Safe Haskell | Safe-Inferred |
---|---|
Language | Haskell2010 |
Utilities related to Monad and Applicative classes Mostly for backwards compatibility.
Synopsis
- class Functor f => Applicative (f :: Type -> Type) where
- (<$>) :: Functor f => (a -> b) -> f a -> f b
- class Monad m => MonadFix (m :: Type -> Type) where
- mfix :: (a -> m a) -> m a
- class Monad m => MonadIO (m :: Type -> Type) where
- zipWith3M :: Monad m => (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m [d]
- zipWith3M_ :: Monad m => (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m ()
- zipWith4M :: Monad m => (a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e]
- zipWithAndUnzipM :: Monad m => (a -> b -> m (c, d)) -> [a] -> [b] -> m ([c], [d])
- mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c])
- mapAndUnzip3M :: Monad m => (a -> m (b, c, d)) -> [a] -> m ([b], [c], [d])
- mapAndUnzip4M :: Monad m => (a -> m (b, c, d, e)) -> [a] -> m ([b], [c], [d], [e])
- mapAndUnzip5M :: Monad m => (a -> m (b, c, d, e, f)) -> [a] -> m ([b], [c], [d], [e], [f])
- mapAccumLM :: Monad m => (acc -> x -> m (acc, y)) -> acc -> [x] -> m (acc, [y])
- liftFstM :: Monad m => (a -> b) -> m (a, r) -> m (b, r)
- liftSndM :: Monad m => (a -> b) -> m (r, a) -> m (r, b)
- mapSndM :: Monad m => (b -> m c) -> [(a, b)] -> m [(a, c)]
- concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b]
- mapMaybeM :: Applicative m => (a -> m (Maybe b)) -> [a] -> m [b]
- fmapMaybeM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)
- fmapEitherM :: Monad m => (a -> m b) -> (c -> m d) -> Either a c -> m (Either b d)
- anyM :: Monad m => (a -> m Bool) -> [a] -> m Bool
- allM :: Monad m => (a -> m Bool) -> [a] -> m Bool
- orM :: Monad m => m Bool -> m Bool -> m Bool
- foldlM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b
- foldlM_ :: (Monad m, Foldable t) => (a -> b -> m a) -> a -> t b -> m ()
- foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b
- maybeMapM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)
- whenM :: Monad m => m Bool -> m () -> m ()
- unlessM :: Monad m => m Bool -> m () -> m ()
- filterOutM :: Applicative m => (a -> m Bool) -> [a] -> m [a]
Documentation
class Functor f => Applicative (f :: Type -> Type) where Source #
A functor with application, providing operations to
A minimal complete definition must include implementations of pure
and of either <*>
or liftA2
. If it defines both, then they must behave
the same as their default definitions:
(<*>
) =liftA2
id
liftA2
f x y = f<$>
x<*>
y
Further, any definition must satisfy the following:
- Identity
pure
id
<*>
v = v- Composition
pure
(.)<*>
u<*>
v<*>
w = u<*>
(v<*>
w)- Homomorphism
pure
f<*>
pure
x =pure
(f x)- Interchange
u
<*>
pure
y =pure
($
y)<*>
u
The other methods have the following default definitions, which may be overridden with equivalent specialized implementations:
As a consequence of these laws, the Functor
instance for f
will satisfy
It may be useful to note that supposing
forall x y. p (q x y) = f x . g y
it follows from the above that
liftA2
p (liftA2
q u v) =liftA2
f u .liftA2
g v
If f
is also a Monad
, it should satisfy
(which implies that pure
and <*>
satisfy the applicative functor laws).
Lift a value.
(<*>) :: f (a -> b) -> f a -> f b infixl 4 Source #
Sequential application.
A few functors support an implementation of <*>
that is more
efficient than the default one.
Example
Used in combination with (
, <$>
)(
can be used to build a record.<*>
)
>>>
data MyState = MyState {arg1 :: Foo, arg2 :: Bar, arg3 :: Baz}
>>>
produceFoo :: Applicative f => f Foo
>>>
produceBar :: Applicative f => f Bar
>>>
produceBaz :: Applicative f => f Baz
>>>
mkState :: Applicative f => f MyState
>>>
mkState = MyState <$> produceFoo <*> produceBar <*> produceBaz
liftA2 :: (a -> b -> c) -> f a -> f b -> f c Source #
Lift a binary function to actions.
Some functors support an implementation of liftA2
that is more
efficient than the default one. In particular, if fmap
is an
expensive operation, it is likely better to use liftA2
than to
fmap
over the structure and then use <*>
.
This became a typeclass method in 4.10.0.0. Prior to that, it was
a function defined in terms of <*>
and fmap
.
Example
>>>
liftA2 (,) (Just 3) (Just 5)
Just (3,5)
(*>) :: f a -> f b -> f b infixl 4 Source #
Sequence actions, discarding the value of the first argument.
Examples
If used in conjunction with the Applicative instance for Maybe
,
you can chain Maybe computations, with a possible "early return"
in case of Nothing
.
