Safe Haskell | Safe |
---|---|
Language | Haskell2010 |
Synopsis
- guard :: Alternative f => Bool -> f ()
- join :: Monad m => m (m a) -> m a
- class Applicative m => Monad (m :: * -> *) where
- class Functor (f :: * -> *) where
- mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b)
- sequence :: (Traversable t, Monad m) => t (m a) -> m (t a)
- mfilter :: MonadPlus m => (a -> Bool) -> m a -> m a
- (<$!>) :: Monad m => (a -> b) -> m a -> m b
- unless :: Applicative f => Bool -> f () -> f ()
- replicateM_ :: Applicative m => Int -> m a -> m ()
- replicateM :: Applicative m => Int -> m a -> m [a]
- foldM_ :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m ()
- foldM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b
- zipWithM_ :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m ()
- zipWithM :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c]
- mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c])
- forever :: Applicative f => f a -> f b
- (<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c
- (>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c
- filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a]
- forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b)
- msum :: (Foldable t, MonadPlus m) => t (m a) -> m a
- sequence_ :: (Foldable t, Monad m) => t (m a) -> m ()
- forM_ :: (Foldable t, Monad m) => t a -> (a -> m b) -> m ()
- mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
- void :: Functor f => f a -> f ()
- ap :: Monad m => m (a -> b) -> m a -> m b
- liftM5 :: Monad m => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
- liftM4 :: Monad m => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
- liftM3 :: Monad m => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
- liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
- liftM :: Monad m => (a1 -> r) -> m a1 -> m r
- when :: Applicative f => Bool -> f () -> f ()
- (=<<) :: Monad m => (a -> m b) -> m a -> m b
- class (Alternative m, Monad m) => MonadPlus (m :: * -> *) where
- module Control.Monad.Fail
- class (Functor f, Monad m) => MonadFree f m where
- data Free f a
- isPure :: Free f a -> Bool
- isImpure :: Free f a -> Bool
- foldFree :: Functor f => (a -> b) -> (f b -> b) -> Free f a -> b
- evalFree :: (a -> b) -> (f (Free f a) -> b) -> Free f a -> b
- mapFree :: (Functor f, Functor g) => (f (Free g a) -> g (Free g a)) -> Free f a -> Free g a
- mapFreeM :: (Traversable f, Functor g, Monad m) => (f (Free g a) -> m (g (Free g a))) -> Free f a -> m (Free g a)
- mapFreeM' :: (Functor f, Traversable g, Monad m) => (forall a. f a -> m (g a)) -> Free f a -> m (Free g a)
- foldFreeM :: (Traversable f, Monad m) => (a -> m b) -> (f b -> m b) -> Free f a -> m b
- induce :: (Functor f, Monad m) => (forall a. f a -> m a) -> Free f a -> m a
- newtype FreeT f m a = FreeT {}
- foldFreeT :: (Traversable f, Monad m) => (a -> m b) -> (f b -> m b) -> FreeT f m a -> m b
- foldFreeT' :: (Traversable f, Monad m) => (a -> b) -> (f b -> b) -> FreeT f m a -> m b
- mapFreeT :: (Functor f, Functor m) => (forall a. m a -> m' a) -> FreeT f m a -> FreeT f m' a
- foldFreeA :: (Traversable f, Applicative m) => (a -> m b) -> m (f b -> b) -> Free f a -> m b
- mapFreeA :: (Traversable f, Functor g, Applicative m) => m (f (Free g a) -> g (Free g a)) -> Free f a -> m (Free g a)
- trans :: MonadFree f m => Free f a -> m a
- trans' :: (Functor f, Monad m) => m (Free f a) -> FreeT f m a
- untrans :: (Traversable f, Monad m) => FreeT f m a -> m (Free f a)
- liftFree :: (Functor f, Monad m) => (a -> Free f b) -> a -> FreeT f m b
Documentation
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)
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.
class Applicative m => Monad (m :: * -> *) where #
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.
Instances of Monad
should satisfy the following laws:
Furthermore, the Monad
and Applicative
operations should relate as follows:
The above laws imply:
and that pure
and (<*>
) satisfy the applicative functor laws.
The instances of Monad
for lists, Maybe
and IO
defined in the Prelude satisfy these laws.
(>>=) :: m a -> (a -> m b) -> m b infixl 1 #
Sequentially compose two actions, passing any value produced by the first as an argument to the second.
(>>) :: m a -> m b -> m b infixl 1 #
Sequentially compose two actions, discarding any value produced by the first, like sequencing operators (such as the semicolon) in imperative languages.
Inject a value into the monadic type.
