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1832 :: How do I get rid of this warning? Simple code...

• enjoysmath (orginal) [2015-11-28T22:35:11+01:00] view original

  Warning: '(x): Hash' has no type. Typeless parameters are deprecated; only allowed for 'template' [TypelessParam]

  Code:

  
  import sets, hashes
  
  proc hash(x): Hash =
    var h = 0
    for i in x: h = h !& hash(i)
    result = !$h
  
  proc powerset[T](inset: HashSet[T]): auto =
    result = toSet([initSet[T]()])
    
    for i in inset:
      var tmp = result
      for j in result:
        var k = j
        k.incl(i)
        tmp.incl(k)
      result = tmp
  
  echo powerset(toSet([1,2,3,4]))
  
  Run

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• ephja (orginal) [2015-11-28T22:48:15+01:00] view original

  You can avoid that warning by specifying a type for 'x'. "x: HashSet" Should work

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• cblake (orginal) [2015-11-29T00:22:15+01:00] view original

  I realize that your post's question was only how to silence the warning, but it may also matter to you that `!&` is not commutative and also that the loop order of "for i in x" is not guaranteed. So, two HashSet[T]'s built by different insertion orders can get different hashes. Nim's sets.`==` would correctly say they are the same, though. I haven't really analyzed whether this would arise in your specific powerset algorithm or if it happens to work out that the same order is always used. It could surely matter, though, if you ever used this "proc hashT: Hash" in some more general context. So, I thought I would point it out. You could iterate over items in sorted order or else use some element-level hash that is commutative (e.g. xor or + them all together or something).

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• cblake (orginal) [2015-11-29T22:27:51+01:00] view original

  Oh, and of course Nim HashSet[T]s are also not immutable. The implication there is that you could also mess things up by inserting or deleting after putting the set within the outer set (in perhaps some other use of sets of sets).

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