Severity of message. Based on monad-logger.

A basic LogEntry with a severity and a (Text) message

The Logger effect

Prefix Effect

Add one log entry of arbitrary type. If you want to log with another type besides @LogEntry.

Add one log-entry of the LogEntry type.

Add a prefix for the block of code.

Run the Logger and PrefixLog effects using the preferred handler and filter output in any Polysemy monad with IO in the union.

LogSeverity list used in order to output everything.

LogSeverity list used to output all but Diagnostic. Diagnostic messages are sometimes useful for debugging but can get noisy depending on how you use it.

Constraint helper for logging with prefixes

Constraint helper for LogEntry type with prefixes

The Semantic monad handles computations of arbitrary extensible effects. A value of type Semantic r describes a program with the capabilities of r. For best results, r should always be kept polymorphic, but you can add capabilities via the Member constraint.

The value of the Semantic monad is that it allows you to write programs against a set of effects without a predefined meaning, and provide that meaning later. For example, unlike with mtl, you can decide to interpret an Error effect tradtionally as an Either, or instead significantly faster as an IO Exception. These interpretations (and others that you might add) may be used interchangably without needing to write any newtypes or Monad instances. The only change needed to swap interpretations is to change a call from runError to runErrorInIO.

The effect stack r can contain arbitrary other monads inside of it. These monads are lifted into effects via the Lift effect. Monadic values can be lifted into a Semantic via sendM.

A Semantic can be interpreted as a pure value (via run) or as any traditional Monad (via runM). Each effect E comes equipped with some interpreters of the form:

runE :: Semantic (E ': r) a -> Semantic r a

which is responsible for removing the effect E from the effect stack. It is the order in which you call the interpreters that determines the monomorphic representation of the r parameter.

After all of your effects are handled, you'll be left with either a Semantic '[] a or a Semantic '[ Lift m ] a value, which can be consumed respectively by run and runM.

Examples

As an example of keeping r polymorphic, we can consider the type

Member (State String) r => Semantic r ()

to be a program with access to

get :: Semantic r String
put :: String -> Semantic r ()

methods.

By also adding a

Member (Error Bool) r

constraint on r, we gain access to the

throw :: Bool -> Semantic r a
catch :: Semantic r a -> (Bool -> Semantic r a) -> Semantic r a

functions as well.

In this sense, a Member (State s) r constraint is analogous to mtl's MonadState s m and should be thought of as such. However, unlike mtl, a Semantic monad may have an arbitrary number of the same effect.

For example, we can write a Semantic program which can output either Ints or Bools:

foo :: ( Member (Output Int) r
       , Member (Output Bool) r
       )
    => Semantic r ()
foo = do
  output @Int  5
  output True

Notice that we must use -XTypeApplications to specify that we'd like to use the (Output Int) effect.

A proof that the effect e is available somewhere inside of the effect stack r.

Handlers are mechanisms to interpret the meaning of logging as an action in the underlying monad. They are simply functions from log messages to m-actions.