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lexgen

Description

lexgen is a lexer generator comprised in its core of only four small procedures. The programmer combines these procedures into regular expression pattern matchers.

A pattern matcher procedure takes a list of streams, and returns a new list of streams advanced by every combination allowed by the pattern matcher function. A stream is defined as a list that contains a list of characters consumed by the pattern matcher, and a list of characters not yet consumed.

Note that the number of streams returned by a pattern matcher typically won't match the number of streams passed in. If the pattern doesn't match at all, the empty list is returned.

Library Procedures

Every combinator procedure in this library returns a procedure that takes in a list of streams as an argument.

Basic procedures

[procedure] (tok TOKEN PROC) => MATCHER

Procedure tok builds a pattern matcher function that, for each stream given, applies a procedure to the given token TOKEN and an input character. If the procedure returns a true value, that value is prepended to the list of consumed elements, and the input character is removed from the list of input elements.

[procedure] (seq MATCHER1 MATCHER2) => MATCHER

seq builds a matcher that matches a sequence of patterns.

[procedure] (bar MATCHER1 MATCHER2) => MATCHER

bar matches either of two patterns. It's analogous to patterns separated by | in traditional regular expressions.

[procedure] (star MATCHER) => MATCHER

star is an implementation of the Kleene closure. It is analogous to * in traditional regular expressions.

Convenience procedures

These procedures are built from the previous four and are provided for convenience.

[procedure] (try PROC) => PROC

Converts a binary predicate procedure to a binary procedure that returns its right argument when the predicate is true, and false otherwise.

[procedure] (char CHAR) => MATCHER

Matches a single character.

[procedure] (lst MATCHER-LIST) => MATCHER

Constructs a matcher for the sequence of matchers in MATCHER-LIST.

[procedure] (pos MATCHER) => MATCHER

Positive closure. Analogous to +.

[procedure] (opt MATCHER) => MATCHER

Optional pattern. Analogous to ?.

[procedure] (set CHAR-SET) => MATCHER

Matches any of a SRFI-14 set of characters.

[procedure] (range CHAR CHAR) => MATCHER

Matches a range of characters. Analogous to character class [].

[procedure] (lit STRING) => MATCHER

Matches a literal string s.

Lexer procedures

[procedure] (longest STREAM-LIST) => STREAM

Takes the resulting streams produced by the application of a pattern on a stream (or streams) and selects the longest match if one exists. If STREAM-LIST is empty, it returns #F.

[procedure] (lex MATCHER ERROR STRING) => CHAR-LIST

lex takes a pattern and a string, turns the string into a list of streams (containing one stream), applies the pattern, and returns the longest match. Argument ERROR is a single-argument procedure called when the pattern does not match anything.

Examples

 
 ;; A pattern to match floating point numbers. 
 ;; "-"?(([0-9]+(\\.[0-9]+)?)|(\\.[0-9]+))([eE][+-]?[0-9]+)? 

 (define (err s)
   (print "lexical error on stream: " s)
   (list))

 (define numpat
   (let* ((digit        (range #\0 #\9))
          (digits       (pos digit))
          (fraction     (seq `(,(char #\.) ,digits)))
          (significand  (bar `(,(seq `(,digits ,(opt fraction))) ,fraction)))
          (exp          (seq `(,(set "eE") ,(opt (set "+-")) ,digits)))
          (sign         (opt (char #\-)) ))     
    (seq `(,sign ,(seq `(,significand ,(opt exp)))))))

 (print (lex numpat err "3.45e-6"))

Requires

Version History

License

Based on the SML lexer generator by Thant Tessman.

 Copyright 2009 Ivan Raikov.
 All rights reserved.
 
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 Neither the name of the author nor the names of its contributors may
 be used to endorse or promote products derived from this software
 without specific prior written permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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