>>>
Just 2 *> Just 3
Just 3
>>>
Nothing *> Just 3
Nothing
Of course a more interesting use case would be to have effectful computations instead of just returning pure values.
>>>
import Data.Char
>>>
import Text.ParserCombinators.ReadP
>>>
let p = string "my name is " *> munch1 isAlpha <* eof
>>>
readP_to_S p "my name is Simon"
[("Simon","")]
(<*) :: f a -> f b -> f a infixl 4 Source #
Sequence actions, discarding the value of the second argument.
Instances
Applicative ZipList | f <$> ZipList xs1 <*> ... <*> ZipList xsN = ZipList (zipWithN f xs1 ... xsN) where (\a b c -> stimes c [a, b]) <$> ZipList "abcd" <*> ZipList "567" <*> ZipList [1..] = ZipList (zipWith3 (\a b c -> stimes c [a, b]) "abcd" "567" [1..]) = ZipList {getZipList = ["a5","b6b6","c7c7c7"]} Since: base-2.1 |
Applicative Complex | Since: base-4.9.0.0 |
Applicative Identity | Since: base-4.8.0.0 |
Defined in Data.Functor.Identity | |
Applicative First | Since: base-4.8.0.0 |
Applicative Last | Since: base-4.8.0.0 |
Applicative Down | Since: base-4.11.0.0 |
Applicative First | Since: base-4.9.0.0 |
Applicative Last | Since: base-4.9.0.0 |
Applicative Max | Since: base-4.9.0.0 |
Applicative Min | Since: base-4.9.0.0 |
Applicative Dual | Since: base-4.8.0.0 |
Applicative Product | Since: base-4.8.0.0 |
Defined in Data.Semigroup.Internal | |
Applicative Sum | Since: base-4.8.0.0 |
Applicative STM | Since: base-4.8.0.0 |
Applicative Par1 | Since: base-4.9.0.0 |
Applicative P | Since: base-4.5.0.0 |
Applicative ReadP | Since: base-4.6.0.0 |
Applicative ReadPrec | Since: base-4.6.0.0 |
Defined in Text.ParserCombinators.ReadPrec | |
Applicative Get | |
Applicative PutM | |
Applicative Put | |
Applicative Seq | Since: containers-0.5.4 |
Applicative Tree | |
Applicative PD Source # | |
Applicative NatM Source # | |
Applicative LlvmM Source # | |
Applicative CoreM Source # | |
Applicative SimplM Source # | |
Applicative UnifyResultM Source # | |
Defined in GHC.Core.Unify pure :: a -> UnifyResultM a Source # (<*>) :: UnifyResultM (a -> b) -> UnifyResultM a -> UnifyResultM b Source # liftA2 :: (a -> b -> c) -> UnifyResultM a -> UnifyResultM b -> UnifyResultM c Source # (*>) :: UnifyResultM a -> UnifyResultM b -> UnifyResultM b Source # (<*) :: UnifyResultM a -> UnifyResultM b -> UnifyResultM a Source # | |
Applicative Pair Source # | |
Applicative Hsc Source # | |
Applicative Ghc Source # | |
Applicative CompPipeline Source # | |
Defined in GHC.Driver.Pipeline.Monad pure :: a -> CompPipeline a Source # (<*>) :: CompPipeline (a -> b) -> CompPipeline a -> CompPipeline b Source # liftA2 :: (a -> b -> c) -> CompPipeline a -> CompPipeline b -> CompPipeline c Source # (*>) :: CompPipeline a -> CompPipeline b -> CompPipeline b Source # (<*) :: CompPipeline a -> CompPipeline b -> CompPipeline a Source # | |
Applicative MatchResult Source # | Product is an "or" on falliblity---the combined match result is infallible only if the left and right argument match results both were. This is useful for combining a bunch of alternatives together and then
getting the overall falliblity of the entire group. See |
Defined in GHC.HsToCore.Monad pure :: a -> MatchResult a Source # (<*>) :: MatchResult (a -> b) -> MatchResult a -> MatchResult b Source # liftA2 :: (a -> b -> c) -> MatchResult a -> MatchResult b -> MatchResult c Source # (*>) :: MatchResult a -> MatchResult b -> MatchResult b Source # (<*) :: MatchResult a -> MatchResult b -> MatchResult a Source # | |
Applicative P Source # | |
Applicative PV Source # | |
Applicative CpsRn Source # | |
Applicative LiftM Source # | |
Applicative CmmParse Source # | |
Defined in GHC.StgToCmm.ExtCode | |
Applicative FCode Source # | |
Applicative TcS Source # | |
Applicative TcPluginM Source # | |
Defined in GHC.Tc.Types | |
Applicative UniqSM Source # | |
Applicative IO | Since: base-2.1 |
Applicative Q | |
