Control.Comonad.Trans.Env

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

The strict environment comonad

type Env e = EnvT e Identity

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

Create an Env using an environment and a value

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

The strict environment comonad transformer

data EnvT e w a

Constructors

EnvT e (w a)

Instances

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)

Comonad w => Comonad (EnvT e w)

(Semigroup e, ComonadApply w) => ComonadApply (EnvT e w)

(Semigroup e, Apply w) => Apply (EnvT e w)

Extend w => Extend (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)

lowerEnvT :: EnvT e w a -> w a

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 -> e

Retrieves the environment.

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

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

 asks = f . ask

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

Modifies the environment using the specified function.