transformers-0.4.2.0: Concrete functor and monad transformers

Copyright | (c) Ross Paterson 2010
License | BSD-style (see the file LICENSE)
Maintainer | ross@soi.city.ac.uk
Stability | experimental
Portability | portable
Safe Haskell | Safe
Language | Haskell98

Control.Applicative.Lift

Contents

- Lifting an applicative
- Collecting errors

Description

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

Synopsis

- data Lift f a
  - = Pure a
  - | Other (f a)
- unLift :: Applicative f => Lift f a -> f a
- mapLift :: (f a -> g a) -> Lift f a -> Lift g a
- type Errors e = Lift (Constant e)
- runErrors :: Errors e a -> Either e a
- failure :: Monoid e => e -> Errors e a

# 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.