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9169 :: Mysterious error message

• Sixte (orginal) [2022-05-24T16:28:04+02:00] view original

  The following program does not compile:

  proc addoneb(a : distinct string, b : distinct string) :  distinct string =
    when typeof(a) is distinct string :
        let v = a & b
        result = v
    else :
        result = cast[distinct string]("")
  
  proc run() =
      let a = "alice"
      let b = "bob"
      let c = addone(a,b)
      echo cast[string](c)
  
  run()
  
  
  
  Run

  and I got:

  
  Error: type mismatch:
   got 'distinct string' for 'cast[distinct string]("")' [string declared in /home/gsixte/.choosenim/toolchains/nim-1.6.2/lib/system.nim(34, 3)]
   but expected 'distinct string' [string declared in /home/gsixte/.choosenim/toolchains/nim-1.6.2/lib/system.nim(34, 3)]
  
  Run

  So, the compiler complains like: "I ordered a coffee and you brought me a coffee". Hard to understand.

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• huantian (orginal) [2022-05-24T16:36:42+02:00] view original

  Each time you write distinct string it represents a different distinct string. First define a type to be distinct string once and replace all the other occurrences with it

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• Sixte (orginal) [2022-05-24T16:46:07+02:00] view original

  Well, the concatenator '&' works well with two distinct strings. So, their types match. The else branch of the when``should not be seen - it is statically clear that the first branch gets used. Since the ``else branch is not used, it should not create a new distinct string. And it is a simple cast from (distinct string) to (distinct string).

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• huantian (orginal) [2022-05-24T17:03:57+02:00] view original

  The first branch of the when is never reached because the distinct string of the parameter isn’t the same as the distinct string you’re comparing too, I believe.

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• Sixte (orginal) [2022-05-24T17:42:17+02:00] view original

  So it turns out that a distinct returntype is pretty much useless and should be avoided.

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• Yardanico (orginal) [2022-05-24T17:45:17+02:00] view original

  I think you might be misunderstanding what distinct is meant for. It's meant to create a new type from some existing type, yet make it so that you can't use any existing procs of that type on your new type without explicitly converting (not casting!) the distinct'd type to its base one. It's not meant to act as "I want the values to be different", and it seems like you understood it this way.

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