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14070Re: "ocaml_beginners"::[] lexer and parser problem

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  • Philippe Veber
    Jun 28, 2014

      Hi Arash,

      The rule for "plus" should come before the rule for identifier tokens, otherwise all occurrences of "plus" will be read as identifiers. Regarding your other question, there's a Lexing.from_channel function you might find useful.


      Le 29 juin 2014 00:50, "Arash Alavi alavi.arash@... [ocaml_beginners]" <ocaml_beginners@yahoogroups.com> a écrit :

      I have a question about my program which is divided in two files: lexer.mll and parser.mly.
      The first file is lexer.mll:

      rule lex = parse
        | [' ' '\n' '\t'] { lex lexbuf }
        | ['0'-'9']+ as s { INT(int_of_string s) }
        | ['a'-'z' 'A'-'Z'] ['a'-'z' 'A'-'Z' '0'-'9']* as word { IDENT word }
        | '+'             { ADD } 
        | "plus"          { PLUS } 
        | eof             { EOF  }

      The second file is parser.mly:

      %{ type expr = 
      Int of int
      |String of string
      | Add of expr * expr 


      %token <int> INT
      %token <string> IDENT
      %token ADD 
      %token PLUS 
      %token EOF

      %start expr
      %type <expr> expr

      %left PLUS ADD 


      IDENT             { String $1 }
      |INT              { Int $1 }
      |expr PLUS expr   { Add($1, $3) }
      |expr ADD expr    { Add($1, $3) }

      So as you can see, for example I want to get the same results for both "1+2" and "1plus2" like :
      Add (Int 1, Int 2) 
      But I don't know why it works for "1+2" and it doesn't work for "1plus2".
      I use these commands to run the program:

      (*commands in Windows PowerShell*):
      ocamllex lexer.mll
      ocamlyacc parser.mly

      (*commands and results in OCamlWinPlus*):
      #use "parser.ml";;
      #use "lexer.ml";;
      # expr lex (Lexing.from_string "1+2");;
      - : expr = Add (Int 1, Int 2)
      # expr lex (Lexing.from_string "1plus2");;
      Exception: Parsing.Parse_error.

      Why it works correctly for 1+2 but not for 1plus2? and also how can I use a main class for this program which can get the inputs as a .txt file and return the result in another .txt file so I don't need to use this command (expr lex (Lexing.from_string "1plus2");;) for each code line?


      Arash Alavi
      Department of Computer Engineering & Information Technology
      Amirkabir University of Technology
      Home page: http://ceit.aut.ac.ir/~arash.alavi

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