Control.Applicative

Copyright	Conor McBride and Ross Paterson 2005
License	BSD-style (see the LICENSE file in the distribution)
Maintainer	libraries@haskell.org
Stability	stable
Portability	portable
Safe Haskell	Trustworthy
Language	Haskell2010

Description

This module describes a structure intermediate between a functor and a monad (technically, a strong lax monoidal functor). Compared with monads, this interface lacks the full power of the binding operation >>=, but

it has more instances.
it is sufficient for many uses, e.g. context-free parsing, or the Traversable class.
instances can perform analysis of computations before they are executed, and thus produce shared optimizations.

This interface was introduced for parsers by Niklas Röjemo, because it admits more sharing than the monadic interface. The names here are mostly based on parsing work by Doaitse Swierstra.

For more details, see Applicative Programming with Effects, by Conor McBride and Ross Paterson.

Applicative functors

class Functor f => Applicative f where Source

A functor with application, providing operations to

embed pure expressions (pure), and
sequence computations and combine their results (<*> and liftA2).

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:

```
u *> v = (id <$ u) <*> v
```
```
u <* v = liftA2 const u v
```

As a consequence of these laws, the Functor instance for f will satisfy

```
fmap f x = pure f <*> x
```

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

```
pure = return
```

m1 <*> m2 = m1 >>= (\x1 -> m2 >>= (\x2 -> return (x1 x2)))

```
(*>) = (>>)
```

(which implies that pure and <*> satisfy the applicative functor laws).

Minimal complete definition

pure, ((<*>) | liftA2)

