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

Lifting an applicative

data Lift f a Source

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 Source

Projection to the other functor.

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

Apply a transformation to the other computation.

Collecting errors

type Errors e = Lift (Constant e) Source

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 Source

Extractor for computations with accumulating errors.

```
runErrors (pure x) = Right x
```
```
runErrors (failure e) = Left e
```

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

Report an error.