Safe Haskell | None |
---|---|
Language | Haskell2010 |
Control-flow
Synopsis
- class Monad m => MonadIO (m :: * -> *) where
- class MonadIO m => MonadInIO (m :: * -> *) where
- (|>) :: a -> (a -> b) -> b
- (<|) :: (a -> b) -> a -> b
- (||>) :: Functor f => f a -> (a -> b) -> f b
- (<||) :: Functor f => (a -> b) -> f a -> f b
- when :: Applicative f => Bool -> f () -> f ()
- unless :: Applicative f => Bool -> f () -> f ()
- whenM :: Monad m => m Bool -> m () -> m ()
- unlessM :: Monad m => m Bool -> m () -> m ()
- ifM :: Monad m => m Bool -> m a -> m a -> m a
- guard :: Alternative f => Bool -> f ()
- void :: Functor f => f a -> f ()
- forever :: Applicative f => f a -> f b
- foldM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b
- foldM_ :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m ()
- forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b)
- forM_ :: (Foldable t, Monad m) => t a -> (a -> m b) -> m ()
- mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b)
- mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
- sequence :: (Traversable t, Monad m) => t (m a) -> m (t a)
- replicateM :: Applicative m => Int -> m a -> m [a]
- replicateM_ :: Applicative m => Int -> m a -> m ()
- filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a]
- join :: Monad m => m (m a) -> m a
- (<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c
- (>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c
- loopM :: Monad m => (a -> m (Either a b)) -> a -> m b
- whileM :: Monad m => m Bool -> m ()
- module Haskus.Utils.Variant.Flow
Documentation
class Monad m => MonadIO (m :: * -> *) where #
Monads in which IO
computations may be embedded.
Any monad built by applying a sequence of monad transformers to the
IO
monad will be an instance of this class.
Instances should satisfy the following laws, which state that liftIO
is a transformer of monads:
Instances
MonadIO IO | Since: base-4.9.0.0 |
Defined in Control.Monad.IO.Class | |
MonadIO Q | |
Defined in Language.Haskell.TH.Syntax | |
MonadIO m => MonadIO (ListT m) | |
MonadIO m => MonadIO (IdentityT m) | |
Defined in Control.Monad.Trans.Identity | |
MonadIO m => MonadIO (StateT s m) | |
Defined in Control.Monad.Trans.State.Lazy | |
(Error e, MonadIO m) => MonadIO (ErrorT e m) | |
Defined in Control.Monad.Trans.Error |
class MonadIO m => MonadInIO (m :: * -> *) where #
liftWith :: (forall c. (a -> IO c) -> IO c) -> (a -> m b) -> m b #
Lift with*-like functions into IO (alloca, etc.)
liftWith2 :: (forall c. (a -> b -> IO c) -> IO c) -> (a -> b -> m e) -> m e #
Lift with*-like functions into IO (alloca, etc.)
Basic operators
Monadic/applicative operators
when :: Applicative f => Bool -> f () -> f () #
Conditional execution of Applicative
expressions. For example,
when debug (putStrLn "Debugging")
will output the string Debugging
if the Boolean value debug
is True
, and otherwise do nothing.
unless :: Applicative f => Bool -> f () -> f () #
The reverse of when
.
guard :: Alternative f => Bool -> f () #
Conditional failure of Alternative
computations. Defined by
guard True =pure
() guard False =empty
Examples
Common uses of guard
include conditionally signaling an error in
an error monad and conditionally rejecting the current choice in an
Alternative
-based parser.
As an example of signaling an error in the error monad Maybe
,
consider a safe division function safeDiv x y
that returns
Nothing
when the denominator y
is zero and
otherwise. For example:Just
(x `div`
y)
>>> safeDiv 4 0 Nothing >>> safeDiv 4 2 Just 2
A definition of safeDiv
using guards, but not guard
:
safeDiv :: Int -> Int -> Maybe Int safeDiv x y | y /= 0 = Just (x `div` y) | otherwise = Nothing
A definition of safeDiv
using guard
and Monad
do
-notation:
safeDiv :: Int -> Int -> Maybe Int safeDiv x y = do guard (y /= 0) return (x `div` y)
void :: Functor f => f a -> f () #
discards or ignores the result of evaluation, such
as the return value of an void
valueIO
action.
Examples
Replace the contents of a
with unit:Maybe
Int
>>>
void Nothing
Nothing>>>
void (Just 3)
Just ()
Replace the contents of an
with unit,
resulting in an Either
Int
Int
:Either
Int
'()'
>>>
void (Left 8675309)
Left 8675309>>>
void (Right 8675309)
Right ()
Replace every element of a list with unit:
>>>
void [1,2,3]
[(),(),()]
Replace the second element of a pair with unit:
>>>
void (1,2)
(1,())
Discard the result of an IO
action:
>>>
mapM print [1,2]
1 2 [(),()]>>>
void $ mapM print [1,2]
1 2
forever :: Applicative f => f a -> f b #
repeats the action infinitely.forever
act
foldM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b #
The foldM
function is analogous to foldl
, except that its result is
encapsulated in a monad. Note that foldM
works from left-to-right over
the list arguments. This could be an issue where (
and the `folded
function' are not commutative.>>
)
foldM f a1 [x1, x2, ..., xm] == do a2 <- f a1 x1 a3 <- f a2 x2 ... f am xm
If right-to-left evaluation is required, the input list should be reversed.
foldM_ :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m () #
Like foldM
, but discards the result.
forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b) #
mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) #
Map each element of a structure to a monadic action, evaluate
these actions from left to right, and collect the results. For
a version that ignores the results see mapM_
.
sequence :: (Traversable t, Monad m) => t (m a) -> m (t a) #
Evaluate each monadic action in the structure from left to
right, and collect the results. For a version that ignores the
results see sequence_
.
replicateM :: Applicative m => Int -> m a -> m [a] #
performs the action replicateM
n actn
times,
gathering the results.
replicateM_ :: Applicative m => Int -> m a -> m () #
Like replicateM
, but discards the result.
filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a] #
This generalizes the list-based filter
function.
join :: Monad m => m (m a) -> m a #
The join
function is the conventional monad join operator. It
is used to remove one level of monadic structure, projecting its
bound argument into the outer level.
(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c infixr 1 #
Left-to-right Kleisli composition of monads.
Variant based operators
module Haskus.Utils.Variant.Flow