Methods

pure :: a -> f a Source

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

Expand

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

Expand

>>> 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

Expand

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

Instances details

Applicative ZipList Source	`f <$> ZipList xs1 <> ... <> ZipList xsN = ZipList (zipWithN f xs1 ... xsN)` where `zipWithN` refers to the `zipWith` function of the appropriate arity (`zipWith`, `zipWith3`, `zipWith4`, ...). For example: `(\a b c -> stimes c [a, b]) <$> 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
Instance details Defined in Control.Applicative Methods pure :: a -> ZipList a Source (<>) :: ZipList (a -> b) -> ZipList a -> ZipList b Source liftA2 :: (a -> b -> c) -> ZipList a -> ZipList b -> ZipList c Source (>) :: ZipList a -> ZipList b -> ZipList b Source (<*) :: ZipList a -> ZipList b -> ZipList a Source
Applicative Complex Source	Since: base-4.9.0.0
Instance details Defined in Data.Complex Methods pure :: a -> Complex a Source (<>) :: Complex (a -> b) -> Complex a -> Complex b Source liftA2 :: (a -> b -> c) -> Complex a -> Complex b -> Complex c Source (>) :: Complex a -> Complex b -> Complex b Source (<*) :: Complex a -> Complex b -> Complex a Source
Applicative Identity Source	Since: base-4.8.0.0
Instance details Defined in Data.Functor.Identity Methods pure :: a -> Identity a Source (<>) :: Identity (a -> b) -> Identity a -> Identity b Source liftA2 :: (a -> b -> c) -> Identity a -> Identity b -> Identity c Source (>) :: Identity a -> Identity b -> Identity b Source (<*) :: Identity a -> Identity b -> Identity a Source
Applicative First Source	Since: base-4.8.0.0
Instance details Defined in Data.Monoid Methods pure :: a -> First a Source (<>) :: First (a -> b) -> First a -> First b Source liftA2 :: (a -> b -> c) -> First a -> First b -> First c Source (>) :: First a -> First b -> First b Source (<*) :: First a -> First b -> First a Source
Applicative Last Source	Since: base-4.8.0.0
Instance details Defined in Data.Monoid Methods pure :: a -> Last a Source (<>) :: Last (a -> b) -> Last a -> Last b Source liftA2 :: (a -> b -> c) -> Last a -> Last b -> Last c Source (>) :: Last a -> Last b -> Last b Source (<*) :: Last a -> Last b -> Last a Source
Applicative Down Source	Since: base-4.11.0.0
Instance details Defined in Data.Ord Methods pure :: a -> Down a Source (<>) :: Down (a -> b) -> Down a -> Down b Source liftA2 :: (a -> b -> c) -> Down a -> Down b -> Down c Source (>) :: Down a -> Down b -> Down b Source (<*) :: Down a -> Down b -> Down a Source
Applicative First Source	Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods pure :: a -> First a Source (<>) :: First (a -> b) -> First a -> First b Source liftA2 :: (a -> b -> c) -> First a -> First b -> First c Source (>) :: First a -> First b -> First b Source (<*) :: First a -> First b -> First a Source
Applicative Last Source	Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods pure :: a -> Last a Source (<>) :: Last (a -> b) -> Last a -> Last b Source liftA2 :: (a -> b -> c) -> Last a -> Last b -> Last c Source (>) :: Last a -> Last b -> Last b Source (<*) :: Last a -> Last b -> Last a Source
Applicative Max Source	Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods pure :: a -> Max a Source (<>) :: Max (a -> b) -> Max a -> Max b Source liftA2 :: (a -> b -> c) -> Max a -> Max b -> Max c Source (>) :: Max a -> Max b -> Max b Source (<*) :: Max a -> Max b -> Max a Source
Applicative Min Source	Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods pure :: a -> Min a Source (<>) :: Min (a -> b) -> Min a -> Min b Source liftA2 :: (a -> b -> c) -> Min a -> Min b -> Min c Source (>) :: Min a -> Min b -> Min b Source (<*) :: Min a -> Min b -> Min a Source
Applicative Dual Source	Since: base-4.8.0.0
Instance details Defined in Data.Semigroup.Internal Methods pure :: a -> Dual a Source (<>) :: Dual (a -> b) -> Dual a -> Dual b Source liftA2 :: (a -> b -> c) -> Dual a -> Dual b -> Dual c Source (>) :: Dual a -> Dual b -> Dual b Source (<*) :: Dual a -> Dual b -> Dual a Source
Applicative Product Source	Since: base-4.8.0.0
Instance details Defined in Data.Semigroup.Internal Methods pure :: a -> Product a Source (<>) :: Product (a -> b) -> Product a -> Product b Source liftA2 :: (a -> b -> c) -> Product a -> Product b -> Product c Source (>) :: Product a -> Product b -> Product b Source (<*) :: Product a -> Product b -> Product a Source
Applicative Sum Source	Since: base-4.8.0.0
Instance details Defined in Data.Semigroup.Internal Methods pure :: a -> Sum a Source (<>) :: Sum (a -> b) -> Sum a -> Sum b Source liftA2 :: (a -> b -> c) -> Sum a -> Sum b -> Sum c Source (>) :: Sum a -> Sum b -> Sum b Source (<*) :: Sum a -> Sum b -> Sum a Source
Applicative STM Source	Since: base-4.8.0.0
Instance details Defined in GHC.Conc.Sync Methods pure :: a -> STM a Source (<>) :: STM (a -> b) -> STM a -> STM b Source liftA2 :: (a -> b -> c) -> STM a -> STM b -> STM c Source (>) :: STM a -> STM b -> STM b Source (<*) :: STM a -> STM b -> STM a Source
