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519 :: Method resolution error with {.discardable.} pragma

• ajitsingh (orginal) [2014-08-09T17:09:28+02:00] view original

  Hi, I have recently taken an interest in Nimrod and I must say I am really impressed with this language. Yesterday I tried implementing inheriting constructors (i hope its the right term) and I ran into this weird error. I have added the code that caused this below:

  type
      TPerson = object of TObject
          name: string
          age: int
  
  # if I remove the discardable pragma here
  proc initTPerson(name: string, age: int, obj: var TPerson) {.discardable.} =
      obj.name = name
      obj.age = age
  
  
  proc initTPerson(name: string, age: int) : TPerson =
      # the following line fails compilation (even with the discard keyword) with
      # Error: expression initTPerson(name, age, result) has no type (or is ambiguos)
      # With the discardable pragma it compiles
      
      initTPerson(name, age, result)
  
  var
      person = initTPerson("Heisenberg", 33)
  
  echo person.age
  Run

  I am unable to find out why this is happening. Any help here would be greatly appreciated.

  The reason I'm trying to do this is so that child class constructors can then call the first initTPerson with an object and get super like behavior. Generation can then be moved into a template (maybe called constructor heh) which will generate the first form automatically when provided with the second form (and its body).

  Thanks guys. As Ive said before any help will be appreciated.

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• Demos (orginal) [2014-08-09T20:32:55+02:00] view original

  this is really strange, however may I recommend just getting rid of the initTPerson version with the var param. Just use result = initTPerson(name, age) in the subclass initialization functions, one of the reasons nimrod has the implicit result variable is so that the compiler can ALWAYS transform result = initTPerson(name, age) to avoid the copy.

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• gradha (orginal) [2014-08-09T20:58:28+02:00] view original

  I removed the discardable pragma and the example compiles fine for me on osx with 0.9.4 and 0.9.5 404ada2aff51517c56f8beb5735b27a12d3f2485. For style reasons I prefer the var prameter to be first, though:

  
  type
      TPerson = object of TObject
          name: string
          age: int
  
  proc initTPerson(obj: var TPerson, name: string, age: int) =
      obj.name = name
      obj.age = age
  
  proc initTPerson(name: string, age: int) : TPerson =
      result.initTPerson(name, age)
  
  var
      person = initTPerson("Heisenberg", 33)
  
  echo person.age
  Run

  Much of Nimrod code chains procs like this, I'd be surprised if they would not compile. Maybe you should report this as a bug detailing your platform, version and compiler.

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• Stefan_Salewski (orginal) [2014-08-09T22:37:09+02:00] view original

  Yes, compiles and works fine with discardable pragma removed.

  $ nimrod -v Nimrod Compiler Version 0.9.5 (2014-07-13) [Linux: amd64]

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• ajitsingh (orginal) [2014-08-10T04:12:09+02:00] view original

  @gradha: Thanks :) your example compiled for me. My incorrect use of "discard" with a function that does not return anything in the offending line was causing the error (which is probably why the discardable pragma fixed the problem) . I would never have guessed. The program works as expected now :)

  @demos: Your idea seems like exactly the thing I want to accomplish. However, I tried the following code and received this error: type mismatch: got (TPerson) but expected 'TStudent'. (line 14, char 24) Am I doing something incorrectly?

  type
      TPerson = object of TObject
          name: string
          age: int
      
      TStudent = object of TPerson
          id: int
  
  proc initTPerson(name: string, age: int) : TPerson =
      result.name = name
      result.age = age
  
  proc initTStudent(name: string, age: int, id: int) : TStudent =
      # the following line raises the error
      result = initTPerson(name, age)
      result.id = id
  
  var
      student = initTStudent("Ajit Singh", 32, 44)
      person = initTPerson("Heisenberg", 33)
  
  echo student.id
  echo student.name
  echo person.age
  Run

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• Araq (orginal) [2014-08-10T13:06:26+02:00] view original

  Well this is the reason why gradha should you the proc initTPerson(obj: var TPerson, name: string, age: int) version. By the way the old, deprecated convention is TFoo and then initFoo, not initTFoo.

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