That's not ideal because if there is white space after BB#0:, it will match COMMENT again. Is there a better way to do this?
On Wed, Jul 23, 2014 at 10:05 PM, Jon Zeppieri <[email protected]> wrote: > Sorry, I sent that early by mistake. More below: > > On Thu, Jul 24, 2014 at 1:02 AM, Jon Zeppieri <[email protected]> wrote: > > Your example string is "\n; BB#0;\n" > > So, I'd expect the lexer to match: > > - whitespace > > - line-comment > > > > Yes, `block-comment` matches, but `line-comment' > > ... gives the longer match, because it includes the newline at the > end, whereas `block-comment` will not match that newline. Since the > ending newline will be taken care of by the whitespace rule, perhaps > you could simply remove the final newline from the `line-comment` > definition? It will still match everything up to (but not including) > the newline. > > -Jon > > > > > > > > On Thu, Jul 24, 2014 at 12:46 AM, Mangpo Phitchaya Phothilimthana > > <[email protected]> wrote: > >> Hi, > >> > >> I try to write a lexer and parser, but I cannot figure out how to set > >> priority to lexer's tokens. My simplified lexer (shown below) has only 2 > >> tokens BLOCK, and COMMENT. BLOCK is in fact a subset of COMMENT. BLOCK > >> appears first in the lexer, but when I parse something that matches > BLOCK, > >> it always matches to COMMENT instead. Below is my program. In this > >> particular example, I expect to get a BLOCK token, but I get COMMENT > token > >> instead. If I comment out (line-comment (token-COMMENT lexeme)) in the > >> lexer, I then get the BLOCK token. > >> > >> Can anyone tell me how to work around this issue? I can only find this > in > >> the documentation > >> "When multiple patterns match, a lexer will choose the longest match, > >> breaking ties in favor of the rule appearing first." > >> > >> #lang racket > >> > >> (require parser-tools/lex > >> (prefix-in re- parser-tools/lex-sre) > >> parser-tools/yacc) > >> > >> (define-tokens a (BLOCK COMMENT)) > >> (define-empty-tokens b (EOF)) > >> > >> (define-lex-trans number > >> (syntax-rules () > >> ((_ digit) > >> (re-: (uinteger digit) > >> (re-? (re-: "." (re-? (uinteger digit)))))))) > >> > >> (define-lex-trans uinteger > >> (syntax-rules () > >> ((_ digit) (re-+ digit)))) > >> > >> (define-lex-abbrevs > >> (block-comment (re-: "; BB#" number10 ":")) > >> (line-comment (re-: ";" (re-* (char-complement #\newline)) #\newline)) > >> (digit10 (char-range "0" "9")) > >> (number10 (number digit10))) > >> > >> (define my-lexer > >> (lexer-src-pos > >> (block-comment (token-BLOCK lexeme)) > >> (line-comment (token-COMMENT lexeme)) > >> (whitespace (position-token-token (my-lexer input-port))) > >> ((eof) (token-EOF)))) > >> > >> (define my-parser > >> (parser > >> (start code) > >> (end EOF) > >> (error > >> (lambda (tok-ok? tok-name tok-value start-pos end-pos) > >> (raise-syntax-error 'parser > >> (format "syntax error at '~a' in src l:~a c:~a" > >> tok-name > >> (position-line start-pos) > >> (position-col start-pos))))) > >> (tokens a b) > >> (src-pos) > >> (grammar > >> (unit ((BLOCK) $1) > >> ((COMMENT) $1)) > >> (code ((unit) (list $1)) > >> ((unit code) (cons $1 $2)))))) > >> > >> (define (lex-this lexer input) > >> (lambda () > >> (let ([token (lexer input)]) > >> (pretty-display token) > >> token))) > >> > >> (define (ast-from-string s) > >> (let ((input (open-input-string s))) > >> (ast input))) > >> > >> (define (ast input) > >> (my-parser (lex-this my-lexer input))) > >> > >> (ast-from-string " > >> ; BB#0: > >> ") > >> > >> ____________________ > >> Racket Users list: > >> http://lists.racket-lang.org/users > >> >
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