Documentation

Polymorphic and very general logging action type.

• msg type variables is an input for logger. It can be Text or custom logging messsage with different fields that you want to format in future.
• m type variable is for monadic action inside which logging is happening. It can be either IO or some custom pure monad.

Key design point here is that LogAction is:

• Semigroup
• Monoid
• Contravariant
• Divisible
• Decidable
• Comonad

Operator version of unLogAction. Note that because of the types, something like:

action <& msg1 <& msg2

doesn't make sense. Instead you want:

action <& msg1 >> action <& msg2

In addition, because <& has higher precedence than the other operators in this module, the following:

f >$< action <& msg

is equivalent to:

(f >$< action) <& msg

This combinator is contramap from contravariant functor. It is useful when you have something like

data LogRecord = LR
    { lrName    :: LoggerName
    , lrMessage :: Text
    }

and you need to provide LogAction which consumes LogRecord

logRecordAction :: LogAction m LogRecord

when you only have action that consumes Text

logTextAction :: LogAction m Text

With cmap you can do the following:

logRecordAction :: LogAction m LogRecord
logRecordAction = cmap lrMesssage logTextAction

This action will print only lrMessage from LogRecord. But if you have formatting function like this:

formatLogRecord :: LogRecord -> Text

you can apply it instead of lrMessage to log formatted LogRecord as Text.

Takes predicate and performs given logging action only if predicate returns True on input logging message.