Command Line Flags in Prolog

Command-line flags are a common way to specify options for command-line programs. For example, in wc -l the -l is a command-line flag.

Prolog doesn’t have built-in support for command-line flag parsing like Go’s flag package. However, we can implement a simple version using Prolog’s argument handling and list processing capabilities.

:- use_module(library(optparse)).

main :-
    opt_arguments(
        [
            [opt(word), type(atom), default(foo), help('a string')],
            [opt(numb), type(integer), default(42), help('an int')],
            [opt(fork), type(boolean), default(false), help('a bool')],
            [opt(svar), type(atom), default(bar), help('a string var')]
        ],
        Opts,
        PositionalArgs
    ),
    % Extract values from the parsed options
    member(word(Word), Opts),
    member(numb(Numb), Opts),
    member(fork(Fork), Opts),
    member(svar(Svar), Opts),
    
    % Print the parsed options and positional arguments
    format('word: ~w~n', [Word]),
    format('numb: ~w~n', [Numb]),
    format('fork: ~w~n', [Fork]),
    format('svar: ~w~n', [Svar]),
    format('tail: ~w~n', [PositionalArgs]).

In this Prolog implementation:

1. We use the optparse library, which provides functionality similar to Go’s flag package.

2. We define our command-line options using opt_arguments/3. Each option is specified with its name, type, default value, and help text.

3. After parsing, we extract the values of the options using the member/2 predicate.

4. Finally, we print out the parsed options and any trailing positional arguments.

To run this Prolog program:

$ swipl -q -s command_line_flags.pl -g main -- -word=opt -numb=7 -fork -svar=flag
word: opt
numb: 7
fork: true
svar: flag
tail: []

You can omit flags, and they will take their default values:

$ swipl -q -s command_line_flags.pl -g main -- -word=opt
word: opt
numb: 42
fork: false
svar: bar
tail: []

Trailing positional arguments can be provided after any flags:

$ swipl -q -s command_line_flags.pl -g main -- -word=opt a1 a2 a3
word: opt
numb: 42
fork: false
svar: bar
tail: [a1,a2,a3]

To get help text for the command-line program, you can add a custom help option or modify the optparse library to provide automatic help text generation.

Note that Prolog’s approach to command-line argument parsing is different from Go’s, but this implementation provides similar functionality. The optparse library handles much of the complexity that Go’s flag package manages internally.