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5532 :: Multi-method wrongly dispatches to base

• konradmb (orginal) [2019-11-16T00:04:41+01:00] view original

  I'm writing a simple math equation lexer and I've come to a point where I need to join two terminal tokens to make a non-terminal token.

  Here's an excerpt:

  type
    Token* = ref object of RootObj
      position*: int
    DigitToken = ref object of Token
      value: uint
    NumberToken = ref object of Token
      value: int
  
  method joinTokens*(a, b: Token): Token {.base.} =
    quit "joinTokens base called " & repr(a) & "\n" & repr(b)
  method joinTokens*(a, b: DigitToken): NumberToken =
    result = NumberToken()
    result.position = max(a.position, b.position)
    result.value = int(a.value*10 + b.value)
  method joinTokens*(a: NumberToken, b: DigitToken): NumberToken =
    result = NumberToken()
    result.position = max(a.position, b.position)
    result.value = a.value*10 + int(b.value)
  
  var digits: seq[Token]
  
  digits.add DigitToken(value:1, position:0)
  digits.add DigitToken(value:2, position:1)
  
  var numberA = joinTokens(digits[0], digits[1])
  Run

  I've compiled it with --multimethods:on. I wanted method joinTokens*(a, b: DigitToken): NumberToken to be called, but instead method joinTokens*(a, b: Token): Token {.base.} gets called.

  What can I do? I've tried type case of and I'd rather prefer to use this parent-children hierachy I have now.

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• mratsim (orginal) [2019-11-16T09:29:32+01:00] view original

  Seems like there is a bug, if you comment out the first method you get

  type
    Token* = ref object of RootObj
      position*: int
    DigitToken* = ref object of Token
      value*: uint
    NumberToken* = ref object of Token
      value*: int
  
  # method joinTokens*(a, b: Token): Token {.base.} =
  #   quit "joinTokens base called " & repr(a) & "\n" & repr(b)
  method joinTokens*(a: DigitToken, b: DigitToken): NumberToken =
    result = NumberToken()
    result.position = max(a.position, b.position)
    result.value = int(a.value*10 + b.value)
  method joinTokens*(a: NumberToken, b: DigitToken): NumberToken =
    result = NumberToken()
    result.position = max(a.position, b.position)
    result.value = a.value*10 + int(b.value)
  
  var digits: seq[Token]
  
  digits.add DigitToken(value:1, position:0)
  digits.add DigitToken(value:2, position:1)
  
  var numberA = joinTokens(digits[0], digits[1])
  Run

  Error: type mismatch: got <Token, Token>
  but expected one of:
  method joinTokens(a: DigitToken; b: DigitToken): NumberToken
    first type mismatch at position: 1
    required type for a: DigitToken
    but expression 'digits[0]' is of type: Token
  method joinTokens(a: NumberToken; b: DigitToken): NumberToken
    first type mismatch at position: 1
    required type for a: NumberToken
    but expression 'digits[0]' is of type: Token
  
  expression: joinTokens(digits[0], digits[1])
  Run

  You can raise it in the issue tracker: https://github.com/nim-lang/Nim/issues

  Apart from that I really don't like to pull the "actually what you are asking is not what you want" card, but this is not how you should write a lexer.

  I suggest you use the npegs module which will let you define a grammar in a declarative way: https://github.com/zevv/npeg

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• konradmb (orginal) [2019-11-16T17:15:04+01:00] view original

  Apart from that I really don't like to pull the "actually what you are asking is not what you want" card, but this is not how you should write a lexer.

  I'd like to, but it's for a class assignment, so I have to do it by hand. Also, I want to learn how it can be done on lower level of abstraction. No problem pulling this card. It's always nice to show the other way.

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