Applicative Capability | |
Defined in System.Console.Terminfo.Base pure :: a -> Capability a Source # (<*>) :: Capability (a -> b) -> Capability a -> Capability b Source # liftA2 :: (a -> b -> c) -> Capability a -> Capability b -> Capability c Source # (*>) :: Capability a -> Capability b -> Capability b Source # (<*) :: Capability a -> Capability b -> Capability a Source # | |
Applicative NonEmpty | Since: base-4.9.0.0 |
Applicative Maybe | Since: base-2.1 |
Applicative Solo | Since: base-4.15 |
Applicative [] | Since: base-2.1 |
Monad m => Applicative (WrappedMonad m) | Since: base-2.1 |
Defined in Control.Applicative pure :: a -> WrappedMonad m a Source # (<*>) :: WrappedMonad m (a -> b) -> WrappedMonad m a -> WrappedMonad m b Source # liftA2 :: (a -> b -> c) -> WrappedMonad m a -> WrappedMonad m b -> WrappedMonad m c Source # (*>) :: WrappedMonad m a -> WrappedMonad m b -> WrappedMonad m b Source # (<*) :: WrappedMonad m a -> WrappedMonad m b -> WrappedMonad m a Source # | |
Arrow a => Applicative (ArrowMonad a) | Since: base-4.6.0.0 |
Defined in Control.Arrow pure :: a0 -> ArrowMonad a a0 Source # (<*>) :: ArrowMonad a (a0 -> b) -> ArrowMonad a a0 -> ArrowMonad a b Source # liftA2 :: (a0 -> b -> c) -> ArrowMonad a a0 -> ArrowMonad a b -> ArrowMonad a c Source # (*>) :: ArrowMonad a a0 -> ArrowMonad a b -> ArrowMonad a b Source # (<*) :: ArrowMonad a a0 -> ArrowMonad a b -> ArrowMonad a a0 Source # | |
Applicative (Either e) | Since: base-3.0 |
Defined in Data.Either | |
Applicative (StateL s) | Since: base-4.0 |
Defined in Data.Functor.Utils | |
Applicative (StateR s) | Since: base-4.0 |
Defined in Data.Functor.Utils | |
Applicative (Proxy :: Type -> Type) | Since: base-4.7.0.0 |
Applicative (U1 :: Type -> Type) | Since: base-4.9.0.0 |
Applicative (ST s) | Since: base-4.4.0.0 |
Applicative (SetM s) | |
Applicative (RegM freeRegs) Source # | |
Defined in GHC.CmmToAsm.Reg.Linear.State pure :: a -> RegM freeRegs a Source # (<*>) :: RegM freeRegs (a -> b) -> RegM freeRegs a -> RegM freeRegs b Source # liftA2 :: (a -> b -> c) -> RegM freeRegs a -> RegM freeRegs b -> RegM freeRegs c Source # (*>) :: RegM freeRegs a -> RegM freeRegs b -> RegM freeRegs b Source # (<*) :: RegM freeRegs a -> RegM freeRegs b -> RegM freeRegs a Source # | |
Applicative (IOEnv m) Source # | |
Applicative (MaybeErr err) Source # | |
Defined in GHC.Data.Maybe pure :: a -> MaybeErr err a Source # (<*>) :: MaybeErr err (a -> b) -> MaybeErr err a -> MaybeErr err b Source # liftA2 :: (a -> b -> c) -> MaybeErr err a -> MaybeErr err b -> MaybeErr err c Source # (*>) :: MaybeErr err a -> MaybeErr err b -> MaybeErr err b Source # (<*) :: MaybeErr err a -> MaybeErr err b -> MaybeErr err a Source # | |
Applicative (CmdLineP s) Source # | |
Defined in GHC.Driver.CmdLine pure :: a -> CmdLineP s a Source # (<*>) :: CmdLineP s (a -> b) -> CmdLineP s a -> CmdLineP s b Source # liftA2 :: (a -> b -> c) -> CmdLineP s a -> CmdLineP s b -> CmdLineP s c Source # (*>) :: CmdLineP s a -> CmdLineP s b -> CmdLineP s b Source # (<*) :: CmdLineP s a -> CmdLineP s b -> CmdLineP s a Source # | |
Monad m => Applicative (EwM m) Source # | |
Applicative m => Applicative (GhcT m) Source # | |
Applicative (State s) Source # | |
Applicative m => Applicative (ListT m) | |
Defined in Control.Monad.Trans.List | |
(Functor m, Monad m) => Applicative (MaybeT m) | |
Defined in Control.Monad.Trans.Maybe | |
Monoid a => Applicative ((,) a) | For tuples, the ("hello ", (+15)) <*> ("world!", 2002) ("hello world!",2017) Since: base-2.1 |
Arrow a => Applicative (WrappedArrow a b) | Since: base-2.1 |
Defined in Control.Applicative pure :: a0 -> WrappedArrow a b a0 Source # (<*>) :: WrappedArrow a b (a0 -> b0) -> WrappedArrow a b a0 -> WrappedArrow a b b0 Source # liftA2 :: (a0 -> b0 -> c) -> WrappedArrow a b a0 -> WrappedArrow a b b0 -> WrappedArrow a b c Source # (*>) :: WrappedArrow a b a0 -> WrappedArrow a b b0 -> WrappedArrow a b b0 Source # (<*) :: WrappedArrow a b a0 -> WrappedArrow a b b0 -> WrappedArrow a b a0 Source # | |
