4.3.2.1 Errors and warnings during compilation

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).