Control.Applicative.Lift

Copyright	(c) Ross Paterson 2010
License	BSD-style (see the file LICENSE)
Maintainer	[email protected]
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell98

Lifting an applicative

data Lift f a Source

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

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.