This library provides pure list-based I/O processing for Prolog,
where the communication to the actual I/O device is performed
transparently through coroutining. This module itself is just an
interface to the actual implementation modules.
- To be done
- Provide support for alternative input readers, e.g. reading terms,
tokens, etc.
This module is part of pio.pl, dealing with pure input:
processing input streams from the outside world using pure predicates,
notably grammar rules (DCG). Using pure predicates makes
non-deterministic processing of input much simpler.
Pure input uses attributed variables to read input from the external
source into a list on demand. The overhead of lazy reading is
more than compensated for by using block reads based on read_pending_codes/3.
Ulrich Neumerkel came up with the idea to use coroutining for
creating a
lazy list. His implementation repositioned the file to deal with
re-reading that can be necessary on backtracking. The current
implementation uses destructive assignment together with more low-level
attribute handling to realise pure input on any (buffered) stream.
- [nondet]phrase_from_file(:Grammar,
+File)
- Process the content of File using the DCG rule Grammar.
The space usage of this mechanism depends on the length of the not
committed part of Grammar. Committed parts of the temporary
list are reclaimed by the garbage collector, while the list is extended
on demand due to unification of the attributed tail variable. Below is
an example that counts the number of times a string appears in a file.
The library dcg/basics provides
string//1 matching an
arbitrary string and remainder//1
which matches the remainder of the input without parsing.
:- use_module(library(dcg/basics)).
file_contains(File, Pattern) :-
phrase_from_file(match(Pattern), File).
match(Pattern) -->
string(_),
string(Pattern),
remainder(_).
match_count(File, Pattern, Count) :-
aggregate_all(count, file_contains(File, Pattern), Count).
This can be called as (note that the pattern must be a string (code
list)):
?- match_count('pure_input.pl', `file`, Count).
- [nondet]phrase_from_file(:Grammar,
+File, +Options)
- As phrase_from_file/2,
providing additional Options. Options are passed
to open/4.
- phrase_from_stream(:Grammar,
+Stream)
- Run Grammer against the character codes on Stream. Stream
must be buffered.
- syntax_error(+Error)
// - Throw the syntax error Error at the current location of the
input. This predicate is designed to be called from the handler of phrase_from_file/3.
- throws
error(syntax_error(Error), Location)
- [det]lazy_list_location(-Location)
// - Determine current (error) location in a lazy list. True when
Location is an (error) location term that represents the
current location in the DCG list.
| Location | is a term file(Name, Line, LinePos, CharNo)
or
stream(Stream, Line, LinePos, CharNo) if no file is
associated to the stream RestLazyList. Finally, if the Lazy list is
fully materialized (ends in []), Location is
unified with end_of_file-CharCount. |
- See also
- lazy_list_character_count//1
only provides the character count.
- lazy_list_character_count(-CharCount)
// - True when CharCount is the current character count in the
Lazy list. The character count is computed by finding the distance to
the next frozen tail of the lazy list. CharCount is one of:
- An integer
- A term end_of_file-Count
- See also
- lazy_list_location//1
provides full details of the location for error reporting.
- [det]stream_to_lazy_list(+Stream,
-List)
- Create a lazy list representing the character codes in Stream.
List is a partial list ending in an attributed variable.
Unifying this variable reads the next block of data. The block is stored
with the attribute value such that there is no need to re-read it.
- Compatibility
- Unlike the previous version of this predicate this version does not
require a repositionable stream. It does require a buffer size of at
least the maximum number of bytes of a multi-byte sequence (6).