Applicative m => Applicative (Kleisli m a) | Since: base-4.14.0.0 |
Defined in Control.Arrow pure :: a0 -> Kleisli m a a0 Source # (<*>) :: Kleisli m a (a0 -> b) -> Kleisli m a a0 -> Kleisli m a b Source # liftA2 :: (a0 -> b -> c) -> Kleisli m a a0 -> Kleisli m a b -> Kleisli m a c Source # (*>) :: Kleisli m a a0 -> Kleisli m a b -> Kleisli m a b Source # (<*) :: Kleisli m a a0 -> Kleisli m a b -> Kleisli m a a0 Source # | |
Monoid m => Applicative (Const m :: Type -> Type) | Since: base-2.0.1 |
Applicative f => Applicative (Ap f) | Since: base-4.12.0.0 |
Applicative f => Applicative (Alt f) | Since: base-4.8.0.0 |
Applicative f => Applicative (Rec1 f) | Since: base-4.9.0.0 |
(Applicative f, Monad f) => Applicative (WhenMissing f x) | Equivalent to Since: containers-0.5.9 |
Defined in Data.IntMap.Internal pure :: a -> WhenMissing f x a Source # (<*>) :: WhenMissing f x (a -> b) -> WhenMissing f x a -> WhenMissing f x b Source # liftA2 :: (a -> b -> c) -> WhenMissing f x a -> WhenMissing f x b -> WhenMissing f x c Source # (*>) :: WhenMissing f x a -> WhenMissing f x b -> WhenMissing f x b Source # (<*) :: WhenMissing f x a -> WhenMissing f x b -> WhenMissing f x a Source # | |
Applicative (Stream m a) Source # | |
Defined in GHC.Data.Stream pure :: a0 -> Stream m a a0 Source # (<*>) :: Stream m a (a0 -> b) -> Stream m a a0 -> Stream m a b Source # liftA2 :: (a0 -> b -> c) -> Stream m a a0 -> Stream m a b -> Stream m a c Source # (*>) :: Stream m a a0 -> Stream m a b -> Stream m a b Source # (<*) :: Stream m a a0 -> Stream m a b -> Stream m a a0 Source # | |
Monad m => Applicative (StreamS m a) Source # | |
Defined in GHC.Data.Stream pure :: a0 -> StreamS m a a0 Source # (<*>) :: StreamS m a (a0 -> b) -> StreamS m a a0 -> StreamS m a b Source # liftA2 :: (a0 -> b -> c) -> StreamS m a a0 -> StreamS m a b -> StreamS m a c Source # (*>) :: StreamS m a a0 -> StreamS m a b -> StreamS m a b Source # (<*) :: StreamS m a a0 -> StreamS m a b -> StreamS m a a0 Source # | |
(Functor m, Monad m) => Applicative (ErrorT e m) | |
Defined in Control.Monad.Trans.Error pure :: a -> ErrorT e m a Source # (<*>) :: ErrorT e m (a -> b) -> ErrorT e m a -> ErrorT e m b Source # liftA2 :: (a -> b -> c) -> ErrorT e m a -> ErrorT e m b -> ErrorT e m c Source # (*>) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m b Source # (<*) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m a Source # | |
(Functor m, Monad m) => Applicative (ExceptT e m) | |
Defined in Control.Monad.Trans.Except pure :: a -> ExceptT e m a Source # (<*>) :: ExceptT e m (a -> b) -> ExceptT e m a -> ExceptT e m b Source # liftA2 :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c Source # (*>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b Source # (<*) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m a Source # | |
Applicative m => Applicative (IdentityT m) | |
Defined in Control.Monad.Trans.Identity pure :: a -> IdentityT m a Source # (<*>) :: IdentityT m (a -> b) -> IdentityT m a -> IdentityT m b Source # liftA2 :: (a -> b -> c) -> IdentityT m a -> IdentityT m b -> IdentityT m c Source # (*>) :: IdentityT m a -> IdentityT m b -> IdentityT m b Source # (<*) :: IdentityT m a -> IdentityT m b -> IdentityT m a Source # | |
Applicative m => Applicative (ReaderT r m) | |
Defined in Control.Monad.Trans.Reader pure :: a -> ReaderT r m a Source # (<*>) :: ReaderT r m (a -> b) -> ReaderT r m a -> ReaderT r m b Source # liftA2 :: (a -> b -> c) -> ReaderT r m a -> ReaderT r m b -> ReaderT r m c Source # (*>) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m b Source # (<*) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m a Source # | |
(Functor m, Monad m) => Applicative (StateT s m) | |
