| Copyright | (C) 2011-2015 Edward Kmett |
|---|---|
| License | BSD-style (see the file LICENSE) |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Stability | provisional |
| Portability | portable |
| Safe Haskell | Safe |
| Language | Haskell98 |
Data.Functor.Alt
Description
- 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 where Source
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 aErrorT 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)
Minimal complete definition
Methods
(<!>) :: f a -> f a -> f a infixl 3 Source
<|> without a required empty
some :: Applicative f => f a -> f [a] Source
many :: Applicative f => f a -> f [a] Source
Instances
module Data.Functor.Apply