Haskell Logging Framework, Primary Interface
Written by John Goerzen, firstname.lastname@example.org
Welcome to the error and information logging system for Haskell.
This system is patterned after Python's logging module,
http://www.python.org/doc/current/lib/module-logging.html and some of
the documentation here was based on documentation there.
To log a message, you perform operations on Loggers. Each Logger has a
name, and they are arranged hierarchically. Periods serve as separators.
Therefore, a Logger named "foo" is the parent of loggers "foo.printing",
"foo.html", and "foo.io". These names can be anything you want. They're
used to indicate the area of an application or library in which a logged
message originates. Later you will see how you can use this concept to
fine-tune logging behaviors based on specific application areas.
You can also tune logging behaviors based upon how important a message is.
Each message you log will have an importance associated with it. The different
importance levels are given by the Priority type. I've also provided
some convenient functions that correspond to these importance levels:
debugM through emergencyM log messages with the specified importance.
Now, an importance level (or Priority)
is associated not just with a particular message but also
with a Logger. If the Priority of a given log message is lower than
the Priority configured in the Logger, that message is ignored. This
way, you can globally control how verbose your logging output is.
Now, let's follow what happens under the hood when you log a message. We'll
assume for the moment that you are logging something with a high enough
Priority that it passes the test in your Logger. In your code, you'll
call logM or something like debugM to log the message. Your Logger
decides to accept the message. What next?
Well, we also have a notion of handlers (LogHandlers, to be precise).
A LogHandler is a thing that takes a message and sends it somewhere.
That "somewhere" may be your screen (via standard error), your system's
logging infrastructure (via syslog), a file, or other things. Each
Logger can have zero or more LogHandlers associated with it. When your
Logger has a message to log, it passes it to every LogHandler it knows
of to process. What's more, it is also passed to /all handlers of all
ancestors of the Logger/, regardless of whether those Loggers would
normally have passed on the message.
To give you one extra little knob to turn, LogHandlers can also have
importance levels (Priority) associated with them in the same way
that Loggers do. They act just like the Priority value in the
Loggers -- as a filter. It's useful, for instance, to make sure that
under no circumstances will a mere DEBUG message show up in your syslog.
There are three built-in handlers given in two built-in modules:
System.Log.Handler.Simple and System.Log.Handler.Syslog.
There is a special logger known as the root logger that sits at the top
of the logger hierarchy. It is always present, and handlers attached
there will be called for every message. You can use getRootLogger to get
it or rootLoggerName to work with it by name.
Here's an example to illustrate some of these concepts:
-- By default, all messages of level WARNING and above are sent to stderr.
-- Everything else is ignored.
-- "MyApp.Component" is an arbitrary string; you can tune
-- logging behavior based on it later.
main = do
debugM "MyApp.Component" "This is a debug message -- never to be seen"
warningM "MyApp.Component2" "Something Bad is about to happen."
-- Copy everything to syslog from here on out.
s <- openlog "SyslogStuff" [PID] USER DEBUG
updateGlobalLogger rootLoggerName (addHandler s)
errorM "MyApp.Component" "This is going to stderr and syslog."
-- Now we'd like to see everything from BuggyComponent
-- at DEBUG or higher go to syslog and stderr.
-- Also, we'd like to still ignore things less than
-- WARNING in other areas.
-- So, we adjust the Logger for MyApp.Component.
-- This message will go to syslog and stderr
debugM "MyApp.BuggyComponent" "This buggy component is buggy"
-- This message will go to syslog and stderr too.
warningM "MyApp.BuggyComponent" "Still Buggy"
-- This message goes nowhere.
debugM "MyApp.WorkingComponent" "Hello"