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\section[GHC.Base]{Module @GHC.Base@}
simple lhs test
-- to be found Monad
Other Prelude modules are much easier with fewer complex dependencies.
\begin{code}
{- | The 'Functor' class is used for types that can be mapped over.
Instances of 'Functor' should satisfy the following laws:
> fmap id == id
> fmap (f . g) == fmap f . fmap g
The instances of 'Functor' for lists, 'Data.Maybe.Maybe' and 'System.IO.IO'
satisfy these laws.
-}
class Functor f where
fmap :: (a -> b) -> f a -> f b
-- | Replace all locations in the input with the same value.
-- The default definition is @'fmap' . 'const'@, but this may be
-- overridden with a more efficient version.
(<$) :: a -> f b -> f a
(<$) = fmap . const
{- | The 'Monad' class defines the basic operations over a /monad/,
a concept from a branch of mathematics known as /category theory/.
From the perspective of a Haskell programmer, however, it is best to
think of a monad as an /abstract datatype/ of actions.
Haskell's @do@ expressions provide a convenient syntax for writing
monadic expressions.
Minimal complete definition: '>>=' and 'return'.
Instances of 'Monad' should satisfy the following laws:
> return a >>= k == k a
> m >>= return == m
> m >>= (\x -> k x >>= h) == (m >>= k) >>= h
Instances of both 'Monad' and 'Functor' should additionally satisfy the law:
> fmap f xs == xs >>= return . f
The instances of 'Monad' for lists, 'Data.Maybe.Maybe' and 'System.IO.IO'
defined in the "Prelude" satisfy these laws.
-}
class Monad m where
-- | Sequentially compose two actions, passing any value produced
-- by the first as an argument to the second.
(>>=) :: forall a b. m a -> (a -> m b) -> m b
-- | Sequentially compose two actions, discarding any value produced
-- by the first, like sequencing operators (such as the semicolon)
-- in imperative languages.
(>>) :: forall a b. m a -> m b -> m b
-- Explicit for-alls so that we know what order to
-- give type arguments when desugaring
-- | Inject a value into the monadic type.
return :: a -> m a
-- | Fail with a message. This operation is not part of the
-- mathematical definition of a monad, but is invoked on pattern-match
-- failure in a @do@ expression.
fail :: String -> m a
{-# INLINE (>>) #-}
m >> k = m >>= \_ -> k
fail s = error s
instance Functor ((->) r) where
fmap = (.)
instance Monad ((->) r) where
return = const
f >>= k = \ r -> k (f r) r
instance Functor ((,) a) where
fmap f (x,y) = (x, f y)
\end{code}
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