Defined in Control.Monad.Trans.State.Lazy pure :: a -> StateT s m a Source # (<*>) :: StateT s m (a -> b) -> StateT s m a -> StateT s m b Source # liftA2 :: (a -> b -> c) -> StateT s m a -> StateT s m b -> StateT s m c Source # (*>) :: StateT s m a -> StateT s m b -> StateT s m b Source # (<*) :: StateT s m a -> StateT s m b -> StateT s m a Source # | |
(Functor m, Monad m) => Applicative (StateT s m) | |
Defined in Control.Monad.Trans.State.Strict pure :: a -> StateT s m a Source # (<*>) :: StateT s m (a -> b) -> StateT s m a -> StateT s m b Source # liftA2 :: (a -> b -> c) -> StateT s m a -> StateT s m b -> StateT s m c Source # (*>) :: StateT s m a -> StateT s m b -> StateT s m b Source # (<*) :: StateT s m a -> StateT s m b -> StateT s m a Source # | |
(Monoid w, Applicative m) => Applicative (WriterT w m) | |
Defined in Control.Monad.Trans.Writer.Lazy pure :: a -> WriterT w m a Source # (<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b Source # liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c Source # (*>) :: WriterT w m a -> WriterT w m b -> WriterT w m b Source # (<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a Source # | |
(Monoid w, Applicative m) => Applicative (WriterT w m) | |
Defined in Control.Monad.Trans.Writer.Strict pure :: a -> WriterT w m a Source # (<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b Source # liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c Source # (*>) :: WriterT w m a -> WriterT w m b -> WriterT w m b Source # (<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a Source # | |
(Monoid a, Monoid b) => Applicative ((,,) a b) | Since: base-4.14.0.0 |
Defined in GHC.Base | |
(Applicative f, Applicative g) => Applicative (Product f g) | Since: base-4.9.0.0 |
Defined in Data.Functor.Product pure :: a -> Product f g a Source # (<*>) :: Product f g (a -> b) -> Product f g a -> Product f g b Source # liftA2 :: (a -> b -> c) -> Product f g a -> Product f g b -> Product f g c Source # (*>) :: Product f g a -> Product f g b -> Product f g b Source # (<*) :: Product f g a -> Product f g b -> Product f g a Source # | |
(Applicative f, Applicative g) => Applicative (f :*: g) | Since: base-4.9.0.0 |
Defined in GHC.Generics | |
Monoid c => Applicative (K1 i c :: Type -> Type) | Since: base-4.12.0.0 |
(Monad f, Applicative f) => Applicative (WhenMatched f x y) | Equivalent to Since: containers-0.5.9 |
Defined in Data.IntMap.Internal pure :: a -> WhenMatched f x y a Source # (<*>) :: WhenMatched f x y (a -> b) -> WhenMatched f x y a -> WhenMatched f x y b Source # liftA2 :: (a -> b -> c) -> WhenMatched f x y a -> WhenMatched f x y b -> WhenMatched f x y c Source # (*>) :: WhenMatched f x y a -> WhenMatched f x y b -> WhenMatched f x y b Source # (<*) :: WhenMatched f x y a -> WhenMatched f x y b -> WhenMatched f x y a Source # | |
(Applicative f, Monad f) => Applicative (WhenMissing f k x) | Equivalent to Since: containers-0.5.9 |
Defined in Data.Map.Internal pure :: a -> WhenMissing f k x a Source # (<*>) :: WhenMissing f k x (a -> b) -> WhenMissing f k x a -> WhenMissing f k x b Source # liftA2 :: (a -> b -> c) -> WhenMissing f k x a -> WhenMissing f k x b -> WhenMissing f k x c Source # (*>) :: WhenMissing f k x a -> WhenMissing f k x b -> WhenMissing f k x b Source # (<*) :: WhenMissing f k x a -> WhenMissing f k x b -> WhenMissing f k x a Source # | |
Applicative (ContT r m) | |
Defined in Control.Monad.Trans.Cont | |
(Monoid a, Monoid b, Monoid c) => Applicative ((,,,) a b c) | Since: base-4.14.0.0 |
Defined in GHC.Base pure :: a0 -> (a, b, c, a0) Source # (<*>) :: (a, b, c, a0 -> b0) -> (a, b, c, a0) -> (a, b, c, b0) Source # liftA2 :: (a0 -> b0 -> c0) -> (a, b, c, a0) -> (a, b, c, b0) -> (a, b, c, c0) Source # (*>) :: (a, b, c, a0) -> (a, b, c, b0) -> (a, b, c, b0) Source # (<*) :: (a, b, c, a0) -> (a, b, c, b0) -> (a, b, c, a0) Source # | |
