Reading Files in Prolog

Here’s the translated code and explanation in Prolog, formatted for Hugo:

Our first program will read files and demonstrate various file operations. Here’s the full source code and explanation:

:- use_module(library(readutil)).

% Helper predicate to check for errors
check_error(Error) :-
    (   Error \= []
    ->  throw(Error)
    ;   true
    ).

main :-
    % Read entire file contents
    read_file_to_string("/tmp/dat", Content, []),
    write(Content),

    % Open file for reading
    open("/tmp/dat", read, Stream),

    % Read some bytes from the beginning of the file
    read_string(Stream, 5, B1),
    string_length(B1, N1),
    format("~d bytes: ~s~n", [N1, B1]),

    % Seek to a known location and read from there
    seek(Stream, 6, bof, O2),
    read_string(Stream, 2, B2),
    string_length(B2, N2),
    format("~d bytes @ ~d: ~s~n", [N2, O2, B2]),

    % Seek relative to current position
    seek(Stream, 4, current, _),

    % Seek relative to end of file
    seek(Stream, -10, eof, _),

    % Use read_string for more robust reading
    seek(Stream, 6, bof, O3),
    read_string(Stream, 2, B3),
    string_length(B3, N3),
    format("~d bytes @ ~d: ~s~n", [N3, O3, B3]),

    % Rewind to the beginning of the file
    seek(Stream, 0, bof, _),

    % Read first 5 bytes
    read_string(Stream, 5, B4),
    format("5 bytes: ~s~n", [B4]),

    % Close the file
    close(Stream).

This program demonstrates various file reading operations in Prolog:

We start by reading the entire contents of a file using read_file_to_string/3.
Then we open the file for reading with open/3.
We read a specific number of bytes from the beginning of the file using read_string/3.
We use seek/4 to move to different positions in the file and read from there.
We demonstrate seeking relative to the current position and the end of the file.
We show how to rewind to the beginning of the file using seek/4.
Finally, we close the file with close/1.

To run the program, save it as reading_files.pl and use the following commands:

$ echo "hello" > /tmp/dat
$ echo "prolog" >> /tmp/dat
$ swipl -q -t main -s reading_files.pl
hello
prolog
5 bytes: hello
2 bytes @ 6: pr
2 bytes @ 6: pr
5 bytes: hello

This example demonstrates basic file operations in Prolog. The language provides predicates for file handling, but the approach is different from imperative languages. Prolog’s declarative nature means we focus on describing what we want to do rather than how to do it step-by-step.

Next, we’ll look at writing files in Prolog.

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Reading Files in Prolog