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Re: early preview of a recursive descent parser compiler implemented as

From: Mike Mattie
Subject: Re: early preview of a recursive descent parser compiler implemented as a macro
Date: Sun, 13 Jan 2008 20:23:42 -0800

On Fri, 11 Jan 2008 15:07:38 -0800
Mike Mattie <> wrote:

> On Tue, 8 Jan 2008 02:34:36 -0800
> Mike Mattie <> wrote:
> > Hello,
> > 
> > I recently found myself needing a top down parser, so I wrote up a
> > parser compiler as a macro. It is turning out quite nicely for my
> > purposes so I would like to submit it for comments and suggestions.
> > I am still working out a couple of kinks, but it is at the point
> > where I can start integrating feedback.
> > 
> > The goal is a lightweight parser for simple jobs such as scraping a
> > data structure out of essentially static buffers, which is the
> > problem I was solving when I developed it.
> > 
> > I am in-lining the actual parser compiler, and attaching a testing
> > file so people can play with it if they find it interesting.
> > 
> > Status:
> > 
> > It's my first medium sized macro so it may have some thinko's or
> > oddities. criticism welcome.
> > 
> > Simple tests work. The backtracking hasn't been extensively tested
> > yet. The and form doesn't seem to return the full AST it should.
> > Other than that all the basic bits are in place.
> I reworked the runtime functions a bit and the and operator works
> correctly now. test-parser.el has some minor changes so I have
> included the latest version.

quoting is fixed for the production symbols.

> > The catch for semantic-error throws is not in place yet. I need
> > something like (condition-case) however that construct seemed a
> > little over-powered for my purpose. Suggestions on handling multiple
> > error conditions in a single form would be appreciated.

> My next priority is completing the error handling.

I chose the condition-case route. I ended up writing up a quick and dirty 
define-error which
is evidently in XEmacs. Any advice for how to handle this compatibility issue 
would be

After some good suggestions following review on IRC (thanks guys), I have 
cleaned up the
code for readability and re-organized it.

> > TODO:
> > 
> > 1. add ? for optional productions. Not a big deal to implement once
> > the basic implementation is sound.
> > 2. Full Documentation
> 3. add a (define ) list so that tokens and productions can be defined
>    without using them in place.

It's implemented but not tested yet.

> > Review Questions:
> > 
> > Do the Emacs developers consider this tool useful ? Does it fill a
> > niche in Emacs ? If so I will document it.
> > 
> > Is it elegant ?
> > 
> > Is the terminology correct ? I am not entirely sure I used curry in
> > the strict sense.

This version cleans up the use of parsing terminology.

> > Are there any essential features missing ? keeping in mind the goal
> > is simplicity, not a fully optimized parser.
> > 
> > Is it interesting in a creative sense, or have I just ground out
> > another lisp right of passage ?

With some pointers from #emacs on freenode I found one similar work on On Lisp
by Paul Graham. They are technical similarities of course, but the interface
and implementation are quite different. Any other pointers to Related works
would be greatly appreciated.
> > Examples:
> > 
> > A couple of contrived examples I have used for testing. A couple of
> > things to note. Keywords are token|and|or. token defines a regex
> > match, while or and and construct terminals. The definition of a
> > token or terminal creates a match object at the point of definition.
> > 
> > There is a built-in start symbol that is a or-terminal, so the first
> > example will match to the start symbol either token. 
> > 
> > (parser-compile test-parser
> >   (token whitespace "[[:blank:]]+")
> >   (token word "[[:alpha:]]+"))
> > 
> > The parser is compiled to the function value of test-parser. You can
> > call it like this: (test-parser (point))
> > 
> > It parses a single start production. Looping is done outside of the
> > parser itself. 
> > 
> > as the second example shows you can reference previously created
> > tokens.
> > 
> > (parser-compile test-parser
> >   (and indented (token whitespace "[[:blank:]]+") (token word
> > "[[:alpha:]]+")) (and inverted word whitespace))
> > 
> > The attached file test-parser.el has some test drivers for playing
> > with the parser interactively. Some of the features for defining
> > tokens such as defining your own actions for tokens are shown there.
> > 
> > P.S, here is a fun little lisp comic in the spirit of finding
> > amusement in your work:
> > 
> > Without further ado here is parser.el , and enjoy !
> >