Applicative ((->) r) | Since: base-2.1 |
(Applicative f, Applicative g) => Applicative (Compose f g) | Since: base-4.9.0.0 |
Defined in Data.Functor.Compose pure :: a -> Compose f g a Source # (<*>) :: Compose f g (a -> b) -> Compose f g a -> Compose f g b Source # liftA2 :: (a -> b -> c) -> Compose f g a -> Compose f g b -> Compose f g c Source # (*>) :: Compose f g a -> Compose f g b -> Compose f g b Source # (<*) :: Compose f g a -> Compose f g b -> Compose f g a Source # | |
(Applicative f, Applicative g) => Applicative (f :.: g) | Since: base-4.9.0.0 |
Defined in GHC.Generics | |
Applicative f => Applicative (M1 i c f) | Since: base-4.9.0.0 |
Defined in GHC.Generics | |
(Monad f, Applicative f) => Applicative (WhenMatched f k x y) | Equivalent to Since: containers-0.5.9 |
Defined in Data.Map.Internal pure :: a -> WhenMatched f k x y a Source # (<*>) :: WhenMatched f k x y (a -> b) -> WhenMatched f k x y a -> WhenMatched f k x y b Source # liftA2 :: (a -> b -> c) -> WhenMatched f k x y a -> WhenMatched f k x y b -> WhenMatched f k x y c Source # (*>) :: WhenMatched f k x y a -> WhenMatched f k x y b -> WhenMatched f k x y b Source # (<*) :: WhenMatched f k x y a -> WhenMatched f k x y b -> WhenMatched f k x y a Source # | |
(Functor m, Monad m) => Applicative (RWST r w s m) | |
Defined in Control.Monad.Trans.RWS.CPS pure :: a -> RWST r w s m a Source # (<*>) :: RWST r w s m (a -> b) -> RWST r w s m a -> RWST r w s m b Source # liftA2 :: (a -> b -> c) -> RWST r w s m a -> RWST r w s m b -> RWST r w s m c Source # (*>) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m b Source # (<*) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m a Source # | |
(Monoid w, Functor m, Monad m) => Applicative (RWST r w s m) | |
Defined in Control.Monad.Trans.RWS.Lazy pure :: a -> RWST r w s m a Source # (<*>) :: RWST r w s m (a -> b) -> RWST r w s m a -> RWST r w s m b Source # liftA2 :: (a -> b -> c) -> RWST r w s m a -> RWST r w s m b -> RWST r w s m c Source # (*>) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m b Source # (<*) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m a Source # | |
(Monoid w, Functor m, Monad m) => Applicative (RWST r w s m) | |
Defined in Control.Monad.Trans.RWS.Strict pure :: a -> RWST r w s m a Source # (<*>) :: RWST r w s m (a -> b) -> RWST r w s m a -> RWST r w s m b Source # liftA2 :: (a -> b -> c) -> RWST r w s m a -> RWST r w s m b -> RWST r w s m c Source # (*>) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m b Source # (<*) :: RWST r w s m a -> RWST r w s m b -> RWST r w s m a Source # |
(<$>) :: Functor f => (a -> b) -> f a -> f b infixl 4 Source #
An infix synonym for fmap
.
The name of this operator is an allusion to $
.
Note the similarities between their types:
($) :: (a -> b) -> a -> b (<$>) :: Functor f => (a -> b) -> f a -> f b
Whereas $
is function application, <$>
is function
application lifted over a Functor
.
Examples
Convert from a
to a Maybe
Int
using Maybe
String
show
:
>>>
show <$> Nothing
Nothing>>>
show <$> Just 3
Just "3"
Convert from an
to an
Either
Int
Int
Either
Int
String
using show
:
>>>
show <$> Left 17
Left 17>>>
show <$> Right 17
Right "17"
Double each element of a list:
>>>
(*2) <$> [1,2,3]
[2,4,6]
Apply even
to the second element of a pair:
>>>
even <$> (2,2)
(2,True)
class Monad m => MonadFix (m :: Type -> Type) where Source #
Monads having fixed points with a 'knot-tying' semantics.
Instances of MonadFix
should satisfy the following laws:
- Purity
mfix
(return
. h) =return
(fix
h)- Left shrinking (or Tightening)
mfix
(\x -> a >>= \y -> f x y) = a >>= \y ->mfix
(\x -> f x y)- Sliding
, for strictmfix
(liftM
h . f) =liftM
h (mfix
(f . h))h
.- Nesting
mfix
(\x ->mfix
(\y -> f x y)) =mfix
(\x -> f x x)
This class is used in the translation of the recursive do
notation
supported by GHC and Hugs.
