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2060 :: Object variants, not nil

• 😀 (orginal) [2016-02-22T20:10:46+01:00] view original

  Hi,

  I'm very new to Nim and got a question. For a binary tree I tried object variants and the "not nil" annotation. The following compiles (not explicitly initializing each field):

  
  t = VTree(kind: nkNode, value: x)
  Run

  but crashes:

  
  SIGSEGV: Illegal storage access. (Attempt to read from nil?)
  Run

  Why is the "not nil" annotation not checked?

  
  type
      NodeKind = enum
          nkEmpty,
          nkNode
      VTree = ref VNode not nil  # not nil ignored?
      VNode = object
          case kind: NodeKind
          of nkEmpty:
              discard
          of nkNode:
              value: int
              left, right: VTree not nil  # not nil ignored?
  
  let empty = VTree(kind: nkEmpty)
  
  proc insert(t: var VTree, x: int) =
      case t.kind
      of nkEmpty:
          # compiles, but:
          # SIGSEGV: Illegal storage access. (Attempt to read from nil?)
  #        t = VTree(kind: nkNode, value: x)
          
          # works fine:
          t = VTree(kind: nkNode, value: x, left: empty, right: empty)
      of nkNode:
          if x < t.value:
              t.left.insert(x)
          elif x > t.value:
              t.right.insert(x)
  
  var vn: VTree = empty
  vn.insert(3)
  vn.insert(30)
  vn.insert(1)
  vn.insert(5)
  Run

  I would be grateful, if somebody could shed light on this.

  Thanks!

  -Michael

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• wicastC (orginal) [2016-02-23T13:29:31+01:00] view original

  Seems good to me with exactly the same code.

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• 😀 (orginal) [2016-02-23T17:54:06+01:00] view original

  It works, but because of "not nil"...

  
  VTree = ref VNode not nil
  ...
  left, right: VTree not nil
  Run

  ... I was expecting the compiler to catch the problem that this...

  
  let empty = VTree(kind: nkEmpty)
  Run

  ... leaves left and right nil. Otherwise what is the point of "not nil"?

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• wicastC (orginal) [2016-02-24T11:08:37+01:00] view original

  Default value for left and right is always nil except you explicitly construct it with nkEmpty. Not nil is ignored,it's seems useless here.Remove it also works fine.

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• OderWat (orginal) [2016-02-24T13:46:46+01:00] view original

  I think it is a compiler bug. IMHO it should work analog to this:

  type
      XNode = object
        value: int
        left, right: ref XNode  not nil
  
  
  let xn1 = XNode(value: 1)
  #> Error: field not initialized: left
  
  echo xn1.repr
  
  let xn2 = XNode(value: 1, left: nil, right: nil)
  #> Error: type mismatch: got (nil) but expected 'ref XNode not nil'
  
  echo xn2.repr
  
  Run

  If you remove the "not nil" both lines compile and generate the same object (with nil for left and right)

  I don't see why that should be different with an object variant?

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• wicastC (orginal) [2016-02-24T19:38:14+01:00] view original

  I got your point,but the kind value of vn was changed from nkEmpty to nkNode by first insert,and not nil seems only check it at compile time not run time.

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• OderWat (orginal) [2016-02-24T20:02:58+01:00] view original

  Where does it change the node? It will be replaced with the new one. If it would change the node kind it had to use "system.reset()" and modify the kind "field".

  Another example for what I think is a bug. Notice there is only one kind in the object:

  type
    nKind = enum nOne
    XNode = object
      case kind: nKind:
      of nOne:
        value: int
        left, right: ref XNode not nil
  
  
  let xn1 = XNode(value: 1)
  # Works but ends up with nil
  
  echo xn1.repr
  
  let xn2 = XNode(value: 1, left: nil, right: nil)
  #> Still gives Error: type mismatch: got (nil) but expected 'ref XNode not nil'
  
  echo xn2.repr
  Run

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• wicastC (orginal) [2016-02-24T20:31:01+01:00] view original

  @OderWat,you are right,it didn't check for those default nil values.

  And the type was matched nkNode because After insert it has a value field.

  var vn: VTree = empty
  echo vn.kind #it gives nkEmpty.
  vn.insert(3)
  echo vn.kind #it gives nkNode.
  Run

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