Here is the third posting, v669. It will probably be the last mailing list post.
If I continue to publish it I will probably put it on EmacsWiki. I wanted to 
off a decently clean version of the code in case anybody stumbles across this 
and is interested.

;; parser.el
;; Primary Author: Mike Mattie (
;; Copyright: Mike Mattie (2008)

;; ->Summary.

;; parser.el aims to be a lightweight implementation of Recursive
;; Descent parsing. The parser-compile macro is given a symbol and a
;; grammar definition. It returns a compiled parser bound to the
;; function of the symbol.

;; -> Application.

;; Building or extracting a nested data structure by parsing static
;; text.

;; -> Comparison with Emacs parsing facilities.

;; Emacs supports parsing in two forms: Regular Expressions and tools
;; like Syntax Tables geared towards using the parse results to
;; annotate buffers with text-properties and overlays.

;; This Dynamic Programming approach works well for overlaying
;; functions that interpret the meaning of text in the buffer, such as
;; syntax highlighting, on a paradigm of unstructured text. The
;; analysis is preserved when the buffer is edited at a character
;; level.

;; When you need to build an interface that rests entirely on the
;; parse analysis to the degree that the user or program does not
;; modify or traverse the buffer at a character level, this tool
;; simplifies construction of a nested data structure that maps tokens
;; to beginning and ending positions of the match.

;; -> Related Works

;; * CEDET.

;; CEDET appears to be a parser generator implemented on-top of a CLOS
;; emulation. This tool-set has developed towards the problem of
;; analyzing source code as a project which is bloated for simpler
;; requirements.

;; CEDET is likely to offer much higher performance than an un-optimized
;; recursive descent parser. If backtracking needs to be optimized
;; something like CEDET is a better choice than parser-compile.

;; ->Concept

;; A parser compiler that combines lexical (regex) analysis with recursive
;; descent parsing compiled by macro expansion.

;; The concept started with the idea to build a parser as nested cond
;; forms. The matching would go in the predicate part and the action
;; in the body.

;; This essential simplicity is still present in the design but the
;; implementation is now more complex with Match Functions replacing
;; the cond clause form. Match Functions allow previous definitions of
;; a token or rule to be referenced later in later rules. Backtracking
;; was another essential complexity once the and non-terminal was
;; added.

;; ->Requirements

;; 1. Easy to use for trivial problems, powerful enough to solve
;;    most of them.

;; 2. Data structures are orthogonal to the parser and not hard-wired
;;    in. Tokens can perform user defined functions or construct custom
;;    data structures with the beginning and ending positions of the
;;    match in the input.
;;    This allows the user to choose between positions, markers, overlays
;;    according to the requirements.

;; ->Characteristics

;; parser-compile produces a no frills recursive descent parser.

;; ->Terminology

;; My reference for parsing terminology is the Dragon Book:

;; Compilers
;; Principles,Techniques,and Tools
;; Alfred V.Aho, Ravi Sethi, Jeffrey D.Ullman
;; 1986, Addison Wesley

;; ->TODO

;; 1. define list where Matches can be defined without inserting a
;;    match at the definition point [implemented, but not tested]

;; 2. optional matching with ? for Match Function references. [easy]

;; 3. Canonical tree walk implemented as parser-ast-node.

(require 'cl)

;; not defined in Gnu Emacs evidently.

