|Subject:||my current project: new lalr module|
|Date:||Wed, 03 Jun 2015 20:37:17 -0700|
|Having fun with all the great code, and documentation, in guile 2. |
My current project is a lalr module. Last December I wanted to start working on an interpreter for modelica in guile. Guile has a lalr parser generator (system base lalr). When I started looking at it, I was not crazy about the input language. I looked to see if a better interface could be pasted on, but the code impenetrable for me. So I started coding up my own LALR parser generator. I was hoping it would take a month of nights/weekends but ended up taking me close to four months. For now I am calling my implementation "lalr1".
A major difference between my lalr1 and lalr in guile is that the guile version is a translation of the C-coded bison tool and mine is a from-scratch implementation in Scheme based on algorithms from the Dragon Book. (The bison implementation uses the DeRemer-Pennello algorithms, yacc does not. I'd guess the bison-based implementation will generate a parser generator faster, but I think that is a minor advantage.)
Some features of lalr1:
0) It's implemented with syntax-rules and syntax-case (versus define-macro for lalr).
1) There is no specification of terminals. Terminals are detected in rules (via fenders) as quoted symbols or characters.
Example of terminals: 'number, #\+
2) Actions are specified as ($$ action ...). (Identifiers beginning with $ are reserved.)
3) Mid-rule actions (Bison Manual section 3.4.8 ) are supported. (Not supported by lalr in guile.)
4) Some handy "proxy" macros are included. The macro _expression_ results in a generated symbol and additional productions. Examples:
($? a b c) will generate symbol $Pk and production ($Pk () (a b c))
($* a b c) will generate symbol $Pk and production ($Pk () ($Pk a b c))
($+ a b c) will generate symbol $Pk and production ($Pk (a b c) ($Pk a b c))
where $Pk will be $P0, $P1, ...
5) It can generate Bison and guile lalr input languages from lalr1 spec's. (I have been using the Bison converter to debug lookahead generation.)
6) There is a macro to generate lalr1 input language from a lalr-parser specification.
7) Table compaction is optional. (The current compaction procedure will generate a default action if max duplication is more than 3.)
8) The bulk of the code resides in one module of ~ 1500 lines of code and ~ 500 lines of comments.
Here is a partial specification from the C-parser I am working on. It is being implemented with attributed-grammar semantics so that I can process code with functions from the SXML modules. For example, I am using foldts to detect typedefs need to feed back to the lexer.)
(translation-unit-proxy (translation-unit ($$ (tl->list $1))))
(external-declaration ($$ (make-tl 'trans-unit $1)))
(translation-unit external-declaration ($$ (tl-app! $1 $2)))
(declaration-specifier init-declarator-list ($$ (...)) #\; ;; <=mid-rule action to capture typedef's
($$ (list 'decl (tl->list $1) (tl->list $2)))))
($$ (list 'decl (tl->list $1))
('ident ($$ (list 'ident $1)))
(#\( declarator #\) ($$ (list 'scope $2)))
(direct-declarator #\[ constant-_expression_ #\] ($$ (list 'array $3 $1)))
(direct-declarator #\[ #\] ($$ (list 'array $3 $1)))
(direct-declarator #\( parameter-type-list #\) ($$ (list 'function $1 $3)))
(direct-declarator #\( identifier-list #\) ($$ (list 'function $1 $3)))
(direct-declarator #\( #\) ($$ (list 'function $1 $3)))
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