SWI-Prolog defines the predicates catch/3 and throw/1 for ISO compliant raising and catching of exceptions. In the current implementation (4.0.6), most of the built-in predicates generate exceptions, but some obscure predicates merely print a message, start the debugger and fail, which was the normal behaviour before the introduction of exceptions.
The overhead of calling a goal through catch/3 is very comparable to call/1. Recovery from an exception is much slower, especially if the exception-term is large due to the copying thereof.
ISO demands throw/1 to make a copy of Exception, walk up the stack to a catch/3 call, backtrack and try to unify the copy of Exception with Catcher. SWI-Prolog delays making a copy of Exception and backtracking until it actually found a matching catch/3 goal. The advantage is that we can start the debugger at the first possible location while preserving the entire exception context if there is no matching catch/3 goal. This approach can lead to different behaviour if Goal and Catcher of catch/3 call share variables. We assume this to be highly unlikely and could not think of a scenario where this is useful.28I'd like to acknowledge Bart Demoen for his clarifications on these matters.
If an exception is raised in a callback from C (see chapter 9) and not caught in the same call-back, PL_next_solution() fails and the exception context can be retrieved using PL_exception().
Before
the introduction of exceptions in SWI-Prolog a runtime error was handled
by printing an error message, after which the predicate failed. If the
prolog_flag (see current_prolog_flag/2) debug_on_error
was in effect (default), the tracer was switched on. The combination of
the error message and trace information is generally sufficient to
locate the error.
With exception handling, things are different. A programmer may wish to trap an exception using catch/3 to avoid it reaching the user. If the exception is not handled by user-code, the interactive top-level will trap it to prevent termination.
If we do not take special precautions, the context information associated with an unexpected exception (i.e., a programming error) is lost. Therefore, if an exception is raised, which is not caught using catch/3 and the top-level is running, the error will be printed, and the system will enter trace mode.
If the system is in an non-interactive callback from foreign code and there is no catch/3 active in the current context, it cannot determine whether or not the exception will be caught by the external routine calling Prolog. It will then base its behaviour on the prolog_flag debug_on_error:
While looking for the context in which an exception takes place, it
is advised to switch on debug mode using the predicate debug/0.
The hook
prolog_exception_hook/4
can be used to add more debugging facilities to exceptions. An example
is the library library(http/http_error), generating a full
stack trace on errors in the HTTP server library.
Built-in predicates generates exceptions using a term
error(Formal, Context). The first argument is the `formal'
description of the error, specifying the class and generic defined
context information. When applicable, the ISO error-term definition is
used. The second part describes some additional context to help the
programmer while debugging. In its most generic form this is a term of
the form context(Name/Arity, Message), where
Name/Arity describes the built-in predicate that
raised the error, and Message provides an additional
description of the error. Any part of this structure may be a variable
if no information was present.
The predicate print_message/2 may be used to print a message term in a human readable format. The other predicates from this section allow the user to refine and extend the message system. The most common usage of print_message/2 is to print error messages from exceptions. The code below prints errors encountered during the execution of Goal, without further propagating the exception and without starting the debugger.
...,
catch(Goal, E,
( print_message(error, E),
fail
)),
...
Another common use is to defined message_hook/3 for printing messages that are normally silent, suppressing messages, redirecting messages or make something happen in addition to printing the message.
informational, banner, warning, error,
help or silent. A human-readable message is
printed to the stream user_error.
If the
prolog flag (see current_prolog_flag/2) verbose
is
silent, messages with Kind informational,
or banner are treated as silent. See -q.
This predicate first translates the Term into a list of `message lines' (see print_message_lines/3 for details). Next it will call the hook message_hook/3 to allow the user intercepting the message. If message_hook/3 fails it will print the message unless Kind is silent.
The print_message/2
predicate and its rules are in the file
<plhome>/boot/messages.pl, which may be
inspected for more information on the error messages and related error
terms. If you need to report errors from your own predicates, we advise
you to stick to the existing error terms if you can; but should you need
to invent new ones, you can define corresponding error messages by
asserting clauses for prolog:message. You will need to declare the
predicate as multifile.
See also message_to_string/2.
flushat_same_line~N).
format(Stream, Format, []).
See also print_message/2 and message_hook/3.
user to
intercept messages from print_message/2. Term
and Kind are the same as passed to print_message/2. Lines
is a list of format statements as described with print_message_lines/3.
See also
message_to_string/2.
This predicate should be defined dynamic and multifile to allow other modules defining clauses for it too.