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3004 :: Using a custom object as a table's key, compiler doesn't look inside a block to find the hash() proc

• jmoriau (orginal) [2017-06-16T18:59:32+02:00] view original

  Hello!

  I've been slowly learning and very much enjoying Nim, I'm going through dom96's book and while messing with the examples I came across an issue. I'm trying to use a CustomObj as key inside a table, so I simply need to define a hash proc for this CustomObj and I should be able to use it as a key inside a table. And it works :

  import tables, hashes
  
  type
    CustomObj = object
      name: string
  
  proc hash(x: CustomObj): Hash =
      result = x.name.hash()
      result = !$result
  
  var objs = initTable[CustomObj, string]()
  objs[CustomObj(name: "A")] = "B"
  echo objs
  Run

  
  {(name: A): B}
  Run

  But when I tried the exact same thing inside of a block suddenly it doesn't work anymore and complains that it can't find the new hash proc :

  import tables, hashes
  
  block ablock:
    
    type
      CustomObj = object
        name: string
    
    proc hash(x: CustomObj): Hash =
        result = x.name.hash()
        result = !$result
    
    var objs = initTable[CustomObj, string]()
    objs[CustomObj(name: "A")] = "B"
    echo objs
  Run

  which produces the error:

  
  Hint: used config file '/nim/config/nim.cfg' [Conf]
  Hint: system [Processing]
  Hint: in [Processing]
  Hint: tables [Processing]
  Hint: hashes [Processing]
  Hint: strutils [Processing]
  Hint: parseutils [Processing]
  Hint: math [Processing]
  Hint: algorithm [Processing]
  Hint: etcpriv [Processing]
  in.nim(14, 7) template/generic instantiation from here
  lib/pure/collections/tableimpl.nim(98, 21) template/generic instantiation from here
  lib/pure/collections/tableimpl.nim(53, 14) template/generic instantiation from here
  lib/pure/collections/tableimpl.nim(43, 12) Error: type mismatch: got (CustomObj)
  but expected one of:
  proc hash[T: Ordinal](x: T): Hash
  proc hash(x: pointer): Hash
  proc hash[T: proc](x: T): Hash
  proc hash(x: float): Hash
  proc hash[A](x: set[A]): Hash
  proc hash[T: tuple](x: T): Hash
  proc hash(x: string): Hash
  proc hash(x: int): Hash
  proc hash[A](aBuf: openArray[A]; sPos, ePos: int): Hash
  proc hash(x: int64): Hash
  proc hash(x: char): Hash
  proc hash(sBuf: string; sPos, ePos: int): Hash
  proc hash[A](x: openArray[A]): Hash
  Run

  I tried it on both 0.17 and devel. I assume I'm simply misunderstanding how blocks work, I thought they were simply creating a new scope?

  Cheers!

  Joris

  EDIT: Edited the title to better reflect the question

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• Lando (orginal) [2017-06-17T11:10:28+02:00] view original

  The comment in *tables.nim* says that the compiler looks for the right hash proc (bypassing scope rules?!). Apparently that only works if it is defined in module scope, not in a block scope.

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• jmoriau (orginal) [2017-06-17T12:29:30+02:00] view original

  Hi,

  Thanks! That was exactly my question, is it supposed to bypass the scope rule?

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• Lando (orginal) [2017-06-17T13:07:40+02:00] view original

  To clarify: by "bypassing scope rules" I meant that code from the tables module somehow knows about the hash proc defined (privately!) in your code. Seems strange to me, but maybe I don't know all the rules.

  As for whether the behavior is supposed to be as it is: as I recently learned, the answer to this is likely "It is as it is now, this will all be fixed with VTable types." :)

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• jmoriau (orginal) [2017-06-17T17:09:39+02:00] view original

  Hahaha okay then thanks!

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