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7124 :: How to write a macro correctly which contains a 'char'?

• AlectronikHQ (orginal) [2020-11-19T17:09:04+01:00] view original

  I am building a lexer where I have a case statement with many repeated cases where just the character changes and the resulting token.

  Example:

  case l.ch
  of '+':
    tok = Token( Type: tkPlus, Literal: $l.ch )
  of '-':
    tok = Token( Type: tkMinus, Literal: $l.ch )
  ...
  Run

  I would like to have a macro like

  ( '-', tkMinus )
  Run

  doing the work for me. How would I write such a macro? Tried with echo statements but it doesn't work. Also is there a compiler switch to quickly see the expanded macros?

  Sorry for the beginner questions.

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• Stefan_Salewski (orginal) [2020-11-19T17:32:23+01:00] view original

  There exists a few macro tutorials...

  But as I am not really good with macros I would try something like

  let h =
    case l.ch
    of '+': tkPlus
    of '-': tkMinus
  ...
  tok = Token(h, Literal: $l.ch )
  Run

  You may not even need a case statement, I think ascii characters are valid for array index, so you may use an array with tk*** content?

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• AlectronikHQ (orginal) [2020-11-19T17:42:55+01:00] view original

  You're right, there might be better solutions here than a macro - this was just a quick porting of a Go code to Nim, and I was thinking of C preprocessor macros, so ...

  But about the macros, I am pretty intrigued by but so far I don't understand them.. found some tutorials but so far no 1:1 macro -> expanded code samples out of which I'd learn best..

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• geotre (orginal) [2020-11-19T18:07:57+01:00] view original

  You can add echo result.repr at the end of a macro to see the resulting code

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• AlectronikHQ (orginal) [2020-11-19T18:37:19+01:00] view original

  Thanks.

  What I am specially looking for is how to (un)escape strings in macros. How to write like result = echo "of '$ch':" if we had a special $ for including characters in the string. Sorry, English's not my primary language..

  But have to say in general that I begin to love nim. Have ported some code which I did previously to JavaScript and it's equally easy but more fun.

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• Araq (orginal) [2020-11-19T19:43:15+01:00] view original

  I wrote https://github.com/araq/lexim back in the days. The code is unlikely to still compile but it is a classical lexer generator based on macros.

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• AlectronikHQ (orginal) [2020-11-20T10:30:04+01:00] view original

  I get error: type expected when assigning buildAst to result ... ?

  macro simpleToken( ch: char, typ: TokType ): untyped =
    result = buildAst(ofBranch(newLit('l')):
      asgn(ident"tok", objConstr(ident"Token", exprColonExpr(ident"Type", TokType))))
    echo result.repr
  Run

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• Araq (orginal) [2020-11-20T10:45:53+01:00] view original

  Not sure about the error message, but you need to build a case statement, not just an "ofBranch".

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• PMunch (orginal) [2020-11-20T11:04:12+01:00] view original

  Maybe this will help a little bit to get a feel for the workflow of creating a macro: https://peterme.net/metaprogramming-and-read-and-maintainability-in-nim.html.

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• AlectronikHQ (orginal) [2020-11-20T11:16:07+01:00] view original

  I thought of writing the case by hand and having the macros just generating of branches, like:

  case ch
  of '!':
    # handle special case
  simpleToken( '-', tkMinus )
  simpleToken( '+', tkPlus )
  ...
  Run

  Agreed that here a table is more elegant and that's what I'm using now but still curious about how the macro solution would be to learn.. :)

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