimNode): bool {. compileTime .} =
    ...
  Run

  to be used like

  let isString = isT[string](typedNode)
  Run

  but I am not able to write the body of that procedure.

  I have tried many variations, but I always run into the problem that T is a type, while n is a NimNode. I can get the AST of a variable of type T using a macro, but then I am one level of macro up and I am not able to get the type of n anymore

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• Araq (orginal) [2015-08-11T12:31:45+02:00] view original

  if x.gettype.typeKind == ntyString

  Run

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• andrea (orginal) [2015-08-11T12:41:23+02:00] view original

  Thank you. Unfortunately, this only works for simple cases, such as strings. For instance for objects one just gets ntyObject. Is there any way to check for a more general type? The compiler is certainly able to asses whether a typed term is of type T, for any T, however complex.

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• Araq (orginal) [2015-08-11T13:17:43+02:00] view original

  IIRC nim devel has macros.sameType.

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• andrea (orginal) [2015-08-11T16:40:00+02:00] view original

  Great, this is almost what I need! The only missing part, now, would be to be able to produce a typed node with a given type.

  Say I have a type such as, in my case, TimeInfo and a typed node n. If I understand correctly I should be able to tell whether n is of type TimeInfo by doing

  sameType(n, ???)
  Run

  where ??? is a NimNode of type TimeInfo. The problem is that I do not know how to produce it! I have tried

  template helper =
    var x: TimeInfo
  sameType(n, getAst(helper())[0][0][0])
  Run

  which should compare the type of n with that of the variable x defined in the template. Unfortunately, it seems that x is not typed at this point. I am not sure how to produce a typed node given the type. I can do it by going inside one more level of macro, like this

  proc ofTypeM(t: typed) =
    # here `t` is typed of type `T`
    discard
  
  proc ofType(T: typedesc) =
    var t: T
    discard ofTypeM(t)
  Run

  but then I do not know how to compare it with the type of n, which is only in the first level of macro, so to speak.

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• Araq (orginal) [2015-08-11T16:53:34+02:00] view original

  bindSym("TimeInfo").getType should work.

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• andrea (orginal) [2015-08-11T17:08:08+02:00] view original

  Great, this is exactly what I needed! For future reference:

  proc hasType(x: NimNode, t: static[string]): bool {. compileTime .} =
    sameType(x, bindSym(t))
  
  # then, inside a macro...
  if field.hasType("TimeInfo"): # do something
  Run

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