Instances
Monad [] | Since: base-2.1 |
Monad Maybe | Since: base-2.1 |
Monad IO | Since: base-2.1 |
Monad Par1 | Since: base-4.9.0.0 |
Monad Identity | Since: base-4.8.0.0 |
Monad First | |
Monad Last | |
Monad Dual | Since: base-4.8.0.0 |
Monad Sum | Since: base-4.8.0.0 |
Monad Product | Since: base-4.8.0.0 |
Monad ReadP | Since: base-2.1 |
Monad NonEmpty | Since: base-4.9.0.0 |
Monad P | Since: base-2.1 |
Monad (Either e) | Since: base-4.4.0.0 |
Monad (U1 :: * -> *) | Since: base-4.9.0.0 |
Monoid a => Monad ((,) a) | Since: base-4.9.0.0 |
Monad m => Monad (WrappedMonad m) | |
Defined in Control.Applicative (>>=) :: WrappedMonad m a -> (a -> WrappedMonad m b) -> WrappedMonad m b # (>>) :: WrappedMonad m a -> WrappedMonad m b -> WrappedMonad m b # return :: a -> WrappedMonad m a # fail :: String -> WrappedMonad m a # | |
ArrowApply a => Monad (ArrowMonad a) | Since: base-2.1 |
Defined in Control.Arrow (>>=) :: ArrowMonad a a0 -> (a0 -> ArrowMonad a b) -> ArrowMonad a b # (>>) :: ArrowMonad a a0 -> ArrowMonad a b -> ArrowMonad a b # return :: a0 -> ArrowMonad a a0 # fail :: String -> ArrowMonad a a0 # | |
Monad (Proxy :: * -> *) | Since: base-4.7.0.0 |
Functor f => Monad (Free f) # | |
Monad (C mu) # | |
Monad f => Monad (Rec1 f) | Since: base-4.9.0.0 |
Monad f => Monad (Alt f) | |
Monad m => Monad (StateT s m) | |
(Functor f, Monad m) => Monad (FreeT f m) # | |
Monad ((->) r :: * -> *) | Since: base-2.1 |
(Monad f, Monad g) => Monad (f :*: g) | Since: base-4.9.0.0 |
Monad f => Monad (M1 i c f) | Since: base-4.9.0.0 |
class Functor (f :: * -> *) where #
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, Maybe
and IO
satisfy these laws.
Instances
Functor [] | Since: base-2.1 |
Functor Maybe | Since: base-2.1 |
Functor IO | Since: base-2.1 |
Functor Par1 | |
Functor ZipList | |
Functor Identity | Since: base-4.8.0.0 |
Functor First | |
Functor Last | |
Functor Dual | Since: base-4.8.0.0 |
Functor Sum | Since: base-4.8.0.0 |
Functor Product | Since: base-4.8.0.0 |
Functor ReadP | Since: base-2.1 |
Functor NonEmpty | Since: base-4.9.0.0 |
Functor P | |
Defined in Text.ParserCombinators.ReadP | |
Functor (Either a) | Since: base-3.0 |
Functor (V1 :: * -> *) | Since: base-4.9.0.0 |
Functor (U1 :: * -> *) | Since: base-4.9.0.0 |
Functor ((,) a) | Since: base-2.1 |
Functor (Array i) | Since: base-2.1 |
Monad m => Functor (WrappedMonad m) | Since: base-2.1 |
Defined in Control.Applicative fmap :: (a -> b) -> WrappedMonad m a -> WrappedMonad m b # (<$) :: a -> WrappedMonad m b -> WrappedMonad m a # | |
Arrow a => Functor (ArrowMonad a) | Since: base-4.6.0.0 |
Defined in Control.Arrow fmap :: (a0 -> b) -> ArrowMonad a a0 -> ArrowMonad a b # (<$) :: a0 -> ArrowMonad a b -> ArrowMonad a a0 # | |
Functor (Proxy :: * -> *) | Since: base-4.7.0.0 |
Functor f => Functor (Free f) # | |
Functor (C mu) # | |
Functor f => Functor (Rec1 f) | |
Functor (URec Char :: * -> *) | |
Functor (URec Double :: * -> *) | |
Functor (URec Float :: * -> *) | |
Functor (URec Int :: * -> *) | |
Functor (URec Word :: * -> *) | |
Functor (URec (Ptr ()) :: * -> *) | |
Arrow a => Functor (WrappedArrow a b) | Since: base-2.1 |
Defined in Control.Applicative fmap :: (a0 -> b0) -> WrappedArrow a b a0 -> WrappedArrow a b b0 # (<$) :: a0 -> WrappedArrow a b b0 -> WrappedArrow a b a0 # | |
Functor (Const m :: * -> *) | Since: base-2.1 |
Functor f => Functor (Alt f) | |
Functor m => Functor (StateT s m) | |
(Functor f, Functor m) => Functor (FreeT f m) # | |
Functor ((->) r :: * -> *) | Since: base-2.1 |
Functor (K1 i c :: * -> *) | |
(Functor f, Functor g) => Functor (f :+: g) | |
(Functor f, Functor g) => Functor (f :*: g) | |
Functor f => Functor (M1 i c f) | |
(Functor f, Functor g) => Functor (f :.: g) | |
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_
.
