SWI-Prolog -- print

Documentation
- Reference manual
  - Built-in Predicates
    - Printing messages
- Packages

Availability:built-in

print_message(+Kind, +Term)

The predicate print_message/2 is used by the system and libraries to print messages. Kind describes the nature of the message, while Term is a Prolog term that describes the content. Printing messages through this indirection instead of using format/3 to the stream user_error allows displaying the message appropriate to the application (terminal, logfile, graphics), acting on messages based on their content instead of a string (see message_hook/3) and creating language specific versions of the messages. See also section 4.11.1. The following message kinds are known:

banner: The system banner message. Banner messages can be suppressed by setting the Prolog flag verbose to silent.
debug(Topic): Message from library(debug). See debug/3.
error: The message indicates an erroneous situation. This kind is used to print uncaught exceptions of type error(Formal, Context). See section introduction (section 4.11).
help: User requested help message, for example after entering‘h' or‘?' to a prompt.
information: Information that is requested by the user. An example is statistics/0.
informational: Typically messages of events and progress that are considered useful to a developer. Such messages can be suppressed by setting the Prolog flag verbose to silent.
silent: Message that is normally not printed. Applications may define message_hook/3 to act upon such messages.
trace: Messages from the (command line) tracer.
warning: The message indicates something dubious that is not considered fatal. For example, discontiguous predicates (see discontiguous/1).

The predicate print_message/2 first translates the Term into a list of‘message lines' (see print_message_lines/3 for details). Next, it calls the hook message_hook/3 to allow the user to intercept the message. If message_hook/3 fails it prints 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 write messages from your own predicates, it is recommended to reuse the existing message terms if applicable. If no existing message term is applicable, invent a fairly unique term that represents the event and define a rule for the multifile predicate prolog:message//1. See section 4.11.1 for a deeper discussion and examples.