1/* Part of SWI-Prolog 2 3 Author: Jan Wielemaker 4 E-mail: J.Wielemaker@vu.nl 5 WWW: http://www.swi-prolog.org 6 Copyright (C): 2008-2016, University of Amsterdam 7 VU University Amsterdam 8 All rights reserved. 9 10 Redistribution and use in source and binary forms, with or without 11 modification, are permitted provided that the following conditions 12 are met: 13 14 1. Redistributions of source code must retain the above copyright 15 notice, this list of conditions and the following disclaimer. 16 17 2. Redistributions in binary form must reproduce the above copyright 18 notice, this list of conditions and the following disclaimer in 19 the documentation and/or other materials provided with the 20 distribution. 21 22 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 23 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 24 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 25 FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 26 COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 27 INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 28 BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 29 LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 30 CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31 LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN 32 ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 33 POSSIBILITY OF SUCH DAMAGE. 34*/ 35 36:- module(pure_input, 37 [ phrase_from_file/2, % :Grammar, +File 38 phrase_from_file/3, % :Grammar, +File, +Options 39 phrase_from_stream/2, % :Grammar, +Stream 40 stream_to_lazy_list/2, % :Stream -List 41 42 syntax_error//1, % +ErrorTerm 43 % Low level interface 44 lazy_list_location//1, % -Location 45 lazy_list_character_count//1 % -CharacterCount 46 ]). 47:- use_module(library(error)). 48:- set_prolog_flag(generate_debug_info, false). 49 50/** <module> Pure Input from files and streams 51 52This module is part of pio.pl, dealing with _pure_ _input_: processing 53input streams from the outside world using pure predicates, notably 54grammar rules (DCG). Using pure predicates makes non-deterministic 55processing of input much simpler. 56 57Pure input uses attributed variables to read input from the external 58source into a list _|on demand|_. The overhead of lazy reading is more 59than compensated for by using block reads based on read_pending_codes/3. 60 61Ulrich Neumerkel came up with the idea to use coroutining for creating a 62_lazy list_. His implementation repositioned the file to deal with 63re-reading that can be necessary on backtracking. The current 64implementation uses destructive assignment together with more low-level 65attribute handling to realise pure input on any (buffered) stream. 66 67@tbd Provide support for alternative input readers, e.g. reading 68 terms, tokens, etc. 69*/ 70 71:- predicate_options(phrase_from_file/3, 3, 72 [ pass_to(system:open/4, 4) 73 ]). 74 75%! phrase_from_file(:Grammar, +File) is nondet. 76% 77% Process the content of File using the DCG rule Grammar. The 78% space usage of this mechanism depends on the length of the not 79% committed part of Grammar. Committed parts of the temporary list 80% are reclaimed by the garbage collector, while the list is 81% extended on demand due to unification of the attributed tail 82% variable. Below is an example that counts the number of times a 83% string appears in a file. The library dcg/basics provides 84% string//1 matching an arbitrary string and remainder//1 which 85% matches the remainder of the input without parsing. 86% 87% == 88% :- use_module(library(dcg/basics)). 89% 90% file_contains(File, Pattern) :- 91% phrase_from_file(match(Pattern), File). 92% 93% match(Pattern) --> 94% string(_), 95% string(Pattern), 96% remainder(_). 97% 98% match_count(File, Pattern, Count) :- 99% aggregate_all(count, file_contains(File, Pattern), Count). 100% == 101% 102% This can be called as (note that the pattern must be a string 103% (code list)): 104% 105% == 106% ?- match_count('pure_input.pl', `file`, Count). 107% == 108 109:- meta_predicate 110 phrase_from_file(, ), 111 phrase_from_file(, , ), 112 phrase_from_stream(, ). 113 114phrase_from_file(Grammar, File) :- 115 phrase_from_file(, File, []). 116 117%! phrase_from_file(:Grammar, +File, +Options) is nondet. 118% 119% As phrase_from_file/2, providing additional Options. Options are 120% passed to open/4. 121 122phrase_from_file(Grammar, File, Options) :- 123 setup_call_cleanup( 124 open(File, read, In, Options), 125 phrase_from_stream(, In), 126 close(In)). 