unless :: Applicative f => Bool -> f () -> f () #
The reverse of when
.
replicateM_ :: Applicative m => Int -> m a -> m () #
Like replicateM
, but discards the result.
replicateM :: Applicative m => Int -> m a -> m [a] #
performs the action replicateM
n actn
times,
gathering the results.
foldM_ :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m () #
Like foldM
, but discards the result.
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.
zipWithM_ :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m () #
zipWithM :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c] #
mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c]) #
The mapAndUnzipM
function maps its first argument over a list, returning
the result as a pair of lists. This function is mainly used with complicated
data structures or a state-transforming monad.
forever :: Applicative f => f a -> f b #
repeats the action infinitely.forever
act
(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c infixr 1 #
Left-to-right Kleisli composition of monads.
filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a] #
This generalizes the list-based filter
function.
forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b) #
sequence_ :: (Foldable t, Monad m) => t (m a) -> m () #
Evaluate each monadic action in the structure from left to right,
and ignore the results. For a version that doesn't ignore the
results see sequence
.
As of base 4.8.0.0, sequence_
is just sequenceA_
, specialized
to Monad
.
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
liftM5 :: Monad m => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r #
Promote a function to a monad, scanning the monadic arguments from
left to right (cf. liftM2
).
liftM4 :: Monad m => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r #
Promote a function to a monad, scanning the monadic arguments from
left to right (cf. liftM2
).
liftM3 :: Monad m => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r #
Promote a function to a monad, scanning the monadic arguments from
left to right (cf. liftM2
).
liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r #
Promote a function to a monad, scanning the monadic arguments from left to right. For example,
liftM2 (+) [0,1] [0,2] = [0,2,1,3] liftM2 (+) (Just 1) Nothing = Nothing
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.
(=<<) :: Monad m => (a -> m b) -> m a -> m b infixr 1 #
Same as >>=
, but with the arguments interchanged.
class (Alternative m, Monad m) => MonadPlus (m :: * -> *) where #
Monads that also support choice and failure.
The identity of mplus
. It should also satisfy the equations
mzero >>= f = mzero v >> mzero = mzero
The default definition is
mzero = empty
An associative operation. The default definition is
mplus = (<|>
)
Instances
MonadPlus [] | Since: base-2.1 |
MonadPlus Maybe | Since: base-2.1 |
MonadPlus IO | Since: base-4.9.0.0 |
MonadPlus ReadP | Since: base-2.1 |
MonadPlus P | Since: base-2.1 |
Defined in Text.ParserCombinators.ReadP | |
MonadPlus (U1 :: * -> *) | Since: base-4.9.0.0 |
(ArrowApply a, ArrowPlus a) => MonadPlus (ArrowMonad a) | Since: base-4.6.0.0 |
Defined in Control.Arrow mzero :: ArrowMonad a a0 # mplus :: ArrowMonad a a0 -> ArrowMonad a a0 -> ArrowMonad a a0 # | |
MonadPlus (Proxy :: * -> *) | Since: base-4.9.0.0 |
MonadPlus mu => MonadPlus (C mu) # | |
MonadPlus f => MonadPlus (Rec1 f) | Since: base-4.9.0.0 |
MonadPlus f => MonadPlus (Alt f) | |
MonadPlus m => MonadPlus (StateT s m) | |
(Functor f, Monad m, MonadPlus m) => MonadPlus (FreeT f m) # | |
(MonadPlus f, MonadPlus g) => MonadPlus (f :*: g) | Since: base-4.9.0.0 |
MonadPlus f => MonadPlus (M1 i c f) | Since: base-4.9.0.0 |
module Control.Monad.Fail
Free Monads
class (Functor f, Monad m) => MonadFree f m where Source #
This type class generalizes over encodings of Free Monads.
