Generalize filterLogger?

[mgsloan]

Currently, filterLogger :: (LogSource -> LogLevel -> Bool) -> LoggingT m a -> LoggingT m a. I'd like filterLogger :: MonadLogger m => (LogSource -> LogLevel -> Bool) -> m a -> m a.

Unfortunately, I can't see a way to do this without a breaking API change or a new typeclass.

Why do I want this? I have a function that's polymorphic on MonadLogger m, which invokes a library which has lots of log output. I'd like to silence some invocations of this library, but not others. I can do one of the following things:

• Make LoggingT explicit in the type, losing polymorphism, and causing me to plumb this decision all the way through all the callers.
• Create a new typeclass specifically for modifying the logger function.

The new typeclass could look something like this:

class LoggerLens m where
    loggerLens :: Lens' (m a) ((Loc -> LogSource -> LogLevel -> msg -> m ()) -> m a)

type Lens' s a = forall f. Functor f => (a -> f a) -> s -> f s

This would then allow polymorphic implementations of filterLogger and withChannelLogger. NoLoggingT would get an improper lens, but I think that's ok.

Personally, I'd prefer that this capability always be available to MonadLogger instances, but I realize the API breakage would be huge.

Thoughts? Maybe something like this can make it into the next major-major release?

[snoyberg]

If we're going for majorly breaking changes, I think we have better options at our disposal, specifically to model this as a MonadReader, together with a local method. The downside is that we either need to explicitly state the base monad (IO) or use a type family/fundep to establish it. Still, that feels much less ad-hoc than using a lens like this.