Errors and warnings reported while compiling a file are reported
using
print_message/2.
Typical errors are syntax errors, errors during macro expansion by term_expansion/2
and goal_expansion/2,
compiler errors such as illegal clauses or an attempt to redefine a
system predicate and errors caused by executing directives,
notably using
initialization/1
and initialization/2.
Merely reporting error messages and warnings is typically desirable
for interactive usage. Non-interactive applications often require to be
notified of such issues, typically using the exit code of the
process. We can distinguish two types of errors and warnings: (1) those
resulting from loading an invalid program and (2) messages that result
from running the program. A typical example is user code that wishes to
try something and in case of an error report this and continue.
...,
E = error(_,_),
catch(do_something, E,
print_message(error, E)),
...
User code may be (and often is) started from directives, while
running user code may involve compilation due to autoloading, loading of
data files, etc. As a result, it is unclear whether an error message
should merely be printed, should result in a non-zero exit status at the
end or should immediately terminate the process.
The default behaviour is defined by the Prolog flags
on_error and on_warning.
It can be fine tuned by defining the hook predicate message_hook/3.
The compiler calls print_message/2
using the level silent and the message below if errors or
warnings where printed during the execution of
load_files/2.
- load_file_errors(File, Errors, Warnings)
- Here, File is the raw file specification handed to load_files/2,
i.e.,
’myfile.pl’ or library(lists), Errors
is the number of errors printed while loading and Warnings is
the number of warnings printed while loading. Note that these counts
include messages from (initialization) directives.
This allows the user to fine tune the behaviour on errors and, for
example, halt the process on a non-zero error count right after loading
the file wth errors using the code below.
:- multifile user:message_hook/3.
user:message_hook(load_file_errors(_File, Errors, _Warnings),
_Level, _Lines) :-
Errors > 0,
halt(1).
