11.3.162 print_message/2   hookable

Synopsis

print_message(+Severity, +MessageTerm)

Print a Message of a given Severity. The behavior can be customized using the hooks user:portray_message/2, user:generate_message_hook/3 and user:message_hook/3.

Arguments

Severity

atom, must be nonvar

Unless the default system portrayal is overridden with user:message_hook/3, Severity must be one of:

Value

Prefix

informational

‘% ’

warning

‘* ’

error

‘! ’

help

query

silent

no prefix

MessageTerm

term

Description

First print_message/2 calls user:portray_message/2 with the same arguments. If this does not succeed, then the message is processed in the following phases:

• Message generation phase: the abstract message term Message is formatted, i.e. converted to a format-command list. First the hook predicate user:generate_message_hook/3 is tried, then if it does not succeed, then 'SU_messages':generate_message/3 is called. The latter predicate is defined in terms of definite clause grammars in library('SU_messages'). If that also does not succeed, then the built-in default conversion is used, which gives the following result:

  ['~q'-[Message],nl]
  

• Line splitting transformation: the format-command list is converted to format-command lines—the list is broken up into a list of lists, each list containing format-commands for one line.
• Message printing phase: The text of the message (format-command lines generated in the previous stage) is printed. First the hook predicate user:message_hook/3 is tried, then, if it does not succeed, then the built-in predicate print_message_lines/3 is called for the user_error stream.

An unhandled exception message E calls print_message(error, E) before returning to the top level. The convention is that an error message is the result of an unhandled exception. Thus, an error message should only be printed if raise_exception/1 does not find a handler and unwinds to the top level.

All messages from the system are printed using this predicate. Means of intercepting these messages before they are printed are provided.

print_message/2 always prints to user_error. Messages can be redirected to other streams using user:message_hook/3 and print_message_lines/3

Silent messages do not get translated or printed, but they can be intercepted with user:portray_message/2 and user:message_hook/3.

Exceptions

instantiation_error

type_error

domain_error

in Severity

See Also

ref-msg.