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3740 :: Generic types and 'when'

• vv (orginal) [2018-04-10T12:33:00+02:00] view original

  Hi, the following code does not compile :

  type
      A[T] = object
          x: T
      B[X] = object
          when X is A:
              y: int
  
  var b = B[A[int]]()
  Run

  The compiler produces the message:

  .nim(10, 10) Error: cannot instantiate: 'A'
  Run

  How could I test that the generic param X is an instance of the generic class A, without instantiating it?

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• vv (orginal) [2018-04-10T12:42:33+02:00] view original

  I found half of a solution, this code works:

  type
      A[T] = object
          x: T
      B[X] = object
          when X is A[any]:
              z: int
  
  var b = B[A[int]]()
  echo b.z
  Run

  But when adding a second generic parameter to A, it does not work anymore: the code compiles, but X is not matched with A:

  type
      A[T,U] = object
          x: T
          y: U
      B[X] = object
          when X is A[any,any]:
              z: int
  
  var b = B[A[int,float]]()
  echo b.z
  Run

  produces:

  .nim(10, 7) Error: undeclared field: 'z'
  Run

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