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5572 :: Empty sequence of specific type given problems when compiling with "cpp"

• drkameleon (orginal) [2019-11-21T10:31:53+01:00] view original

  I have this line of code:

  var ret = DICT(cast[Context](@[]))
  Run

  DICT takes Context as a param. And a Context is nothing but a seq[(int,int)].

  Obviously, an empty sequence can be anything, so I cast it to Context.

  Now, the thing is: when I compile it with "c", everything goes smooth; when I compile it with "cpp" (clang++ on macOS), I have the following error:

  
  error: assigning to 'TGenericSeq *' from incompatible type 'tySequence__77YVzYb2AOu2vP0iI0b8Dw *'
                  T11_ = (tySequence__77YVzYb2AOu2vP0iI0b8Dw*)0;
                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Run

  Any workarounds?

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• Vindaar (orginal) [2019-11-21T10:46:42+01:00] view original

  If I'm not missing something, you should call newSeq instead:

  var ret = DICT(newSeq[Context]())
  Run

  But of course, any Nim code you write should either error during nim compilation or compile successfully.

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• drkameleon (orginal) [2019-11-21T11:10:15+01:00] view original

  Spot on! (with one slight difference: given that Context=seq[int,int], it has to be:

  DICT(newSeq[(int,int)]())
  Run

  Thanks! ;-)

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• Vindaar (orginal) [2019-11-21T15:20:15+01:00] view original

  Ah yes, indeed. If DICT is just a proc that takes a Context, this also works btw:

  let ret = DICT(@[])
  Run

  The compiler can deduce the type of the empty seq itself from the args of proc DICT(c: Context).

  Although personally if the use case of an empty seq as the argument comes up more often, I'd set the empty sequence as the default value for the argument proc DICT(s: Context = @[]) instead. And proc newContext(len: int): Context helper is probably also useful to emphasize the intention.

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