[ ⊨ ] Logtalk

Meta-messages for lazy coders

  • Mar 20, 2020

One of the frequent excuses for coders to avoid using the message printing mechanism is that they require defining tokenization rules for the messages and that is too much work when they can just call write/1 instead. We can try to argue about laziness but the coders do have a point. To easy the pain of using the message printing mechanism, Logtalk defines a set of meta-messages that can be used without defining any message_tokens//2 tokenization rules.

The first example is when you want to print a message when execution is at a given point in the code. The @/1 meta-message allows you to write:

| ?- logtalk::print_message(comment, core, @'Phase 1 completed').
% Phase 1 completed
yes

Another common example is printing a key-value pair:

| ?- logtalk::print_message(comment, core, answer-42).
% answer: 42
yes

But sometimes we need to write a complex term using a specific format. In that case, we can use the Format+Arguments meta-message. For example:

| ?- logtalk::print_message(comment, core, 'Position: <~d,~d>'+[42,23]).
% Position: <42,23>
yes

Also common is printing a list of items, with or without a title:

| ?- logtalk::print_message(comment, core, [arthur,ford,marvin]).
% - arthur
% - ford
% - marvin
yes

| ?- logtalk::print_message(comment, core, names::[arthur,ford,marvin]).
% names:
% - arthur
% - ford
% - marvin
yes

Worth mentioning that a message term is only interpreted as a meta-message when the user doesn’t define a message_tokens//2 tokenization rule for it. Thus, no conflicts if the user decides to use a message term that would be otherwise interpreted by default as a meta-message.

A second complain is that the goals are verbose. Sure, we can use implicit message sending to the built-in logtalk object using the directive to shorten the goals a bit. For example:

:- uses(logtalk, [
    print_message/3
]).

enterprise :-
    print_message(comment, core, @('Entering the Neutral Zone...')),
    ...,
    print_message(comment, core, @('Looking for Romulans...')),
    ....

But we still need to write the message kind and component arguments. Not conceding defeat, predicate aliases to the rescue:

:- uses(logtalk, [
    print_message(comment, core, @Message) as log(Message)
]).

enterprise :-
    log('Entering the Neutral Zone...'),
    ...,
    log('Looking for Romulans...'),
    ....

while still benefiting from all the advantages of using the message printing mechanism (see e.g. Abstracting user interaction).

Do you happen to have some existing code using e.g. format/2 for writing messages that you want to port to the message printing mechanism? Note that you can write:

:- uses(logtalk, [
    print_message(comment, core, Format+Arguments) as format(Format, Arguments)
]).

enterprise(Zone, Enemy) :-
    format('Entering the ~w...', [Zone]),
    ...,
    format('Looking for ~w...', [Enemy]),
    ....

In this case, all format/2 goals will be compiled as logtalk::print_message/3 goals. You could use similar aliases for other output predicates, simplifying the port by minimizing the changes required.

P.S. The Format+Arguments meta-message exemplified above requires Logtalk 3.37.0 or a later version.

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