Control.Applicative.Lift

Adding a new kind of pure computation to an applicative functor.

Lifting an applicative

data Lift f a

Applicative functor formed by adding pure computations to a given applicative functor.

Constructors

Pure a

Other (f a)

Instances

Functor f => Functor (Lift f)

Applicative f => Applicative (Lift f)

A combination is Pure only if both parts are.

Foldable f => Foldable (Lift f)

Traversable f => Traversable (Lift f)

Alternative f => Alternative (Lift f)

A combination is Pure only either part is.

Show1 f => Show1 (Lift f)

Read1 f => Read1 (Lift f)

Ord1 f => Ord1 (Lift f)

Eq1 f => Eq1 (Lift f)

(Eq1 f, Eq a) => Eq (Lift f a)

(Ord1 f, Ord a) => Ord (Lift f a)

(Read1 f, Read a) => Read (Lift f a)

(Show1 f, Show a) => Show (Lift f a)

unLift :: Applicative f => Lift f a -> f a

Projection to the other functor.

mapLift :: (f a -> g a) -> Lift f a -> Lift g a

Apply a transformation to the other computation.

Collecting errors

type Errors e = Lift (Constant e)

An applicative functor that collects a monoid (e.g. lists) of errors. A sequence of computations fails if any of its components do, but unlike monads made with ExceptT from Control.Monad.Trans.Except, these computations continue after an error, collecting all the errors.

• pure f <*> pure x = pure (f x)

• pure f <*> failure e = failure e

• failure e <*> pure x = failure e

• failure e1 <*> failure e2 = failure (e1 <> e2)

runErrors :: Errors e a -> Either e a

Extractor for computations with accumulating errors.

• runErrors (pure x) = Right x

• runErrors (failure e) = Left e

failure :: Monoid e => e -> Errors e a

Report an error.