Instances
MonadFix Complex | Since: base-4.15.0.0 |
MonadFix Identity | Since: base-4.8.0.0 |
MonadFix First | Since: base-4.8.0.0 |
MonadFix Last | Since: base-4.8.0.0 |
MonadFix Down | Since: base-4.12.0.0 |
MonadFix First | Since: base-4.9.0.0 |
MonadFix Last | Since: base-4.9.0.0 |
MonadFix Max | Since: base-4.9.0.0 |
MonadFix Min | Since: base-4.9.0.0 |
MonadFix Dual | Since: base-4.8.0.0 |
MonadFix Product | Since: base-4.8.0.0 |
MonadFix Sum | Since: base-4.8.0.0 |
MonadFix Par1 | Since: base-4.9.0.0 |
MonadFix Seq | Since: containers-0.5.11 |
MonadFix Tree | Since: containers-0.5.11 |
MonadFix Ghc Source # | |
MonadFix UniqSM Source # | |
MonadFix IO | Since: base-2.1 |
MonadFix Q | If the function passed to Since: template-haskell-2.17.0.0 |
MonadFix NonEmpty | Since: base-4.9.0.0 |
MonadFix Maybe | Since: base-2.1 |
MonadFix Solo | Since: base-4.15 |
MonadFix [] | Since: base-2.1 |
Defined in Control.Monad.Fix | |
MonadFix (Either e) | Since: base-4.3.0.0 |
MonadFix (ST s) | Since: base-2.1 |
MonadFix (IOEnv env) Source # | |
MonadFix m => MonadFix (ListT m) | |
MonadFix m => MonadFix (MaybeT m) | |
MonadFix f => MonadFix (Ap f) | Since: base-4.12.0.0 |
MonadFix f => MonadFix (Alt f) | Since: base-4.8.0.0 |
MonadFix f => MonadFix (Rec1 f) | Since: base-4.9.0.0 |
(MonadFix m, Error e) => MonadFix (ErrorT e m) | |
MonadFix m => MonadFix (ExceptT e m) | |
MonadFix m => MonadFix (IdentityT m) | |
MonadFix m => MonadFix (ReaderT r m) | |
MonadFix m => MonadFix (StateT s m) | |
MonadFix m => MonadFix (StateT s m) | |
(Monoid w, MonadFix m) => MonadFix (WriterT w m) | |
(Monoid w, MonadFix m) => MonadFix (WriterT w m) | |
(MonadFix f, MonadFix g) => MonadFix (Product f g) | Since: base-4.9.0.0 |
(MonadFix f, MonadFix g) => MonadFix (f :*: g) | Since: base-4.9.0.0 |
MonadFix ((->) r) | Since: base-2.1 |
Defined in Control.Monad.Fix | |
MonadFix f => MonadFix (M1 i c f) | Since: base-4.9.0.0 |
MonadFix m => MonadFix (RWST r w s m) | |
(Monoid w, MonadFix m) => MonadFix (RWST r w s m) | |
(Monoid w, MonadFix m) => MonadFix (RWST r w s m) | |
class Monad m => MonadIO (m :: Type -> Type) where Source #
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:
liftIO :: IO a -> m a Source #
Lift a computation from the IO
monad.
This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations
(i.e. IO
is the base monad for the stack).
Example
import Control.Monad.Trans.State -- from the "transformers" library printState :: Show s => StateT s IO () printState = do state <- get liftIO $ print state
Had we omitted
, we would have ended up with this error:liftIO
• Couldn't match type ‘IO’ with ‘StateT s IO’ Expected type: StateT s IO () Actual type: IO ()
The important part here is the mismatch between StateT s IO ()
and
.IO
()
Luckily, we know of a function that takes an
and returns an IO
a(m a)
:
,
enabling us to run the program and see the expected results:liftIO
> evalStateT printState "hello" "hello" > evalStateT printState 3 3
Instances
MonadIO CoreM Source # | |
MonadIO SimplM Source # | |
MonadIO Hsc Source # | |
MonadIO Ghc Source # | |
MonadIO CompPipeline Source # | |
Defined in GHC.Driver.Pipeline.Monad liftIO :: IO a -> CompPipeline a Source # | |
MonadIO IO | Since: base-4.9.0.0 |
MonadIO Q | |
MonadIO (IOEnv env) Source # | |
MonadIO m => MonadIO (GhcT m) Source # | |
MonadIO m => MonadIO (ListT m) | |
MonadIO m => MonadIO (MaybeT m) | |
MonadIO m => MonadIO (Stream m b) Source # | |
(Error e, MonadIO m) => MonadIO (ErrorT e m) | |
MonadIO m => MonadIO (ExceptT e m) | |
MonadIO m => MonadIO (IdentityT m) | |
MonadIO m => MonadIO (ReaderT r m) | |
MonadIO m => MonadIO (StateT s m) | |
MonadIO m => MonadIO (StateT s m) | |
(Monoid w, MonadIO m) => MonadIO (WriterT w m) | |
(Monoid w, MonadIO m) => MonadIO (WriterT w m) | |
MonadIO m => MonadIO (ContT r m) | |
MonadIO m => MonadIO (RWST r w s m) | |
(Monoid w, MonadIO m) => MonadIO (RWST r w s m) | |
(Monoid w, MonadIO m) => MonadIO (RWST r w s m) | |
zipWith3M_ :: Monad m => (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m () Source #
zipWithAndUnzipM :: Monad m => (a -> b -> m (c, d)) -> [a] -> [b] -> m ([c], [d]) Source #
mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c]) Source #
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 monad.
