Printing messages and asking questions¶
Applications, components, and libraries often print all sorts of messages. These include banners, logging, debugging, and computation results messages but also, in some cases, user interaction messages. However, the authors of applications, components, and libraries often cannot anticipate the context where their software will be used and thus decide which and when messages should be displayed, suppressed, or diverted. Consider the different components in a Logtalk application development and deployment. At the base level, you have the Logtalk compiler and runtime. The compiler writes messages related to e.g. compiling and loading files, compiling entities, compilation warnings and errors. The runtime may write banner messages or throw execution errors that may result in printing human-level messages. The development environment can be console-based or you may be using a GUI tool such as PDT. In the latter case, PDT needs to intercept the Logtalk compiler and runtime messages to present the relevant information using its GUI. Then you have all the other components in a typical application. For example, your own libraries and third-party libraries. The libraries may want to print messages on its own, e.g. banners, debugging information, or logging information. As you assemble all your application components, you want to have the final word on which messages are printed, where, and when. Uncontrolled message printing by libraries could potentially disturb application flow, expose implementation details, spam the user with irrelevant details, or break user interfaces.
The solution is to decouple the calls to print a message from the actual printing of the output text. The same is true for calls to read user input. By decoupling the call to input some data from the actual read of the data, we can easily switched e.g. from a command-line interface to a GUI input dialog or even automate providing the data (e.g. when automating testing of user interaction).
Logtalk provides a solution based on the structured message printing mechanism that was introduced by Quintus Prolog, where it was apparently implemented by Dave Bowen (thanks to Richard O’Keefe for the historical bits). This mechanism gives the programmer full control of message printing, allowing it to filter, rewrite, or redirect any message. Variations of this mechanism can also be found in some Prolog systems including SICStus Prolog, SWI-Prolog, and YAP. Based on this mechanism, Logtalk introduces an extension that also allows abstracting asking a user for input. Both mechanisms are implemented by the logtalk built-in object and described in this section. The message printing mechanism is extensively used by the Logtalk compiler itself and by the developer tools. The question asking mechanism is used in the debugger tool.
The main predicate for printing a message is logtalk::print_message/3. A simple example, using the Logtalk runtime is:
| ?- logtalk::print_message(banner, core, banner). Logtalk 3.23.0 Copyright (c) 1998-2018 Paulo Moura yes
The first argument of the predicate is the kind of message that we
want to print. In this case, we use
banner to indicate that
we are printing a product name and copyright banner. An extensive
list of message kinds is supported by default:
- banner messages (used e.g. when loading tools or main application
components; can be suppressed by setting the report
- messages printed in reply for the user asking for help (mostly for helping port existing Prolog code)
- messages usually printed in reply to a user request for information
- not printed by default (but can be intercepted using the
- useful but usually not essential messages (can be suppressed by
setting the report flag to
- warning messages (generated e.g. by the compiler; can be suppressed by turning off the report flag)
- error messages (generated e.g. by the compiler)
- debugging messages (by default, only printed when the debug flag is turned on; these messages are suppressed by the compiler when the optimize flag is turned on)
- questions to a user
Using a compound term allows easy partitioning of messages of the same kind in different groups. Note that you can define your own alternative message kind identifiers, for your own components, together with suitable definitions for their associated prefixes and output streams.
The second argument of
print_message/3 represents the component
defining the message being printed. Here component is a generic term that
can designate e.g a tool, a library, or some sub-system in a large application.
In our example, the component name is
core, identifying the Logtalk
compiler/runtime. This argument was introduced to provide multiple namespaces
for message terms and thus simplify programming-in-the-large by allowing easy
filtering of all messages from a specific component and also avoiding conflicts
when two components happen to define the same message term (e.g.
Users should choose and use a unique name for a component, which usually is
the name of the component itself. For example, all messages from the
lgtunit tool use
lgtunit for the component argument. The compiler
and runtime are interpreted as a single component designated as
The third argument of
print_message/3 is the message itself, represented
by a term. In the above example, the message term is
banner. Using a
term to represent a message instead of a string with the message text itself
have significant advantages. Notably, it allows using a compound term for
easy parameterization of the message text and simplifies machine-processing,
localization of applications, and message interception. For example:
| ?- logtalk::print_message(comment, core, redefining_entity(object, foo)). % Redefining object foo yes
The advantages of using message terms require a solution for generating the actual messages text. This is supported by defining grammar rules for the logtalk::message_tokens//2 multifile non-terminal, which translates a message term, for a given component, to a list of tokens. For example:
:- multifile(logtalk::message_tokens//2). :- dynamic(logtalk::message_tokens//2). logtalk::message_tokens(redefining_entity(Type, Entity), core) --> ['Redefining ~w ~q'-[Type, Entity], nl].
The following tokens can be used when translating a message:
- Signals a following part to a multi-part message with no line break in between; this token is ignored when it’s not the first in the list of tokens
- Flush the output stream (by calling the
- Change line in the output stream
Formatmust be an atom and
Argumentsmust be a list of format arguments (the token arguments are passed to a call to the
format/3de facto standard predicate)
Termcan be any term and
Optionsmust be a list of valid
write_term/3output options (the token arguments are passed to a call to the
ansi(Attributes, Format, Arguments)
- Taken from SWI-Prolog; by default, do nothing; can be used for styled output
- Taken from SWI-Prolog; by default, do nothing; can be used together
end(Var)to wrap a sequence of message tokens
- Taken from SWI-Prolog; by default, do nothing
logtalk object also defines public predicates for printing a list
of tokens, for hooking into printing an individual token, and for setting
default output stream and message prefixes. For example, the SWI-Prolog
adapter file uses the print message token hook predicate to enable coloring
of messages printed on a console.
