Line Filters in Prolog

Here’s the translation of the Go line filter program to Prolog, along with explanations in Markdown format suitable for Hugo:

A line filter is a common type of program that reads input on stdin, processes it, and then prints some derived result to stdout. grep and sed are common line filters.

Here’s an example line filter in Prolog that writes a capitalized version of all input text. You can use this pattern to write your own Prolog line filters.

:- use_module(library(readutil)).
:- use_module(library(strings)).

main :-
    % Read input lines
    read_line_to_string(user_input, Line),
    process_line(Line).

% Base case: end of file
process_line(end_of_file) :- !.

% Process each line
process_line(Line) :-
    % Convert the line to uppercase
    string_upper(Line, UpperLine),
    % Write out the uppercased line
    writeln(UpperLine),
    % Read the next line and continue processing
    read_line_to_string(user_input, NextLine),
    process_line(NextLine).

In this Prolog implementation:

1. We use the readutil library for reading input lines and the strings library for string manipulation.

2. The main predicate starts the process by reading the first line of input.

3. process_line/1 is a recursive predicate that handles each line:

   • If it encounters end_of_file, it stops.
   • Otherwise, it converts the line to uppercase using string_upper/2, prints it, reads the next line, and calls itself recursively.

4. We use writeln/1 to print each uppercased line.

To try out our line filter, first make a file with a few lowercase lines.

$ echo 'hello'   > /tmp/lines
$ echo 'filter' >> /tmp/lines

Then use the line filter to get uppercase lines. Assuming you’ve saved the Prolog code in a file named line_filter.pl:

$ cat /tmp/lines | swipl -q -s line_filter.pl -t main
HELLO
FILTER

This example demonstrates how to create a simple line filter in Prolog that reads input, processes it line by line, and outputs the result. The pattern can be adapted for various text processing tasks.