[sod] / src / proto-lexer.lisp

;;; -*-lisp-*-
;;;
;;; Protocol for lexical analysis
;;;
;;; (c) 2009 Straylight/Edgeware
;;;

;;;----- Licensing notice ---------------------------------------------------
;;;
;;; This file is part of the Sensble Object Design, an object system for C.
;;;
;;; SOD is free software; you can redistribute it and/or modify
;;; it under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 2 of the License, or
;;; (at your option) any later version.
;;;
;;; SOD 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 General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with SOD; if not, write to the Free Software Foundation,
;;; Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

(cl:in-package #:sod)

;;;--------------------------------------------------------------------------
;;; Accessors.

(export 'lexer-char)
(defgeneric lexer-char (lexer)
  (:documentation
   "Return the current lookahead character from the LEXER.

   When the lexer is first created, there is no lookahead character: you must
   `prime the pump' by calling NEXT-CHAR.  The lexer represents encountering
   the end of its input stream by setting the lookahead character to nil.  At
   this point it is still possible to push back characters."))

;;;--------------------------------------------------------------------------
;;; Formatting tokens.

(defgeneric format-token (token-type &optional token-value)
  (:documentation
   "Return a string describing a token with the specified type and value.")
  (:method ((token-type (eql :eof)) &optional token-value)
    (declare (ignore token-value))
    "<end-of-file>")
  (:method ((token-type (eql :string)) &optional token-value)
    (declare (ignore token-value))
    "<string-literal>")
  (:method ((token-type (eql :char)) &optional token-value)
    (declare (ignore token-value))
    "<character-literal>")
  (:method ((token-type (eql :id)) &optional token-value)
    (format nil "<identifier~@[ `~A'~]>" token-value))
  (:method ((token-type symbol) &optional token-value)
    (declare (ignore token-value))
    (check-type token-type keyword)
    (format nil "`~(~A~)'" token-type))
  (:method ((token-type character) &optional token-value)
    (declare (ignore token-value))
    (format nil "~:[<~:C>~;`~C'~]"
            (and (graphic-char-p token-type)
                 (char/= token-type #\space))
            token-type)))

;;;--------------------------------------------------------------------------
;;; Reading and pushing back characters.

(export 'next-char)
(defgeneric next-char (lexer)
  (:documentation
   "Fetch the next character from the LEXER's input stream.

   Read a character from the input stream, and store it in the LEXER's CHAR
   slot.  The character stored is returned.  If characters have been pushed
   back then pushed-back characters are used instead of the input stream.  If
   there are no more characters to be read then the lookahead character is
   nil.  Returns the new lookahead character.

   (This function is primarily intended for the use of lexer subclasses.)"))

(export 'pushback-char)
(defgeneric pushback-char (lexer char)
  (:documentation
   "Push the CHAR back into the lexer.

   Make CHAR be the current lookahead character (stored in the LEXER's CHAR
   slot).  The previous lookahead character is pushed down, and will be made
   available again once this character is consumed by NEXT-CHAR.

   (This function is primarily intended for the use of lexer subclasses.)"))

(defgeneric fixup-stream* (lexer thunk)
  (:documentation
   "Helper function for WITH-LEXER-STREAM.

   This function does the main work for WITH-LEXER-STREAM.  The THUNK is
   invoked on a single argument, the LEXER's underlying STREAM."))

(export 'with-lexer-stream)
(defmacro with-lexer-stream ((streamvar lexer) &body body)
  "Evaluate BODY with STREAMVAR bound to the LEXER's input stream.

   The STREAM is fixed up so that the next character read (e.g., using
   READ-CHAR) will be the lexer's current lookahead character.  Once the BODY
   completes, the next character in the stream is read and set as the
   lookahead character.  It is an error if the lexer has pushed-back
   characters (since these can't be pushed back into the input stream
   properly)."

  `(fixup-stream* ,lexer (lambda (,streamvar) ,@body)))

;;;--------------------------------------------------------------------------
;;; Reading and pushing back tokens.

(export 'scan-token)
(defgeneric scan-token (lexer)
  (:documentation
   "Internal protocol for scanning tokens from an input stream.

   Implementing a method on this function is the main responsibility of LEXER
   subclasses; it is called by the user-facing NEXT-TOKEN function.

   The method should consume characters (using NEXT-CHAR) as necessary, and
   return two values: a token type and token value.  These will be stored in
   the corresponding slots in the lexer object in order to provide the user
   with one-token lookahead."))

(export 'next-token)
(defgeneric next-token (lexer)
  (:documentation
   "Scan a token from an input stream.

   This function scans a token from an input stream.  Two values are
   returned: a `token type' and a `token value'.  These are opaque to the
   LEXER base class, but the intent is that the token type be significant to
   determining the syntax of the input, while the token value carries any
   additional information about the token's semantic content.  The token type
   and token value are also made available for lookahead via accessors
   TOKEN-TYPE and TOKEN-VALUE on the LEXER object.

   The new lookahead token type and value are returned as two separate
   values.

   If tokens have been pushed back (see PUSHBACK-TOKEN) then they are
   returned one by one instead of scanning the stream."))

(export 'pushback-token)
(defgeneric pushback-token (lexer token-type &optional token-value location)
  (:documentation
   "Push a token back into the lexer.

   Make the given TOKEN-TYPE and TOKEN-VALUE be the current lookahead token.
   The previous lookahead token is pushed down, and will be made available
   agan once this new token is consumed by NEXT-TOKEN.  If LOCATION is
   non-nil then FILE-LOCATION is saved and replaced by LOCATION.  The
   TOKEN-TYPE and TOKEN-VALUE can be anything at all: for instance, they need
   not be values which can actually be returned by NEXT-TOKEN."))

;;;--------------------------------------------------------------------------
;;; Utilities.

(export 'skip-spaces)
(defgeneric skip-spaces (lexer)
  (:documentation
   "Skip over whitespace characters in the LEXER.

   There must be a lookahead character; when the function returns, the
   lookahead character will be a non-whitespace character or nil if there
   were no non-whitespace characters remaining.  Returns the new lookahead
   character."))

(export 'require-token)
(defun require-token
    (lexer wanted-token-type &key (errorp t) (consumep t) default)
  "Require a particular token to appear.

   If the LEXER's current lookahead token has type WANTED-TOKEN-TYPE then
   consume it (using NEXT-TOKEN) and return its value.  Otherwise, if the
   token doesn't have the requested type then signal a continuable error
   describing the situation and return DEFAULT (which defaults to nil).

   If ERRORP is false then no error is signalled; this is useful for
   consuming or checking for optional punctuation.  If CONSUMEP is false then
   a matching token is not consumed; non-matching tokens are never consumed."

  (with-slots (token-type token-value) lexer
    (cond ((eql token-type wanted-token-type)
           (prog1 token-value
             (when consumep (next-token lexer))))
          (errorp
           (cerror* "Expected ~A but found ~A"
                    (format-token wanted-token-type)
                    (format-token token-type token-value))
           default)
          (t
           default))))

;;;----- That's all, folks --------------------------------------------------