(defmacro define-error ( symbol message &rest isa-list )
  "define a error symbol with a isa list and a error message"
     (put ',symbol
       'error-conditions (append '(error ,symbol) ',isa-list))
     (put ',symbol 'error-message ,message)

;; A recursive macro expansion would be nice for creating a hierarchy.
(define-error parser-compile-error  "parser error")
  (define-error parser-syntactic-error  "syntactic error" parser-compile-error)
  (define-error parser-semantic-error   "semantic error" parser-compile-error)

;; Backtracking.

;; parser-position

;; The position of the parser in the input stream is maintained as a
;; stack of indexes into the buffer. As matching progresses the top of
;; the stack (car) is updated. The push and pop operations copy a
;; position to a next or previous stack position. backtrack discards
;; the top of the stack.

(defsubst parser-pos () ;; tested
  "Return the current position of the parser in the buffer"
  (car parser-position))

(defun parser-push ()
  "Copy the parser position to a new stack level so the parser can backtrack 
when necessary."
  (push (parser-pos) parser-position))

(defun parser-pop ()
  "Copy the parser position to a previous stack level When the possibility of a 
   has been eliminated by matching."
    ((current (pop parser-position)))
    (setcar parser-position current)

(defun parser-backtrack ()
  "Restore the previous parser position by discarding the top of the 
parser-position stack."
  (pop parser-position)
  (goto-char (parser-pos)))

(defun parser-advance ( consumed )
  "Advance the input position of the parser to the next un-matched character: 
input consumed + 1."

  (if (> consumed 0)
      ((pos (+ 1 consumed (parser-pos))))
        (setcar parser-position pos)
        (goto-char pos)))

(defun parser-consumed ()
  "The number of input characters consumed by the token's match in the input."
  (- (match-end 0) (match-beginning 0)))

;; Match Result

;; The parser functions have a standard structure for returning the
;; result of a Match Function.

;; nil | (parser . AST ).

;; nil indicates failure to match.

;; A successful match has a parser and AST parts. The parser is
;; the ending position of the match relative to the starting
;; match position.

;; The AST part is created by a token action handler or a combination
;; operator like and.

;; token ( match-symbol . ( start . end ) | "user value" | "user function 
return value")
;; and   ( match-symbol . "a list of Match Result AST parts" )

;; This consistent return value structure allows token/and/or
;; match-functions to nest arbitrarily [except that tokens are
;; strictly leafs].

;; My rationale for using inline functions is that I am essentially naming
;; the car and cdr of my cons cell for readability.

(defsubst parser-make-match ( consumed data )
  "create a match result from the input consumed by the match, and the match 
  (cons consumed data))

(defsubst parser-match-consumed ( match-result )
  "return the input consumed by the match"
  (car match-result))

(defsubst parser-match-data ( match-result )
  "return the data of the match"
  (cdr match-result))

(defsubst parser-make-match-data ( name data )
  (cons name data))

;; Combination Operators

;; The parser uses two combination operators: parser-and, parser-or as
;; nodes in the parser tree. Significantly parser-and can backtrack
;; and parser-or never does.

;; and/or have the same essential meaning as the lisp and/or forms
;; with two specializations. Both functions treat their argument lists
;; as a list of Match Functions. Also the parser-and function returns
;; a production consisting of the AST parts of the Match Results,
;; instead of returning the last Match Result.

(defun parser-or ( &rest match-list )
  "Combine Match Functions by or ; the first successful match is returned.
   nil is returned if no matches are found"
  (catch 'match
    (dolist (match match-list)
        ((production (funcall match)))
        (if production
            (parser-advance (parser-match-consumed production))
            (throw 'match (parser-make-match 0 (parser-match-data production))))
    (throw 'match nil)

(defun parser-and ( &rest match-list )
  "combine the matches with and. all of the match objects must return non-nil
   in the parser part or the parser will backtrack and return nil."

    ((production (catch 'backtrack
            ;; we want to gather all the matches, so mapcar across the match 
              (lambda (match)
                  ((production (funcall match)))

                  (if production
                      (parser-advance (parser-match-consumed production))
                      (parser-match-data production))

                    (throw 'backtrack nil))
                  )) match-list)
   (if production
       (parser-make-match 0 production)) ;; would be nice to filter optional 
matches here.

