Command Line Subcommands in Prolog

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

% Some command-line tools have many subcommands, each with its own set of
% flags. For example, 'git add' and 'git commit' are two different subcommands
% of the 'git' tool. In Prolog, we can implement similar functionality using
% predicates and command-line argument parsing.

:- use_module(library(optparse)).

% Define the main predicate
main :-
    current_prolog_flag(argv, Args),
    (   Args = [Subcommand|RestArgs] ->
        handle_subcommand(Subcommand, RestArgs)
    ;   writeln('expected "foo" or "bar" subcommands'),
        halt(1)
    ).

% Handle the 'foo' subcommand
handle_subcommand(foo, Args) :-
    OptsSpec = [
        [opt(enable), type(boolean), default(false), longflags([enable])],
        [opt(name), type(atom), default(''), longflags([name])]
    ],
    opt_parse(OptsSpec, Args, Opts, PositionalArgs),
    writeln('subcommand "foo"'),
    member(enable(Enable), Opts),
    format('  enable: ~w~n', [Enable]),
    member(name(Name), Opts),
    format('  name: ~w~n', [Name]),
    format('  tail: ~w~n', [PositionalArgs]).

% Handle the 'bar' subcommand
handle_subcommand(bar, Args) :-
    OptsSpec = [
        [opt(level), type(integer), default(0), longflags([level])]
    ],
    opt_parse(OptsSpec, Args, Opts, PositionalArgs),
    writeln('subcommand "bar"'),
    member(level(Level), Opts),
    format('  level: ~w~n', [Level]),
    format('  tail: ~w~n', [PositionalArgs]).

% Handle unknown subcommands
handle_subcommand(_, _) :-
    writeln('expected "foo" or "bar" subcommands'),
    halt(1).

This Prolog code implements a command-line tool with subcommands similar to the Go example. Here’s how it works:

We use the optparse library for parsing command-line arguments.
The main predicate is the entry point. It checks if a subcommand is provided and calls the appropriate handler.
For each subcommand (foo and bar), we define a separate predicate to handle its specific flags and behavior.
The foo subcommand supports --enable and --name flags, while the bar subcommand supports a --level flag.
We use opt_parse/4 to parse the command-line arguments for each subcommand.
After parsing, we print the subcommand name, the values of its flags, and any remaining positional arguments.

To run this Prolog program:

$ swipl -q -s command_line_subcommands.pl -g main -- foo --enable --name=joe a1 a2
subcommand "foo"
  enable: true
  name: joe
  tail: [a1,a2]

$ swipl -q -s command_line_subcommands.pl -g main -- bar --level 8 a1
subcommand "bar"
  level: 8
  tail: [a1]

Note that Prolog doesn’t have a built-in compilation process like Go. Instead, you typically run Prolog programs using a Prolog interpreter such as SWI-Prolog.

This implementation demonstrates how to create a command-line tool with subcommands in Prolog, providing similar functionality to the original Go example.

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Command Line Subcommands in Prolog