Printing messages

The predicate print_message2 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_message2 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.

Another common use is to defined message_hook3 for printing messages that are normally silent, suppressing messages, redirecting messages or make something happen in addition to printing the message.

print_message2+Kind, +Term The predicate print_message2 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_lines3 for details). Next it will call the hook message_hook3 to allow the user intercepting the message. If message_hook3 fails it will print the message unless Kind is silent.

The print_message2 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_string2.

print_message_lines3+Stream, +Prefix, +Lines Print a message (see print_message2) 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 format3. flush If this appears as the last element, Stream is flushed (see flush_output1) 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 format1, ~N). Format Handed to format3 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_message2 and message_hook3.

message_hook3+Term, +Kind, +Lines Hook predicate that may be define in the module user to intercept messages from print_message2. Term and Kind are the same as passed to print_message2. Lines is a list of format statements as described with print_message_lines3. See also message_to_string2.

This predicate should be defined dynamic and multifile to allow other modules defining clauses for it too.

message_to_string2+Term, -String Translates a message-term into a string object (see string. Primarily intended to write messages to Windows in XPCE (see xpce) or other GUI environments.