invalid type: 'seq[int] or iterator (): int{.closure.}' in this context

[tsojtsoj]

This gives a bit more helpful error message: 
    
    
    type Curve*[T] = object
      cp: seq[T] or iterator: T
    
    proc linear*[T](cp: seq[T] or iterator: T): auto =
      result = Curve[T](cp: cp)
    
    let vf = @[0,1,2,3,4,5,6,7,8,9]
    var tlin = linear(vf)
    
    
    Run

`/usercode/in.nim(8, 18) Error: expression 'linear(vf)' has no type (or is 
ambiguous)`

It seems like it doesn't matter with which type you initialize `cp`, `Curve` 
stays the same type. An easy but inelegant solution additionally to your own 
solution: 
    
    
    type Curve*[T] = object
      cp: T
    
    proc linear*[T](cp: seq[T] or iterator: T): auto =
      result = Curve[typeof cp](cp: cp)
    
    let vf = @[0,1,2,3,4,5,6,7,8,9]
    var tlin = linear(vf)
    
    
    Run