Applicative NoIO Source	Since: base-4.8.0.0
Instance details Defined in GHC.GHCi Methods pure :: a -> NoIO a Source (<>) :: NoIO (a -> b) -> NoIO a -> NoIO b Source liftA2 :: (a -> b -> c) -> NoIO a -> NoIO b -> NoIO c Source (>) :: NoIO a -> NoIO b -> NoIO b Source (<*) :: NoIO a -> NoIO b -> NoIO a Source
Applicative Par1 Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> Par1 a Source (<>) :: Par1 (a -> b) -> Par1 a -> Par1 b Source liftA2 :: (a -> b -> c) -> Par1 a -> Par1 b -> Par1 c Source (>) :: Par1 a -> Par1 b -> Par1 b Source (<*) :: Par1 a -> Par1 b -> Par1 a Source
Applicative ReadP Source	Since: base-4.6.0.0
Instance details Defined in Text.ParserCombinators.ReadP Methods pure :: a -> ReadP a Source (<>) :: ReadP (a -> b) -> ReadP a -> ReadP b Source liftA2 :: (a -> b -> c) -> ReadP a -> ReadP b -> ReadP c Source (>) :: ReadP a -> ReadP b -> ReadP b Source (<*) :: ReadP a -> ReadP b -> ReadP a Source
Applicative ReadPrec Source	Since: base-4.6.0.0
Instance details Defined in Text.ParserCombinators.ReadPrec Methods pure :: a -> ReadPrec a Source (<>) :: ReadPrec (a -> b) -> ReadPrec a -> ReadPrec b Source liftA2 :: (a -> b -> c) -> ReadPrec a -> ReadPrec b -> ReadPrec c Source (>) :: ReadPrec a -> ReadPrec b -> ReadPrec b Source (<*) :: ReadPrec a -> ReadPrec b -> ReadPrec a Source
Applicative IO Source	Since: base-2.1
Instance details Defined in GHC.Base Methods pure :: a -> IO a Source (<>) :: IO (a -> b) -> IO a -> IO b Source liftA2 :: (a -> b -> c) -> IO a -> IO b -> IO c Source (>) :: IO a -> IO b -> IO b Source (<*) :: IO a -> IO b -> IO a Source
Applicative NonEmpty Source	Since: base-4.9.0.0
Instance details Defined in GHC.Base Methods pure :: a -> NonEmpty a Source (<>) :: NonEmpty (a -> b) -> NonEmpty a -> NonEmpty b Source liftA2 :: (a -> b -> c) -> NonEmpty a -> NonEmpty b -> NonEmpty c Source (>) :: NonEmpty a -> NonEmpty b -> NonEmpty b Source (<*) :: NonEmpty a -> NonEmpty b -> NonEmpty a Source
Applicative Maybe Source	Since: base-2.1
Instance details Defined in GHC.Base Methods pure :: a -> Maybe a Source (<>) :: Maybe (a -> b) -> Maybe a -> Maybe b Source liftA2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c Source (>) :: Maybe a -> Maybe b -> Maybe b Source (<*) :: Maybe a -> Maybe b -> Maybe a Source
Applicative Solo Source	Since: base-4.15
Instance details Defined in GHC.Base Methods pure :: a -> Solo a Source (<>) :: Solo (a -> b) -> Solo a -> Solo b Source liftA2 :: (a -> b -> c) -> Solo a -> Solo b -> Solo c Source (>) :: Solo a -> Solo b -> Solo b Source (<*) :: Solo a -> Solo b -> Solo a Source
Applicative [] Source	Since: base-2.1
Instance details Defined in GHC.Base Methods pure :: a -> [a] Source (<>) :: [a -> b] -> [a] -> [b] Source liftA2 :: (a -> b -> c) -> [a] -> [b] -> [c] Source (>) :: [a] -> [b] -> [b] Source (<*) :: [a] -> [b] -> [a] Source
Monad m => Applicative (WrappedMonad m) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods 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) Source	Since: base-4.6.0.0
Instance details Defined in Control.Arrow Methods 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 (ST s) Source	Since: base-2.1
Instance details Defined in Control.Monad.ST.Lazy.Imp Methods pure :: a -> ST s a Source (<>) :: ST s (a -> b) -> ST s a -> ST s b Source liftA2 :: (a -> b -> c) -> ST s a -> ST s b -> ST s c Source (>) :: ST s a -> ST s b -> ST s b Source (<*) :: ST s a -> ST s b -> ST s a Source
Applicative (Either e) Source	Since: base-3.0
Instance details Defined in Data.Either Methods pure :: a -> Either e a Source (<>) :: Either e (a -> b) -> Either e a -> Either e b Source liftA2 :: (a -> b -> c) -> Either e a -> Either e b -> Either e c Source (>) :: Either e a -> Either e b -> Either e b Source (<*) :: Either e a -> Either e b -> Either e a Source
Applicative (Proxy :: Type -> Type) Source	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods pure :: a -> Proxy a Source (<>) :: Proxy (a -> b) -> Proxy a -> Proxy b Source liftA2 :: (a -> b -> c) -> Proxy a -> Proxy b -> Proxy c Source (>) :: Proxy a -> Proxy b -> Proxy b Source (<*) :: Proxy a -> Proxy b -> Proxy a Source
Applicative (U1 :: Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> U1 a Source (<>) :: U1 (a -> b) -> U1 a -> U1 b Source liftA2 :: (a -> b -> c) -> U1 a -> U1 b -> U1 c Source (>) :: U1 a -> U1 b -> U1 b Source (<*) :: U1 a -> U1 b -> U1 a Source
Applicative (ST s) Source	Since: base-4.4.0.0
Instance details Defined in GHC.ST Methods pure :: a -> ST s a Source (<>) :: ST s (a -> b) -> ST s a -> ST s b Source liftA2 :: (a -> b -> c) -> ST s a -> ST s b -> ST s c Source (>) :: ST s a -> ST s b -> ST s b Source (<*) :: ST s a -> ST s b -> ST s a Source
Monoid a => Applicative ((,) a) Source	For tuples, the `Monoid` constraint on `a` determines how the first values merge. For example, `String`s concatenate: `("hello ", (+15)) <*> ("world!", 2002) ("hello world!",2017)` Since: base-2.1