;; compiler diagnostics

;; construct meaningful compiler error messages in the
;; "expected: foo got: bar" form.

(defun parser-expr-diagnostic ( form ) ;; tested
  (format "type(%s) %s" (symbol-name (type-of form)) (pp (eval form))))

(defmacro parser-diagnostic ( form from expected ) ;; tested
  "syntax: (parser-diagnostic form from expected)

   Where form is the expr received, from is the component issuing the 
   and expected is a message describing what the component expected"
  `(concat (format "[%s] expected: " ,from)  ,expected " not: " 
,(parser-expr-diagnostic form)))

;; Match Functions

;; make-match and get-match implement a table of match functions.

;; Match functions are lambdas that take no arguments and return Match
;; Result values. They assume that the compiled parser has scoped a
;; parser-position stack to determine the parsing position in input.

;; match-table is objarray created at macro scope, and does not appear
;; in the compiled form.

(defun parser-match-function ( identifier &optional definition )
  "Retrieve or define a Match Function."
    ((id (symbol-name identifier))
      (lookup (intern-soft id match-table)))

    (if lookup
      (if (eq definition nil)
        (signal 'parser-semantic-error (format "illegal redefinition of Match 
Function %s" id)))

        (fset (intern id match-table) (eval definition))
        (intern id match-table))

;; tokens

;; EXPERIMENT: allow other functions to be used for token matching
;;             other than looking-at, such as re-search.

(defun parser-build-token ( identifier ) ;; tested
  "parser-make-token is a built-in constructor that records the analysis
   and the location of the text (id . (begin . end))"
  (parser-make-match-data identifier (cons (match-beginning 0) (match-end 0))))

(defun parser-make-anon-func ( sexp ) ;; tested
  "bind an un-evaluated anonymous function to an un-interned symbol"
    ((anon-func (make-symbol "parser-user-handler")))
    (fset anon-func (eval sexp))

;; the token interpreter was split into two functions to isolate the
;; flexibility of tokens (user functions or return values for
;; constructing AST) from the hard-wired parser part.

(defun parser-interp-token-action ( identifier constructor ) ;; tested
  "Translate the AST constructor part of a token definition into Elisp."

  (unless (symbolp identifier)
    (signal 'parser-syntactic-error
      (parser-diagnostic identifier
        "parser token"
        "identifier: An unbound symbol used as an identifier"

  ;; Warning: this code is very touchy, double quotation of AST
  ;; symbols is required. the saving grace is that the symbols don't
  ;; have a variable value bound so it fails noisily when the
  ;; quotation is incorrect.
    ((eq nil constructor)    `(parser-build-token '',identifier))
    ((listp constructor)     `(,(parser-make-anon-func constructor) 
(match-beginning 0) (match-end 0)))
    ((functionp constructor) `(,constructor (match-beginning 0) (match-end 0)))
    ((symbolp constructor)   `(quote ',constructor))

    ;; all other constructor types are un-handled.
    ((signal 'parser-syntactic-error
       (parser-diagnostic identifier
         "parser token: identifier" "A symbol"))))

(defun parser-interp-token ( syntax ) ;; tested
  "Translate a token definition into a Match Function.

   The matching part is hard-wired into the function, while the
   AST part which contains a great degree of flexibility is
   translated by parser-interp-token-action"

    ((identifier  (car syntax))
     (regex       (cadr syntax))    ;; eval ? many regexs are stored in 
     (constructor (caddr syntax)))

    `(lambda ()
       (if (looking-at ,regex)
         (parser-make-match (parser-consumed) ,(parser-interp-token-action 
identifier constructor))

(defun parser-compile-token ( syntax ) ;; tested
  "Compile a token into a Match Function."
  (parser-match-function (car syntax) (parser-interp-token syntax)))

;; rules

;; rules are non-terminals, with a left side or identifier, and a
;; right side containing matches that recognize a production of the
;; rule.