:: m a | |
-> m (Either a (f (m a))) |
|
:: f (m a) | |
-> m a | Wraps a side effect into a monadic computation |
Instances
Functor f => MonadFree f (Free f) Source # | |
Functor f => MonadFree f (C (Free f)) Source # | |
Functor f => Monad (Free f) Source # | |
Functor f => Functor (Free f) Source # | |
Functor f => Applicative (Free f) Source # | |
(Functor f, Foldable f) => Foldable (Free f) Source # | |
Defined in Control.Monad.Free fold :: Monoid m => Free f m -> m # foldMap :: Monoid m => (a -> m) -> Free f a -> m # foldr :: (a -> b -> b) -> b -> Free f a -> b # foldr' :: (a -> b -> b) -> b -> Free f a -> b # foldl :: (b -> a -> b) -> b -> Free f a -> b # foldl' :: (b -> a -> b) -> b -> Free f a -> b # foldr1 :: (a -> a -> a) -> Free f a -> a # foldl1 :: (a -> a -> a) -> Free f a -> a # elem :: Eq a => a -> Free f a -> Bool # maximum :: Ord a => Free f a -> a # minimum :: Ord a => Free f a -> a # | |
Traversable f => Traversable (Free f) Source # | |
Eq1 f => Eq1 (Free f) Source # | |
Ord1 f => Ord1 (Free f) Source # | |
Defined in Control.Monad.Free | |
(Eq a, Eq1 f) => Eq (Free f a) Source # | |
(Ord a, Ord1 f) => Ord (Free f a) Source # | |
Defined in Control.Monad.Free | |
(Show a, Show1 f) => Show (Free f a) Source # | |
Generic (Free f a) Source # | |
type Rep (Free f a) Source # | |
Defined in Control.Monad.Free type Rep (Free f a) = D1 (MetaData "Free" "Control.Monad.Free" "control-monad-free-0.6.2-D8gRCj9jdnL78y3kztZ3ka" False) (C1 (MetaCons "Impure" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 (f (Free f a)))) :+: C1 (MetaCons "Pure" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 a))) |
mapFree :: (Functor f, Functor g) => (f (Free g a) -> g (Free g a)) -> Free f a -> Free g a Source #
mapFreeM :: (Traversable f, Functor g, Monad m) => (f (Free g a) -> m (g (Free g a))) -> Free f a -> m (Free g a) Source #
mapFreeM' :: (Functor f, Traversable g, Monad m) => (forall a. f a -> m (g a)) -> Free f a -> m (Free g a) Source #
Monad Morphisms
Free Monad Transformers
Instances
(Monad m, Functor f) => MonadFree f (C (FreeT f m)) Source # | |
(Functor f, Monad m) => MonadFree f (FreeT f m) Source # | |
Functor f => MonadTrans (FreeT f) Source # | |
Defined in Control.Monad.Free | |
(Functor f, Monad m) => Monad (FreeT f m) Source # | |
(Functor f, Functor m) => Functor (FreeT f m) Source # | |
(Functor f, Functor a, Monad a) => Applicative (FreeT f a) Source # | |
(Traversable m, Traversable f) => Foldable (FreeT f m) Source # | |
Defined in Control.Monad.Free fold :: Monoid m0 => FreeT f m m0 -> m0 # foldMap :: Monoid m0 => (a -> m0) -> FreeT f m a -> m0 # foldr :: (a -> b -> b) -> b -> FreeT f m a -> b # foldr' :: (a -> b -> b) -> b -> FreeT f m a -> b # foldl :: (b -> a -> b) -> b -> FreeT f m a -> b # foldl' :: (b -> a -> b) -> b -> FreeT f m a -> b # foldr1 :: (a -> a -> a) -> FreeT f m a -> a # foldl1 :: (a -> a -> a) -> FreeT f m a -> a # toList :: FreeT f m a -> [a] # length :: FreeT f m a -> Int # elem :: Eq a => a -> FreeT f m a -> Bool # maximum :: Ord a => FreeT f m a -> a # minimum :: Ord a => FreeT f m a -> a # | |
(Traversable m, Traversable f) => Traversable (FreeT f m) Source # | |
Defined in Control.Monad.Free | |
(Functor f, Monad m, MonadIO m) => MonadIO (FreeT f m) Source # | |
Defined in Control.Monad.Free | |
(Functor f, Functor m, Monad m, MonadPlus m) => Alternative (FreeT f m) Source # | |
(Functor f, Monad m, MonadPlus m) => MonadPlus (FreeT f m) Source # | |
foldFreeT' :: (Traversable f, Monad m) => (a -> b) -> (f b -> b) -> FreeT f m a -> m b Source #
mapFreeT :: (Functor f, Functor m) => (forall a. m a -> m' a) -> FreeT f m a -> FreeT f m' a Source #
foldFreeA :: (Traversable f, Applicative m) => (a -> m b) -> m (f b -> b) -> Free f a -> m b Source #
mapFreeA :: (Traversable f, Functor g, Applicative m) => m (f (Free g a) -> g (Free g a)) -> Free f a -> m (Free g a) Source #