comonad-4.0: Comonads

Portability | portable
Stability | provisional
Maintainer | Edward Kmett <ekmett@gmail.com>
Safe Haskell | Trustworthy

Control.Comonad.Trans.Env

Contents

- The strict environment comonad
- The strict environment comonad transformer
- Combinators

Description

The environment comonad holds a value along with some retrievable context.

This module specifies the environment comonad transformer (aka coreader), which is left adjoint to the reader comonad.

The following sets up an experiment that retains its initial value in the background:

    >>> let initial = env 0 0

Extract simply retrieves the value:

    >>> extract initial
    0

Play around with the value, in our case producing a negative value:

    >>> let experiment = fmap (+ 10) initial
    >>> extract experiment
    10

Oh noes, something went wrong, 10 isn't very negative! Better restore the initial value using the default:

    >>> let initialRestored = experiment =>> ask
    >>> extract initialRestored
    0

Synopsis

- type Env e = EnvT e Identity
- env :: e -> a -> Env e a
- runEnv :: Env e a -> (e, a)
- data EnvT e w a = EnvT e (w a)
- runEnvT :: EnvT e w a -> (e, w a)
- lowerEnvT :: EnvT e w a -> w a
- ask :: EnvT e w a -> e
- asks :: (e -> f) -> EnvT e w a -> f
- local :: (e -> e') -> EnvT e w a -> EnvT e' w a

# The strict environment comonad

type Env e = EnvT e Identity Source

env :: e -> a -> Env e aSource

Create an Env using an environment and a value

runEnv :: Env e a -> (e, a)Source

# The strict environment comonad transformer

data EnvT e w a Source

Constructors

EnvT e (w a) |

Instances

Comonad w => ComonadEnv e (EnvT e w) |
ComonadStore s w => ComonadStore s (EnvT e w) |
ComonadTraced m w => ComonadTraced m (EnvT e w) |
ComonadTrans (EnvT e) |
ComonadHoist (EnvT e) |
Functor w => Functor (EnvT e w) |
(Typeable s, Typeable1 w) => Typeable1 (EnvT s w) |
Foldable w => Foldable (EnvT e w) |
Traversable w => Traversable (EnvT e w) |
(Semigroup e, ComonadApply w) => ComonadApply (EnvT e w) |
Comonad w => Comonad (EnvT e w) |
(Data e, Typeable1 w, Data (w a), Data a) => Data (EnvT e w a) |
(Typeable s, Typeable1 w, Typeable a) => Typeable (EnvT s w a) |

runEnvT :: EnvT e w a -> (e, w a)Source

lowerEnvT :: EnvT e w a -> w aSource

Gets rid of the environment. This differs from `extract` in that it will not continue extracting the value from the contained comonad.

# Combinators

ask :: EnvT e w a -> eSource

Retrieves the environment.

asks :: (e -> f) -> EnvT e w a -> fSource

Like `ask`, but modifies the resulting value with a function.

     asks = f . ask

local :: (e -> e') -> EnvT e w a -> EnvT e' w aSource

Modifies the environment using the specified function.