Mirror of forum.nim-lang.org

6085 :: Type inference as proc return value

• cmc (orginal) [2020-03-18T09:30:38+01:00] view original

  Hi everyone!

  I'm working on getting a better hang of generics. I found generics can use type inference if the object is passed as a parameter, but needs to be explicitly stated if it is a return value.

  type
    Foo = object
      foo: int
      bar: int
  
  proc init[T](): T =
    result = T()
  
  proc stringify[T](o: T):string =
    return $o
  
  # fails with "cannot instantiate"
  # let t:Foo = init()
  
  # works
  let t = init[Foo]()
  
  # works
  echo stringify(t)
  
  
  Run

  Is this how it's supposed to work or did I miss something?

---

• DeletedUser (orginal) [2020-03-18T12:03:26+01:00] view original

  Return value type inference like this does not exist in Nim unfortunately

---

• cmc (orginal) [2020-03-18T21:06:22+01:00] view original

  Thanks for letting me know! Reported the error message (or the behavior) as a bug <https://github.com/nim-lang/Nim/issues/13683> because I thought it too hard to get confirmation of what's going on, fingers crossed.

  As a followup, is it correct that when there are several generic types, and at least one of them is the return type, that all types must be specified to the generic, even if the information would be available to infer both?

  type
    Foo = object
      foo: int
      bar: int
    
    Fuz = object
      fuz: float
      buz: float
  
  proc init[T, U](o: T): U =
    discard o
    result = U()
  
  # works
  let foo = Foo()
  let fuz = init[Foo, Fuz](foo)
  
  # fails with "cannot instantiate U" (expected)
  # let foo = Foo()
  # let fuz:Fuz = init(foo)
  
  # fails with "Error: cannot instantiate: 'init[Fuz]'; got 1 type(s) but expected 2"
  # Can this be made to work with a different syntax?
  # let foo = Foo()
  # let fuz:Fuz = init[Fuz](foo)
  
  echo foo
  echo fuz
  Run

---