Instance details Defined in GHC.Base Methods pure :: a0 -> (a, a0) Source (<>) :: (a, a0 -> b) -> (a, a0) -> (a, b) Source liftA2 :: (a0 -> b -> c) -> (a, a0) -> (a, b) -> (a, c) Source (>) :: (a, a0) -> (a, b) -> (a, b) Source (<*) :: (a, a0) -> (a, b) -> (a, a0) Source
Arrow a => Applicative (WrappedArrow a b) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods 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) Source	Since: base-4.14.0.0
Instance details Defined in Control.Arrow Methods 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) Source	Since: base-2.0.1
Instance details Defined in Data.Functor.Const Methods pure :: a -> Const m a Source (<>) :: Const m (a -> b) -> Const m a -> Const m b Source liftA2 :: (a -> b -> c) -> Const m a -> Const m b -> Const m c Source (>) :: Const m a -> Const m b -> Const m b Source (<*) :: Const m a -> Const m b -> Const m a Source
Applicative f => Applicative (Ap f) Source	Since: base-4.12.0.0
Instance details Defined in Data.Monoid Methods pure :: a -> Ap f a Source (<>) :: Ap f (a -> b) -> Ap f a -> Ap f b Source liftA2 :: (a -> b -> c) -> Ap f a -> Ap f b -> Ap f c Source (>) :: Ap f a -> Ap f b -> Ap f b Source (<*) :: Ap f a -> Ap f b -> Ap f a Source
Applicative f => Applicative (Alt f) Source	Since: base-4.8.0.0
Instance details Defined in Data.Semigroup.Internal Methods pure :: a -> Alt f a Source (<>) :: Alt f (a -> b) -> Alt f a -> Alt f b Source liftA2 :: (a -> b -> c) -> Alt f a -> Alt f b -> Alt f c Source (>) :: Alt f a -> Alt f b -> Alt f b Source (<*) :: Alt f a -> Alt f b -> Alt f a Source
(Generic1 f, Applicative (Rep1 f)) => Applicative (Generically1 f) Source	Since: base-4.17.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> Generically1 f a Source (<>) :: Generically1 f (a -> b) -> Generically1 f a -> Generically1 f b Source liftA2 :: (a -> b -> c) -> Generically1 f a -> Generically1 f b -> Generically1 f c Source (>) :: Generically1 f a -> Generically1 f b -> Generically1 f b Source (<*) :: Generically1 f a -> Generically1 f b -> Generically1 f a Source
Applicative f => Applicative (Rec1 f) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> Rec1 f a Source (<>) :: Rec1 f (a -> b) -> Rec1 f a -> Rec1 f b Source liftA2 :: (a -> b -> c) -> Rec1 f a -> Rec1 f b -> Rec1 f c Source (>) :: Rec1 f a -> Rec1 f b -> Rec1 f b Source (<*) :: Rec1 f a -> Rec1 f b -> Rec1 f a Source
(Monoid a, Monoid b) => Applicative ((,,) a b) Source	Since: base-4.14.0.0
Instance details Defined in GHC.Base Methods pure :: a0 -> (a, b, a0) Source (<>) :: (a, b, a0 -> b0) -> (a, b, a0) -> (a, b, b0) Source liftA2 :: (a0 -> b0 -> c) -> (a, b, a0) -> (a, b, b0) -> (a, b, c) Source (>) :: (a, b, a0) -> (a, b, b0) -> (a, b, b0) Source (<*) :: (a, b, a0) -> (a, b, b0) -> (a, b, a0) Source
(Applicative f, Applicative g) => Applicative (Product f g) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Product Methods 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) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> (f :: g) a Source (<>) :: (f :: g) (a -> b) -> (f :: g) a -> (f :: g) b Source liftA2 :: (a -> b -> c) -> (f :: g) a -> (f :: g) b -> (f :: g) c Source (>) :: (f :: g) a -> (f :: g) b -> (f :: g) b Source (<) :: (f :: g) a -> (f :: g) b -> (f :: g) a Source
Monoid c => Applicative (K1 i c :: Type -> Type) Source	Since: base-4.12.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> K1 i c a Source (<>) :: K1 i c (a -> b) -> K1 i c a -> K1 i c b Source liftA2 :: (a -> b -> c0) -> K1 i c a -> K1 i c b -> K1 i c c0 Source (>) :: K1 i c a -> K1 i c b -> K1 i c b Source (<*) :: K1 i c a -> K1 i c b -> K1 i c a Source
(Monoid a, Monoid b, Monoid c) => Applicative ((,,,) a b c) Source	Since: base-4.14.0.0
Instance details Defined in GHC.Base Methods 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) Source	Since: base-2.1
Instance details Defined in GHC.Base Methods pure :: a -> r -> a Source (<>) :: (r -> (a -> b)) -> (r -> a) -> r -> b Source liftA2 :: (a -> b -> c) -> (r -> a) -> (r -> b) -> r -> c Source (>) :: (r -> a) -> (r -> b) -> r -> b Source (<*) :: (r -> a) -> (r -> b) -> r -> a Source
(Applicative f, Applicative g) => Applicative (Compose f g) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Compose Methods 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) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> (f :.: g) a Source (<>) :: (f :.: g) (a -> b) -> (f :.: g) a -> (f :.: g) b Source liftA2 :: (a -> b -> c) -> (f :.: g) a -> (f :.: g) b -> (f :.: g) c Source (>) :: (f :.: g) a -> (f :.: g) b -> (f :.: g) b Source (<*) :: (f :.: g) a -> (f :.: g) b -> (f :.: g) a Source
Applicative f => Applicative (M1 i c f) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods pure :: a -> M1 i c f a Source (<>) :: M1 i c f (a -> b) -> M1 i c f a -> M1 i c f b Source liftA2 :: (a -> b -> c0) -> M1 i c f a -> M1 i c f b -> M1 i c f c0 Source (>) :: M1 i c f a -> M1 i c f b -> M1 i c f b Source (<*) :: M1 i c f a -> M1 i c f b -> M1 i c f a Source

