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3552 :: Has anyone else been bitten by this?

• StasB (orginal) [2018-02-16T02:41:30+01:00] view original

  Hello! So I have this snippet, which generates an error:

  import streams
  
  proc readbin(s: Stream, x: var int8) =
      x = readInt8(s)
  
  proc readbin(s: Stream, x: var int16) =
      x = readInt16(s)
  
  proc readbin(s: Stream, x: var int32) =
      x = readInt32(s)
  
  proc readbin(s: Stream, x: var int64) =
      x = readInt64(s)
  
  proc test(s: Stream) =
      var n: int
      readbin(s, n) #Error: for a 'var' type a variable needs to be passed
  Run

  The error goes away when I give n a more specific type (exactly matching one of the proc signatures). My guess is that this happens because there's some (redundant) implicit casting going on, so the compiler thinks I'm trying to pass an expression rather than a variable. The source of the error was pretty difficult to diagnose, especially since the offending line was being generated by a macro. If I'm right about the cause, perhaps it would save others some pain if the compiler gave a more helpful error message for this particular case.

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• c0ntribut0r (orginal) [2018-02-16T10:40:40+01:00] view original

  At my (newbie's) point of view, you're right - conversion makes n being an expression. This wouldn't be a problem if there was overloading resolution by return type and you wouldn't need to use those x: var int8, but there's no hope https://forum.nim-lang.org/t/3025

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• luntik2012 (orginal) [2018-02-16T14:07:22+01:00] view original

  somewhy you need to specify size of var you are passing to readbin (e.g. int32)

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• Araq (orginal) [2018-02-16T19:06:27+01:00] view original

  Why would a type conversion preserve lvalue-ness? Strange idea IMO.

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• StasB (orginal) [2018-02-16T19:22:17+01:00] view original

  I'm not saying a type conversion should preserve lvalue-ness. I'm saying the compiler could probably give a more helpful error message, because the situation gives the appearance that a variable is being passed, and such implicit conversions make no sense in this context.

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• Araq (orginal) [2018-02-16T20:41:56+01:00] view original

  Oh, I see. I improved the error message.

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• StasB (orginal) [2018-02-16T22:12:53+01:00] view original

  That happened a lot quicker than I expected! I appreciate the responsiveness.

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