127 128%! phrase_from_stream(:Grammer, +Stream) 129% 130% Run Grammer against the character codes on Stream. Stream must 131% be buffered. 132 133phrase_from_stream(Grammar, In) :- 134 stream_to_lazy_list(In, List), 135 phrase(, List). 136 137%! syntax_error(+Error)// 138% 139% Throw the syntax error Error at the current location of the 140% input. This predicate is designed to be called from the handler 141% of phrase_from_file/3. 142% 143% @throws error(syntax_error(Error), Location) 144 145syntax_error(Error) --> 146 lazy_list_location(Location), 147 { throw(error(syntax_error(Error), Location)) 148 }. 149 150%! lazy_list_location(-Location)// is det. 151% 152% Determine current (error) location in a lazy list. True when 153% Location is an (error) location term that represents the current 154% location in the DCG list. 155% 156% @arg Location is a term file(Name, Line, LinePos, CharNo) or 157% stream(Stream, Line, LinePos, CharNo) if no file is 158% associated to the stream RestLazyList. Finally, if the 159% Lazy list is fully materialized (ends in =|[]|=), Location 160% is unified with `end_of_file-CharCount`. 161% @see lazy_list_character_count//1 only provides the character 162% count. 163 164lazy_list_location(Location, Here, Here) :- 165 lazy_list_location(Here, Location). 166 167lazy_list_location(Here, Location) :- 168 '$skip_list'(Skipped, Here, Tail), 169 ( attvar(Tail) 170 -> get_attr(Tail, pure_input, State), 171 State = lazy_input(Stream, PrevPos, Pos, _), 172 Details = [Line, LinePos, CharNo], 173 ( stream_property(Stream, file_name(File)) 174 -> PosParts = [file, File|Details] 175 ; PosParts = [stream, Stream|Details] 176 ), 177 Location =.. PosParts, 178 ( PrevPos == (-) % nothing is read. 179 -> Line = 1, LinePos = 0, CharNo = 0 180 ; stream_position_data(char_count, Pos, EndRecordCharNo), 181 CharNo is EndRecordCharNo - Skipped, 182 set_stream_position(Stream, PrevPos), 183 stream_position_data(char_count, PrevPos, StartRecordCharNo), 184 Skip is CharNo-StartRecordCharNo, 185 forall(between(1, Skip, _), get_code(Stream, _)), 186 stream_property(Stream, position(ErrorPos)), 187 stream_position_data(line_count, ErrorPos, Line), 188 stream_position_data(line_position, ErrorPos, LinePos) 189 ) 190 ; Tail == [] 191 -> Location = end_of_file-Skipped 192 ; type_error(lazy_list, Here) 193 ). 194 195 196%! lazy_list_character_count(-CharCount)// 197% 198% True when CharCount is the current character count in the Lazy 199% list. The character count is computed by finding the distance to 200% the next frozen tail of the lazy list. CharCount is one of: 201% 202% - An integer 203% - A term end_of_file-Count 204% 205% @see lazy_list_location//1 provides full details of the location 206% for error reporting. 207 208lazy_list_character_count(Location, Here, Here) :- 209 lazy_list_character_count(Here, Location). 210 211lazy_list_character_count(Here, CharNo) :- 212 '$skip_list'(Skipped, Here, Tail), 213 ( attvar(Tail) 214 -> get_attr(Tail, pure_input, State), 215 arg(3, State, Pos), 216 stream_position_data(char_count, Pos, EndRecordCharNo), 217 CharNo is EndRecordCharNo - Skipped 218 ; Tail == [] 219 -> CharNo = end_of_file-Skipped 220 ; type_error(lazy_list, Here) 221 ). 222 223 224%! stream_to_lazy_list(+Stream, -List) is det. 225% 226% Create a lazy list representing the character codes in Stream. 227% List is a partial list ending in an attributed variable. 228% Unifying this variable reads the next block of data. The block 229% is stored with the attribute value such that there is no need to 230% re-read it. 231% 232% @compat Unlike the previous version of this predicate this 233% version does not require a repositionable stream. It 234% does require a buffer size of at least the maximum 235% number of bytes of a multi-byte sequence (6). 236 237stream_to_lazy_list(Stream, List) :- 238 ( stream_property(Stream, buffer(false)) 239 -> permission_error(create, lazy_list, Stream) 240 ; true 241 ), 242 stream_to_lazy_list(Stream, -, List). 243 244stream_to_lazy_list(Stream, PrevPos, List) :- 245 stream_property(Stream, position(Pos)), 246 put_attr(List, pure_input, lazy_input(Stream, PrevPos, Pos, _)). 247 248attr_unify_hook(State, Value) :- 249 State = lazy_input(Stream, _PrevPos, Pos, Read), 250 ( var(Read) 251 -> fill_buffer(Stream), 252 read_pending_codes(Stream, NewList, Tail), 253 ( Tail == [] 254 -> nb_setarg(4, State, []), 255 Value = [] 256 ; stream_to_lazy_list(Stream, Pos, Tail), 257 nb_linkarg(4, State, NewList), 258 Value = NewList 259 ) 260 ; Value = Read 261 ).