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;;; Guile Emacs Lisp
;;; Copyright (C) 2009, 2010 Free Software Foundation, Inc.
;;;
;;; This library is free software; you can redistribute it and/or
;;; modify it under the terms of the GNU Lesser General Public
;;; License as published by the Free Software Foundation; either
;;; version 3 of the License, or (at your option) any later version.
;;;
;;; This library is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;;; Lesser General Public License for more details.
;;;
;;; You should have received a copy of the GNU Lesser General Public
;;; License along with this library; if not, write to the Free Software
;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
;;; Code:
(define-module (language elisp parser)
#:use-module (language elisp lexer)
#:export (read-elisp))
;;; The parser (reader) for elisp expressions.
;;;
;;; It is hand-written (just as the lexer is) instead of using some
;;; parser generator because this allows easier transfer of source
;;; properties from the lexer ((text parse-lalr) seems not to allow
;;; access to the original lexer token-pair) and is easy enough anyways.
;;; Report a parse error. The first argument is some current lexer
;;; token where source information is available should it be useful.
(define (parse-error token msg . args)
(apply error msg args))
;;; For parsing circular structures, we keep track of definitions in a
;;; hash-map that maps the id's to their values. When defining a new
;;; id, though, we immediately fill the slot with a promise before
;;; parsing and setting the real value, because it must already be
;;; available at that time in case of a circular reference. The promise
;;; refers to a local variable that will be set when the real value is
;;; available through a closure. After parsing the expression is
;;; completed, we work through it again and force all promises we find.
;;; The definitions themselves are stored in a fluid and their scope is
;;; one call to read-elisp (but not only the currently parsed
;;; expression!).
(define circular-definitions (make-fluid))
(define (make-circular-definitions)
(make-hash-table))
(define (circular-ref token)
(if (not (eq? (car token) 'circular-ref))
(error "invalid token for circular-ref" token))
(let* ((id (cdr token))
(value (hashq-ref (fluid-ref circular-definitions) id)))
(if value
value
(parse-error token "undefined circular reference" id))))
;;; Returned is a closure that, when invoked, will set the final value.
;;; This means both the variable the promise will return and the
;;; hash-table slot so we don't generate promises any longer.
(define (circular-define! token)
(if (not (eq? (car token) 'circular-def))
(error "invalid token for circular-define!" token))
(let ((value #f)
(table (fluid-ref circular-definitions))
(id (cdr token)))
(hashq-set! table id (delay value))
(lambda (real-value)
(set! value real-value)
(hashq-set! table id real-value))))
;;; Work through a parsed data structure and force the promises there.
;;; After a promise is forced, the resulting value must not be recursed
;;; on; this may lead to infinite recursion with a circular structure,
;;; and additionally this value was already processed when it was
;;; defined. All deep data structures that can be parsed must be
;;; handled here!
(define (force-promises! data)
(cond
((pair? data)
(begin
(if (promise? (car data))
(set-car! data (force (car data)))
(force-promises! (car data)))
(if (promise? (cdr data))
(set-cdr! data (force (cdr data)))
(force-promises! (cdr data)))))
((vector? data)
(let ((len (vector-length data)))
(let iterate ((i 0))
(if (< i len)
(let ((el (vector-ref data i)))
(if (promise? el)
(vector-set! data i (force el))
(force-promises! el))
(iterate (1+ i)))))))
;; Else nothing needs to be done.
))
;;; We need peek-functionality for the next lexer token, this is done
;;; with some single token look-ahead storage. This is handled by a
;;; closure which allows getting or peeking the next token. When one
;;; expression is fully parsed, we don't want a look-ahead stored here
;;; because it would miss from future parsing. This is verified by the
;;; finish action.
(define (make-lexer-buffer lex)
(let ((look-ahead #f))
(lambda (action)
(if (eq? action 'finish)
(if look-ahead
(error "lexer-buffer is not empty when finished")
#f)
(begin
(if (not look-ahead)
(set! look-ahead (lex)))
(case action
((peek) look-ahead)
((get)
(let ((result look-ahead))
(set! look-ahead #f)
result))
(else (error "invalid lexer-buffer action" action))))))))
;;; Get the contents of a list, where the opening parentheses has
;;; already been found. The same code is used for vectors and lists,
;;; where lists allow the dotted tail syntax and vectors not;
;;; additionally, the closing parenthesis must of course match. The
;;; implementation here is not tail-recursive, but I think it is clearer
;;; and simpler this way.
(define (get-list lex allow-dot close-square)
(let* ((next (lex 'peek))
(type (car next)))
(cond
((eq? type (if close-square 'square-close 'paren-close))
(begin
(if (not (eq? (car (lex 'get)) type))
(error "got different token than peeked"))
'()))
((and allow-dot (eq? type 'dot))
(begin
(if (not (eq? (car (lex 'get)) type))
(error "got different token than peeked"))
(let ((tail (get-list lex #f close-square)))
(if (not (= (length tail) 1))
(parse-error next
"expected exactly one element after dot"))
(car tail))))
(else
;; Do both parses in exactly this sequence!
(let* ((head (get-expression lex))
(tail (get-list lex allow-dot close-square)))
(cons head tail))))))
;;; Parse a single expression from a lexer-buffer. This is the main
;;; routine in our recursive-descent parser.
(define quotation-symbols '((quote . quote)
(backquote . #{`}#)
(unquote . #{,}#)
(unquote-splicing . #{,@}#)))
(define (get-expression lex)
(let* ((token (lex 'get))
(type (car token))
(return (lambda (result)
(if (pair? result)
(set-source-properties!
result
(source-properties token)))
result)))
(case type
((eof)
(parse-error token "end of file during parsing"))
((integer float symbol character string)
(return (cdr token)))
((function)
(return `(function ,(get-expression lex))))
((quote backquote unquote unquote-splicing)
(return (list (assq-ref quotation-symbols type)
(get-expression lex))))
((paren-open)
(return (get-list lex #t #f)))
((square-open)
(return (list->vector (get-list lex #f #t))))
((circular-ref)
(circular-ref token))
((circular-def)
;; The order of definitions is important!
(let* ((setter (circular-define! token))
(expr (get-expression lex)))
(setter expr)
(force-promises! expr)
expr))
((set-lexical-binding-mode!)
(return `(%set-lexical-binding-mode ,(cdr token))))
(else
(parse-error token "expected expression, got" token)))))
;;; Define the reader function based on this; build a lexer, a
;;; lexer-buffer, and then parse a single expression to return. We also
;;; define a circular-definitions data structure to use.
(define (read-elisp port)
(with-fluids ((circular-definitions (make-circular-definitions)))
(let* ((lexer (get-lexer port))
(lexbuf (make-lexer-buffer lexer))
(next (lexbuf 'peek)))
(if (eq? (car next) 'eof)
(cdr next)
(let ((result (get-expression lexbuf)))
(lexbuf 'finish)
result)))))
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