Documentation

The Sem monad handles computations of arbitrary extensible effects. A value of type Sem 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 Sem 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 Sem via sendM.

A Sem 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 :: Sem (E ': r) a -> Sem 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 Sem '[] a or a Sem '[ 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 => Sem r ()

to be a program with access to

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

methods.

By also adding a

Member (Error Bool) r

constraint on r, we gain access to the

throw :: Bool -> Sem r a
catch :: Sem r a -> (Bool -> Sem r a) -> Sem 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 Sem monad may have an arbitrary number of the same effect.

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

foo :: ( Member (Output Int) r
       , Member (Output Bool) r
       )
    => Sem 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.

Since: 0.1.2.0

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

Makes constraints of functions that use multiple effects shorter by translating single list of effects into multiple Member constraints:

foo :: Members '[ Output Int
                , Output Bool
                , State String
                ] r
    => Sem r ()

translates into:

foo :: ( Member (Output Int) r
       , Member (Output Bool) r
       , Member (State String) r
       )
    => Sem r ()

Since: 0.1.2.0

Lift an effect into a Sem. This is used primarily via makeSem to implement smart constructors.

Lift a monadic action m into Sem.

Run a Sem containing no effects as a pure value.

Lower a Sem containing only a single lifted Monad into that monad.

Introduce an effect into Sem. Analogous to lift in the mtl ecosystem

An effect which allows a regular Monad m into the Sem ecosystem. Monadic actions in m can be lifted into Sem via sendM.

For example, you can use this effect to lift IO actions directly into Sem:

sendM (putStrLn "hello") :: Member (Lift IO) r => Sem r ()

That being said, you lose out on a significant amount of the benefits of Sem by using sendM directly in application code; doing so will tie your application code directly to the underlying monad, and prevent you from interpreting it differently. For best results, only use Lift in your effect interpreters.

Consider using trace and runTraceIO as a substitute for using putStrLn directly.

Like runSem but flipped for better ergonomics sometimes.

Some interpreters need to be able to lower down to the base monad (often IO) in order to function properly --- some good examples of this are runErrorInIO and runResource.

However, these interpreters don't compose particularly nicely; for example, to run runResource, you must write:

runM . runErrorInIO runM

Notice that runM is duplicated in two places here. The situation gets exponentially worse the more intepreters you have that need to run in this pattern.

Instead, .@ performs the composition we'd like. The above can be written as

(runM .@ runErrorInIO)

The parentheses here are important; without them you'll run into operator precedence errors.

Like .@, but for interpreters which change the resulting type --- eg. runErrorInIO.