;;; Guile Parser Combinators
;;; Copyright © 2015 David Thompson <davet@gnu.org>
;;;
;;; This module 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 module 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 module.  If not, see
;;; <http://www.gnu.org/licenses/>.

;;; Commentary:
;;
;; Monadic parser combinators.
;;
;;; Code:

(define-module (parser-combinators)
  #:use-module (ice-9 match)
  #:use-module (ice-9 q)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-26)
  #:use-module (srfi srfi-41)
  #:export (parse-result
            parse-result?
            parse-result-value
            parse-result-index
            parse-failure?
            parse-success?
            %parse-failure
            parse-fail
            parse-end
            parse-bind
            parse-return
            parse-lift
            parse-map
            parse-any
            parse-each
            parse-zero-or-more
            parse-one-or-more
            parse-maybe
            parse))

(define-record-type <parse-result>
  (parse-result value stream)
  parse-result?
  (value parse-result-value)
  (stream parse-result-stream))

(define (parse-failure? result)
  "Return #t if RESULT represents a failed parse."
  (not (parse-result-stream result)))

(define (parse-success? result)
  "Return #t if RESULT represents a successful parse."
  (stream? (parse-result-stream result)))

(define (parse-done? result)
  "Return #t if the remainder of RESULT is the empty stream."
  (stream-null? (parse-result-stream result)))

(define %parse-failure (parse-result #f #f))

(define (parse-fail stream)
  "Always fail to parse STREAM."
  %parse-failure)

(define (parse-end stream)
  (if (stream-null? stream)
      (parse-result #t stream-null)
      %parse-failure))

(define (parse-bind proc parser)
  (lambda (stream)
    (match (parser stream)
      ((? parse-failure? _) %parse-failure)
      (($ <parse-result> value stream)
       ((proc value) stream)))))

(define (parse-return x)
  "Return a parser that always yields X as the parse result."
  (lambda (stream)
    (parse-result x stream)))

(define (parse-lift proc)
  "Return a procedure that wraps the result of PROC in a parser."
  (lambda args
    (parse-return (apply proc args))))

(define (parse-map proc parser)
  "Return a new parser that applies PROC to result of PARSER."
  (parse-bind (parse-lift proc) parser))

(define (%parse-any . parsers)
  (lambda (stream)
    (let loop ((parsers parsers))
      (match parsers
        (() %parse-failure)
        ((parser . rest)
         (match ((force parser) stream)
           ((? parse-failure? _)
            (loop rest))
           (result result)))))))

;; This is a special form due to the lazy evaluation used to handle
;; right recursive grammars.
(define-syntax-rule (parse-any parser ...)
  "Create a disjunctive parser that succeeds if any of the input
parsers succeed."
  (%parse-any (delay parser) ...))

(define (%parse-each . parsers)
  (lambda (stream)
    (let loop ((stream stream)
               (parsers parsers)
               (result '()))
      (match parsers
        (() (parse-result (reverse result) stream))
        ((parser . rest)
         (match ((force parser) stream)
           ((? parse-failure?) %parse-failure)
           (($ <parse-result> value stream)
            (loop stream rest (cons value result)))))))))

(define-syntax-rule (parse-each parser ...)
  (%parse-each (delay parser) ...))

(define (parse-zero-or-more parser)
  (lambda (stream)
    (let loop ((stream stream)
               (result '()))
      (match (parser stream)
        ((? parse-failure?)
         (parse-result (reverse result) stream))
        (($ <parse-result> value stream)
         (loop stream (cons value result)))))))

(define (parse-one-or-more parser)
  (lambda (stream)
    (let loop ((stream stream)
               (result '()))
      (match (parser stream)
        ((? parse-failure?)
         (if (null? result)
             %parse-failure
             (parse-result (reverse result) stream)))
        (($ <parse-result> value stream)
         (loop stream (cons value result)))))))

(define* (parse-maybe parser #:optional (default #f))
  (lambda (stream)
    (match (parser stream)
      ((? parse-failure?)
       (parse-result default stream))
      (result result))))

(define (string->stream str)
  "Convert the string STR into a stream of characters."
  (stream-map (lambda (i)
                (string-ref str i))
              (stream-range 0 (string-length str))))

(define* (parse parser obj #:optional (fail-value #f))
  "Parse the contents of OBJ with PARSER.  OBJ may be either a string, port,
or stream."
  (let ((stream (match obj
                  ;; Handle strings and ports as a convenience.
                  ((? string? str) (string->stream str))
                  ((? port? port) (port->stream port))
                  ((? stream? stream) stream))))
    (match (parser stream)
      ((or (? parse-failure?)
           (not (? parse-done?)))
       fail-value)
      (($ <parse-result> value _) value))))