Portability | portable |
---|---|
Stability | provisional |
Maintainer | Edward Kmett <ekmett@gmail.com> |
Safe Haskell | Safe |
- class Functor f => Alt f where
- (<!>) :: f a -> f a -> f a
- some :: Applicative f => f a -> f [a]
- many :: Applicative f => f a -> f [a]
- module Data.Functor.Apply
Documentation
class Functor f => Alt f whereSource
Laws:
<!> is associative: (a <!> b) <!> c = a <!> (b <!> c) <$> left-distributes over <!>: f <$> (a <!> b) = (f <$> a) <!> (f <$> b)
If extended to an Alternative
then <!>
should equal <|>
.
Ideally, an instance of Alt
also satisfies the "left distributon" law of
MonadPlus with respect to .:
<.> right-distributes over <!>: (a <!> b) <.> c = (a <.> c) <!> (b <.> c)
But Maybe
, IO
,
, Either
a
, and ErrorT
e mSTM
satisfy the alternative
"left catch" law instead:
pure a <!> b = pure a
However, this variation cannot be stated purely in terms of the dependencies of Alt
.
When and if MonadPlus is successfully refactored, this class should also be refactored to remove these instances.
The right distributive law should extend in the cases where the a Bind
or Monad
is
provided to yield variations of the right distributive law:
(m <!> n) >>- f = (m >>- f) <!> (m >>- f) (m <!> n) >>= f = (m >>= f) <!> (m >>= f)
(<!>) :: f a -> f a -> f aSource
(|)
without a required empty
some :: Applicative f => f a -> f [a]Source
many :: Applicative f => f a -> f [a]Source
Alt [] | |
Alt IO | This instance does not actually satisfy the (.) right distributive law It instead satisfies the Left-Catch law |
Alt Maybe | |
Alt Seq | |
Alt IntMap | |
Alt Option | |
Alt NonEmpty | |
Alt (Either a) | |
MonadPlus m => Alt (WrappedMonad m) | |
Alt f => Alt (IdentityT f) | |
Ord k => Alt (Map k) | |
(Bind f, Monad f) => Alt (MaybeT f) | |
Apply f => Alt (ListT f) | |
Alternative f => Alt (WrappedApplicative f) | |
ArrowPlus a => Alt (WrappedArrow a b) | |
Alt f => Alt (WriterT w f) | |
Alt f => Alt (WriterT w f) | |
(Bind f, Monad f) => Alt (ErrorT e f) | |
Alt f => Alt (StateT e f) | |
Alt f => Alt (StateT e f) | |
Alt f => Alt (ReaderT e f) | |
Alt f => Alt (Static f a) | |
Alt f => Alt (RWST r w s f) | |
Alt f => Alt (RWST r w s f) |
module Data.Functor.Apply