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3.9 ISO compliant Exception handling

SWI-Prolog defines the predicates catch/3 and throw/1 for ISO compliant raising and catching of exceptions. In the current implementation (2.9.0), only part of the built-in predicates generate exceptions. In general, exceptions are implemented for I/O and arithmetic.

catch(:Goal, +Catcher, :Recover)
Behaves as call/1 if no exception is raised when executing Goal. If a exception is raised using throw/1 while Goal executes, and the Goal is the innermost goal for which Catcher unifies with the argument of throw/1, all choicepoints generated by Goal are cut, and Recover is called as in call/1.

The overhead of calling a goal through catch/3 is very comparable to call/1. Recovery from an exception has a similar overhead.

throw(+Exception)
Raise an exception. The system will look for the innermost catch/3 ancestor for which Exception unifies with the Catcher argument of the catch/3 call. See catch/3 for details.

If there is no catch/3 willing to catch the error in the current Prolog context, the toplevel (prolog/0) catches the error and prints a warning message. If an exception was raised in a callback from C (see chapter 5), PL_next_solution() will fail and the exception context can be retrieved using PL_exception().

3.9.1 Debugging and exceptions

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 toplevel 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 toplevel 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 adviced to switch on debug mode using the predicate debug/0.

3.9.2 The exception term

Builtin 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.

3.9.3 Printing messages

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.

print_message(+Kind, +Term)
The predicate print_message/2 is used to print messages, notably from exceptions in a human-readable format. Kind is one of informational, warning, error, help or silent. A human-readable message is printed to the stream user_error.

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.

See also message_to_string/2.

print_message_lines(+Stream, +Prefix, +Lines)
Print a message (see print_message/2) that has been translated to a list of message elements. The elements of this list are:
<Format>-<Args>
Where Format is an atom and Args is a list of format argument. Handed to format/3.
flush
If this appears as the last element, Stream is flushed (see flush_output/1) and no final newline is generated.
at_same_line
If this appears as first element, no prefix is printed for the first line and the line-position is not forced to 0 (see format/1, ~N).
<Format>
Handed to format/3 as format(Stream, Format, []).
nl
A new line is started and if the message is not complete the Prefix is printed too.

See also print_message/2 and message_hook/3.

message_hook(+Term, +Kind, +Lines)
Hook predicate that may be define in the module 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.

message_to_string(+Term, -String)
Translates a message-term into a string object (see section ??. Primarily intended to write messages to Windows in XPCE (see section 1.5) or other GUI environments.