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generate_message/3
hook:- multifile 'SU_messages':generate_message/3.
'SU_messages':generate_message(+MessageTerm, -S0, -S)
For a given MessageTerm, generates a list composed of
Control-Arg pairs and the atom nl
. This can be translated into a
nested list of Control-Arg pairs, which can be used as input to
print_message_lines/3
.
term
May be any term.
list of pair
The resulting list of Control-Args pairs.
list of pair
The remaining list.
Clauses for 'SU_messages':generate_message/3
underlie all messages from Prolog. They may
be examined and altered. They are found in library('SU_messages')
.
The purpose of this predicate is to allow you to redefine how Prolog’s messages are displayed. For example, to translate all the messages from English into some other language.
This predicate should not be modified if all you want to do is modify or
add a few messages: user:generate_message_hook/3
is provided for that purpose.
The Prolog system uses the built-in predicate print_message/2
to print all its messages. When print_message/2
is called, it
calls user:generate_message_hook(Message,L,[])
to generate the
message. If that fails, then
'SU_messages':generate_message(Message,L,[])
is called instead.
If that succeeds, then L is assumed to have been bound to a list whose
elements are either Control-Args pairs or the atom nl
. Each Control-Arg
pair should be such that the call
format(user_error, Control, Args)
is valid. The atom nl
is used for breaking the message into
lines. Using the format specification ‘~n’ (new-line) is
discouraged, since the routine that actually prints the message (see
user:message_hook/3
and print_message_lines/3
) may need
to have control over newlines.
'SU_messages':generate_message/3
is not included by default in runtime systems,
since end-users of application programs should probably not be seeing any
messages from the Prolog system.
If there is a call to print_message/2
when
when 'SU_messages':generate_message/3
does not succeed
for some reason, then the message term itself is printed, for example:
| ?- print_message(error,unexpected_error(37)). ! unexpected_error(37)
'SU_messages':generate_message/3
failed because the message term was not recognized. In
the following example print_message/2
is being called by the default
exception handler:
| ?- writeq(A,B). ! Instantiation error in argument 1 of writeq/2 ! goal: writeq(_2107,_2108)
:- multifile user:generate_message_hook/3. user:generate_message_hook(hello_world) --> ['hello world'-[],nl].
Note that the terminating nl
is required.
print_message/2
checks that the generated list is a valid parse.