mapAndUnzip3M :: Monad m => (a -> m (b, c, d)) -> [a] -> m ([b], [c], [d]) Source #
mapAndUnzipM for triples
mapAndUnzip4M :: Monad m => (a -> m (b, c, d, e)) -> [a] -> m ([b], [c], [d], [e]) Source #
mapAndUnzip5M :: Monad m => (a -> m (b, c, d, e, f)) -> [a] -> m ([b], [c], [d], [e], [f]) Source #
:: Monad m | |
=> (acc -> x -> m (acc, y)) | combining function |
-> acc | initial state |
-> [x] | inputs |
-> m (acc, [y]) | final state, outputs |
Monadic version of mapAccumL
concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b] Source #
Monadic version of concatMap
mapMaybeM :: Applicative m => (a -> m (Maybe b)) -> [a] -> m [b] Source #
Applicative version of mapMaybe
fmapEitherM :: Monad m => (a -> m b) -> (c -> m d) -> Either a c -> m (Either b d) Source #
Monadic version of fmap
anyM :: Monad m => (a -> m Bool) -> [a] -> m Bool Source #
Monadic version of any
, aborts the computation at the first True
value
allM :: Monad m => (a -> m Bool) -> [a] -> m Bool Source #
Monad version of all
, aborts the computation at the first False
value
foldlM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b Source #
Left-to-right monadic fold over the elements of a structure.
Given a structure t
with elements (a, b, ..., w, x, y)
, the result of
a fold with an operator function f
is equivalent to:
foldlM f z t = do aa <- f z a bb <- f aa b ... xx <- f ww x yy <- f xx y return yy -- Just @return z@ when the structure is empty
For a Monad m
, given two functions f1 :: a -> m b
and f2 :: b -> m c
,
their Kleisli composition (f1 >=> f2) :: a -> m c
is defined by:
(f1 >=> f2) a = f1 a >>= f2
Another way of thinking about foldlM
is that it amounts to an application
to z
of a Kleisli composition:
foldlM f z t = flip f a >=> flip f b >=> ... >=> flip f x >=> flip f y $ z
The monadic effects of foldlM
are sequenced from left to right.
If at some step the bind operator (
short-circuits (as with, e.g.,
>>=
)mzero
in a MonadPlus
), the evaluated effects will be from an initial
segment of the element sequence. If you want to evaluate the monadic
effects in right-to-left order, or perhaps be able to short-circuit after
processing a tail of the sequence of elements, you'll need to use foldrM
instead.
If the monadic effects don't short-circuit, the outermost application of
f
is to the rightmost element y
, so that, ignoring effects, the result
looks like a left fold:
((((z `f` a) `f` b) ... `f` w) `f` x) `f` y
Examples
Basic usage:
>>>
let f a e = do { print e ; return $ e : a }
>>>
foldlM f [] [0..3]
0 1 2 3 [3,2,1,0]
foldlM_ :: (Monad m, Foldable t) => (a -> b -> m a) -> a -> t b -> m () Source #
Monadic version of foldl that discards its result
foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b Source #
Right-to-left monadic fold over the elements of a structure.
Given a structure t
with elements (a, b, c, ..., x, y)
, the result of
a fold with an operator function f
is equivalent to:
foldrM f z t = do yy <- f y z xx <- f x yy ... bb <- f b cc aa <- f a bb return aa -- Just @return z@ when the structure is empty
For a Monad m
, given two functions f1 :: a -> m b
and f2 :: b -> m c
,
their Kleisli composition (f1 >=> f2) :: a -> m c
is defined by:
(f1 >=> f2) a = f1 a >>= f2
Another way of thinking about foldrM
is that it amounts to an application
to z
of a Kleisli composition:
foldrM f z t = f y >=> f x >=> ... >=> f b >=> f a $ z
The monadic effects of foldrM
are sequenced from right to left, and e.g.
folds of infinite lists will diverge.
If at some step the bind operator (
short-circuits (as with, e.g.,
>>=
)mzero
in a MonadPlus
), the evaluated effects will be from a tail of the
element sequence. If you want to evaluate the monadic effects in
left-to-right order, or perhaps be able to short-circuit after an initial
sequence of elements, you'll need to use foldlM
instead.
If the monadic effects don't short-circuit, the outermost application of
f
is to the leftmost element a
, so that, ignoring effects, the result
looks like a right fold:
a `f` (b `f` (c `f` (... (x `f` (y `f` z))))).
Examples
Basic usage:
>>>
let f i acc = do { print i ; return $ i : acc }
>>>
foldrM f [] [0..3]
3 2 1 0 [0,1,2,3]
maybeMapM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b) Source #
Monadic version of fmap specialised for Maybe
whenM :: Monad m => m Bool -> m () -> m () Source #
Monadic version of when
, taking the condition in the monad
unlessM :: Monad m => m Bool -> m () -> m () Source #
Monadic version of unless
, taking the condition in the monad
filterOutM :: Applicative m => (a -> m Bool) -> [a] -> m [a] Source #
Like filterM
, only it reverses the sense of the test.