SWI-Prolog 7.6.4 Reference Manual: Section A.24

## A.24 library(pio): Pure I/O

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.

### A.24.1 library(pure_input): Pure Input from files and streams

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/3 matching an arbitrary string and remainder/3 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/3 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/3 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).