| Portability | non-portable (rank-2 polymorphism) |
|---|---|
| Stability | provisional |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Safe Haskell | Trustworthy |
Control.Monad.Codensity
Description
- newtype Codensity m a = Codensity {
- runCodensity :: forall b. (a -> m b) -> m b
- lowerCodensity :: Monad m => Codensity m a -> m a
- codensityToAdjunction :: Adjunction f g => Codensity g a -> g (f a)
- adjunctionToCodensity :: Adjunction f g => g (f a) -> Codensity g a
- codensityToRan :: Codensity g a -> Ran g g a
- ranToCodensity :: Ran g g a -> Codensity g a
- codensityToComposedRep :: Representable u => Codensity u a -> u (Key u, a)
- composedRepToCodensity :: (Representable u, Functor h) => u (Key u, a) -> Codensity u a
- improve :: Functor f => (forall m. MonadFree f m => m a) -> Free f a
Documentation
Codensity fFunctor f along itself (Ran f f).
This can often be more "efficient" to construct than f itself using
repeated applications of (>>=).
See "Asymptotic Improvement of Computations over Free Monads" by Janis Voightländer for more information about this type.
Constructors
| Codensity | |
Fields
| |
Instances
| MonadTrans Codensity | |
| (Functor f, MonadFree f m) => MonadFree f (Codensity m) | |
| MonadReader r m => MonadState r (Codensity m) | |
| Monad (Codensity f) | |
| Functor (Codensity k) | |
| MonadPlus v => MonadPlus (Codensity v) | |
| Applicative (Codensity f) | |
| Alternative v => Alternative (Codensity v) | |
| MonadIO m => MonadIO (Codensity m) | |
| Plus v => Plus (Codensity v) | |
| Alt v => Alt (Codensity v) | |
| Apply (Codensity f) | |
| MonadSpec (Codensity m) |
lowerCodensity :: Monad m => Codensity m a -> m aSource
This serves as the *left*-inverse (retraction) of lift.
'lowerCodensity . lift' ≡ id
In general this is not a full 2-sided inverse, merely a retraction, as
Codensity mm.
e.g. Codensity ((->) s)) a ~ forall r. (a -> s -> r) -> s -> rMonadState s(->) s
is limited to MonadReader s
codensityToAdjunction :: Adjunction f g => Codensity g a -> g (f a)Source
The Codensity monad of a right adjoint is isomorphic to the
monad obtained from the Adjunction.
codensityToAdjunction.adjunctionToCodensity≡idadjunctionToCodensity.codensityToAdjunction≡id
adjunctionToCodensity :: Adjunction f g => g (f a) -> Codensity g aSource
codensityToRan :: Codensity g a -> Ran g g aSource
The Codensity Monad of a Functor g is the right Kan extension (Ran)
of g along itself.
codensityToRan.ranToCodensity≡idranToCodensity.codensityToRan≡id
ranToCodensity :: Ran g g a -> Codensity g aSource
codensityToComposedRep :: Representable u => Codensity u a -> u (Key u, a)Source
The Codensity monad of a representable Functor is isomorphic to the
monad obtained from the Adjunction for which that Functor is the right
adjoint.
codensityToComposedRep.composedRepToCodensity≡idcomposedRepToCodensity.codensityToComposedRep≡id
codensityToComposedRep =ranToComposedRep.codensityToRan
composedRepToCodensity :: (Representable u, Functor h) => u (Key u, a) -> Codensity u aSource
improve :: Functor f => (forall m. MonadFree f m => m a) -> Free f aSource
Right associate all binds in a computation that generates a free monad
This can improve the asymptotic efficiency of the result, while preserving semantics.
See "Asymptotic Improvement of Computations over Free Monads" by Janis Voightländer for more information about this combinator.