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|debug.pl -- Print debug messages and test assertions|
This library is a replacement for format/3 for printing debug messages.
Messages are assigned a topic. By dynamically enabling or disabling
topics the user can select desired messages. Calls to debug/3 and
assertion/1 are removed when the code is compiled for optimization
unless the Prolog flag
optimise_debug is set to
Using the predicate assertion/1 you can make assumptions about your program explicit, trapping the debugger if the condition does not hold.
Output and actions by these predicates can be configured using hooks to fit your environment. With XPCE, you can use the call below to start a graphical monitoring tool.
debugging(+Topic)may be used to perform more complex debugging tasks. A typical usage skeleton is:
( debugging(mytopic) -> <perform debugging actions> ; true ), ...
The other two calls are intended to examine existing and enabled debugging tokens and are typically not used in user programs.
nodebug(_)removes all topics. Gives a warning if the topic is not defined unless it is used from a directive. The latter allows placing debug topics at the start of a (load-)file without warnings.
For debug/1, Topic can be a term
Topic > Out, where Out is
either a stream or stream-alias or a filename (an atom). This
redirects debug information on this topic to the given output. On
Linux systems redirection can be used to make the message appear,
even if the
user_error stream is redefined using
?- debug(Topic > '/proc/self/fd/2').
A platform independent way to get debug messages in the current
console (for example, a
swipl-win window, or login using
Prolog running an SSH server from the
libssh pack) is to use:
?- stream_property(S, alias(user_error)), debug(Topic > S).
Do not forget to disable the debugging using nodebug/1 before quitting the console if Prolog must remain running.
[search(String)]or a normal option list. Defined options are:
true) or inactive (
user_error, but only prints if Topic is activated through debug/1. Args is a meta-argument to deal with goal for the @-command. Output is first handed to the hook prolog:debug_print_hook/3. If this fails, Format+Args is translated to text using the message-translation (see print_message/2) for the term
debug(Format, Args)and then printed to every matching destination (controlled by debug/1) using print_message_lines/3.
The message is preceded by '% ' and terminated with a newline.
assert()macro. It has no effect if Goal succeeds. If Goal fails or throws an exception, the following steps are taken:
error(assertion_error(Reason, G),_)where Reason is one of
failor the exception raised.
failif Goal simply failed or an exception call otherwise. If this hook fails, the default behaviour is activated. If the hooks throws an exception it will be propagated into the caller of assertion/1.
The following predicates are exported, but not or incorrectly documented.