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4977 :: A problem of object inheritence

• i_am_hal (orginal) [2019-06-29T03:49:59+02:00] view original

  I'm seeming to be having problems with the type system and object inheritance. First, here's the code and I'll explain what I am trying to do.

  type
    Object = ref object of RootObj
      name, desc: string
      collectable: bool
    
    Container = ref object of Object
      contents: seq[Object]
  
  
  proc newContainer(name="", desc="", collectable=false, contents=newSeq[Object](0)): Container =
    return Container(name:name, desc:desc, collectable:collectable, contents:contents)
  
  #Raises an error: "Type mistmatch: got <contents seq[Container]>
  discard newContainer(contents= @[newContainer()])
  Run

  In short, I would like the object type Container to be recursive in that the contents field could take either another container, or a regular object. I've done other little code snippets like this and they've seemed to work- so I am unsure why exactly Nim complains about this in particular. The only solution I see at this time is to wrap the two types into a single one as an object variant. But is there a way to get around that? Thanks for your time.

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• Jehan (orginal) [2019-06-29T04:24:09+02:00] view original

  The compiler infers the type of @[newContainer()] to be seq[Container], which does not conform to @seq[Object] (covariance).

  To make the type of an array or sequence explicit, cast the first element, e.g.:

  discard newContainer(contents= @[Object newContainer()])
  Run

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• i_am_hal (orginal) [2019-06-29T04:42:56+02:00] view original

  Thank you very much for the help!

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