Re: Bison string processing

[Hans Aberg]

At 12:10 +0300 0-11-29, Vladimir Rykov wrote:
>I have to write  a program which converts the input string:
>
>   vv implies ww  ;
>
>in the output line  :

Bison parses lexemes, or "words", of a language, which are produced by the
yylex functions. A simple way to produce such lexemes is by using the a
lexical analyzer compiler like Flex, see
  ftp://ftp.digital.com/pub/GNU/non-gnu/flex/flex-2.5.4a.tar.gz
which from a regular-word-like definition writes a definition of yylex.

In Flex one could write a definition say like

[[:alpha:]]+  { /* save (yytext, yyleng) say in yylval */ return NAME; }
"=>"          { return IMPLIES; }
\n            { ++line_number; return yytext[0]; }
.             { return yytext[0]; /* Single character becomes code. */ }

In Bison, one would define

%token IMPLIES, NAME, ...

and use in variable definitions as

exp:
    NAME { ...  }
  | exp IMPLIES exp  { ... }

The Bison manual 3.6.1 also says that if you declare

%token IMPLIES "=>"

then you can later use that name as in

exp:
    NAME { ...  }
  | exp "=>" exp  { ... }

But here "=>" is just a symbol for IMPLIES, a way to make your Bison
definition file look nicer; yylex still needs to recognize the token
IMPLIES, which could be something else than "=>".

-- The Bison manual 4.2.1 indicates a way for yylex to use token table
lookups, but this is said to be slow.

  Hans Aberg