Define tokenization rules for every message is not always necessary, however. Logtalk defines several meta-messages that are handy for simple cases and temporary messages only used to help developing, notably debugging messages. See the Debugging messages section and the logtalk built-in object remarks section for details.
Calls to the logtalk::print_message/3 predicate can be intercepted by defining clauses for the logtalk::message_hook/4 multifile hook predicate. This predicate can suppress, rewrite, and divert messages.
As a first example, assume that you want to make Logtalk startup less verbose by suppressing printing of the default compiler flag values. This can be easily accomplished by defining the following category in a settings file:
:- category(my_terse_logtalk_startup_settings). :- multifile(logtalk::message_hook/4). :- dynamic(logtalk::message_hook/4). logtalk::message_hook(default_flags, comment(settings), core, _). :- end_category.
The printing message mechanism automatically calls the
hook predicate. When this call succeeds, the mechanism assumes that the
message have been successfully handled.
As another example, assume that you want to print all otherwise silent compiler messages:
:- category(my_verbose_logtalk_message_settings). :- multifile(logtalk::message_hook/4). :- dynamic(logtalk::message_hook/4). logtalk::message_hook(_Message, silent, core, Tokens) :- logtalk::message_prefix_stream(comment, core, Prefix, Stream), logtalk::print_message_tokens(Stream, Prefix, Tokens). logtalk::message_hook(_Message, silent(Key), core, Tokens) :- logtalk::message_prefix_stream(comment(Key), core, Prefix, Stream), logtalk::print_message_tokens(Stream, Prefix, Tokens). :- end_category.
This example calls the logtalk::message_prefix_stream/4 hook predicate, which can be used to define a message line prefix and an output stream for printing messages for a given component.
Logtalk structured question asking mechanism complements the message printing mechanism. It provides an abstraction for the common task of asking a user a question and reading back its reply. By default, this mechanism writes the question, writes a prompt, and reads the answer using the current user input and output streams but allows all steps to be intercepted, filtered, rewritten, and redirected. Two typical examples are using a GUI dialog for asking questions and automatically providing answers to specific questions.
The question asking mechanism works in tandem with the message printing
mechanism, using it to print the question text and a prompt. It provides
an asking predicate and a hook predicate, both declared and defined in
logtalk built-in object. The asking predicate,
is used for ask a question and read the answer. Assume that we defined
the following message tokenization and question prompt and stream:
:- category(hitchhikers_guide_to_the_galaxy). :- multifile(logtalk::message_tokens//2). :- dynamic(logtalk::message_tokens//2). % abstract the question text using the atom ultimate_question % the second argument, hitchhikers, is the application component logtalk::message_tokens(ultimate_question, hitchhikers) --> ['The answer to the ultimate question of life, the universe and everything is?'-, nl]. :- multifile(logtalk::question_prompt_stream/4). :- dynamic(logtalk::question_prompt_stream/4). % the prompt is specified here instead of being part of the question text % as it will be repeated if the answer doesn't satisfy the question closure logtalk::question_prompt_stream(question, hitchhikers, '> ', user_input). :- end_category.
After compiling and loading this category, we can now ask the ultimate question:
| ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N). The answer to the ultimate question of life, the universe and everything is? > 42. N = 42 yes
Note that the fourth argument,
'=='(42) in our example, is a closure
that is used to check the answers provided by the user. The question is repeated
until the goal constructed by extending the closure with the user answer
succeeds. For example:
| ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N). The answer to the ultimate question of life, the universe and everything is? > icecream. > tea. > 42. N = 42 yes
Practical usage examples of this mechanism can be found e.g. in the
debugger tool where it’s used to abstract the user interaction when
tracing a goal execution in debug mode.
Calls to the logtalk::ask_question/5 predicate can be intercepted by defining clauses for the logtalk::question_hook/6 multifile hook predicate. This predicate can suppress, rewrite, and divert questions. For example, assume that we want to automate testing and thus cannot rely on someone manually providing answers:
:- category(hitchhikers_fixed_answers). :- multifile(logtalk::question_hook/6). :- dynamic(logtalk::question_hook/6). logtalk::question_hook(ultimate_question, question, hitchhikers, _, _, 42). :- end_category.
After compiling and loading this category, trying the question again will now skip asking the user:
| ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(42), N). N = 42 yes
In a practical case, the fixed answer would be used for followup goals
being tested. The question answer read loop (which calls the question
check closure) is not used when a fixed answer is provided using the
logtalk::question_hook/6 predicate thus preventing the creation
of endless loops. For example, the following query succeeds:
| ?- logtalk::ask_question(question, hitchhikers, ultimate_question, '=='(41), N). N = 42 yes
Note that the
logtalk::question_hook/6 predicate takes as argument
the closure specified in the
logtalk::ask_question/5 call, allowing
a fixed answer to be checked before being returned.