(defun list-filter-nil ( list )
  "filter nil symbols from a list"
  (if (consp list)
      ((head (car list)))

      (if (eq head 'nil)
        (list-filter-nil (cdr list))
        (cons head (list-filter-nil (cdr list)))

(defun parser-rule-left ( prod-left combine-operator prod-right )
  "Translate the Left Side of a Rule into a Match Function
   currying the Right Side of a rule."

  (unless (symbolp prod-left)
    (parser-diagnostic prod-left
      "Rule Left interpreter"
      "left side of the production or an identifier"))

    ((matchf-list (list-filter-nil prod-right))) ;; filter nils created by 
define lists.

    (parser-match-function prod-left
      `(lambda ()
           ((production (apply ',combine-operator ',matchf-list)))
           (if production
               (parser-match-consumed production)
               (parser-make-match-data '',prod-left (parser-match-data 

(defun parser-rule-right ( rule )
  "Translate a match in the Right Side of the rule into a
   compiled Match Function by retrieving the Match Function or
   recursively interpreting the grammar definition."

    ((listp rule) (parser-compile-definition rule))
    ((symbolp rule) (parser-match-function rule))

    (signal 'parser-syntactic-error
      (parser-daignostic rule
        "Rule Right interpreter"
        "expected a grammar list e.g: token,and,or ; or a symbol as a rule or 
token reference"))

(defun parser-compile-rule ( combine-function prod-right )
  (parser-rule-left      ;; make a match function
    (car prod-right)     ;; the identifier of the production
    combine-function     ;; the combine operator
    (mapcar 'parser-rule-right (cdr prod-right)) ;; interpret the matching 

;; grammar definition.

(defun parser-compile-definition ( term )
  "parser-compile-definition is the recursive heart of the compiler."
  (unless (listp term)
    (signal 'parser-syntactic-error
      (parser-diagnostic term
        "parser definition"
        "expected a definition of token|or|and|define")))

    ((keyword (car term))
      (syntax (cdr term)))

    (if (listp keyword)
      (parser-compile-definition keyword)

        ((eq keyword 'token)  (parser-compile-token syntax))
        ((eq keyword 'or)     (parser-compile-rule 'parser-or syntax))
        ((eq keyword 'and)    (parser-compile-rule 'parser-and syntax))

        ((eq keyword 'define)
          ;; define discards the match functions as a return value so
          ;; tokens and rules can be defined before they are used.
          (mapcar 'parser-rule-right syntax)

        ((signal 'parser-syntactic-error
           (parser-diagnostic term
             "parser definition"
             "definition keyword token|or|and|define")))

(defvar parser-mtable-init-size 13
  "initial size of the match-table objarray for storing match functions. the 
   was chosen based on the recommendation of prime numbers for good hashing.")

(defmacro parser-compile ( parser &rest definition )
  "compile a LL parser from the given grammar."
    ;; create a symbol table to store compiled terminal and
    ;; non-terminal match functions
    ((match-table (make-vector parser-mtable-init-size 0)))

    (condition-case diagnostic
        (fset parser
            `(lambda ( start-pos )
                 ((parser-position (cons start-pos nil))) ;; initialize the 
backtrack stack
                   (goto-char start-pos)
                   ;; note that the start symbol of the grammar is built in as 
an or combination
                   ;; of the top-level definitions.
                     ((parse (,(parser-rule-left 'start 'parser-or
                                 (mapcar 'parser-compile-definition definition))
                     (if parse
                       ;; if we have a production return the position at which 
                       ;; parser stopped along with the AST.
                       (parser-make-match (parser-pos) (parser-match-data 

          (message "parser-compile Syntax Error %s" diagnostic)
          (message "parser-compile invalid statement %s" diagnostic)

(provide 'parser)

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