Alternatives

class Applicative f => Alternative f where Source

A monoid on applicative functors.

If defined, some and many should be the least solutions of the equations:

```
some v = (:) <$> v <*> many v
```
```
many v = some v <|> pure []
```

Minimal complete definition

empty, (<|>)

Methods

empty :: f a Source

The identity of <|>

(<|>) :: f a -> f a -> f a infixl 3 Source

An associative binary operation

some :: f a -> f [a] Source

One or more.

many :: f a -> f [a] Source

Zero or more.

Instances

Instances details

Alternative ZipList Source	Since: base-4.11.0.0
Instance details Defined in Control.Applicative Methods empty :: ZipList a Source (<\|>) :: ZipList a -> ZipList a -> ZipList a Source some :: ZipList a -> ZipList [a] Source many :: ZipList a -> ZipList [a] Source
Alternative STM Source	Since: base-4.8.0.0
Instance details Defined in GHC.Conc.Sync Methods empty :: STM a Source (<\|>) :: STM a -> STM a -> STM a Source some :: STM a -> STM [a] Source many :: STM a -> STM [a] Source
Alternative ReadP Source	Since: base-4.6.0.0
Instance details Defined in Text.ParserCombinators.ReadP Methods empty :: ReadP a Source (<\|>) :: ReadP a -> ReadP a -> ReadP a Source some :: ReadP a -> ReadP [a] Source many :: ReadP a -> ReadP [a] Source
Alternative ReadPrec Source	Since: base-4.6.0.0
Instance details Defined in Text.ParserCombinators.ReadPrec Methods empty :: ReadPrec a Source (<\|>) :: ReadPrec a -> ReadPrec a -> ReadPrec a Source some :: ReadPrec a -> ReadPrec [a] Source many :: ReadPrec a -> ReadPrec [a] Source
Alternative IO Source	Since: base-4.9.0.0
Instance details Defined in GHC.Base Methods empty :: IO a Source (<\|>) :: IO a -> IO a -> IO a Source some :: IO a -> IO [a] Source many :: IO a -> IO [a] Source
Alternative Maybe Source	Since: base-2.1
Instance details Defined in GHC.Base Methods empty :: Maybe a Source (<\|>) :: Maybe a -> Maybe a -> Maybe a Source some :: Maybe a -> Maybe [a] Source many :: Maybe a -> Maybe [a] Source
Alternative [] Source	Since: base-2.1
Instance details Defined in GHC.Base Methods empty :: [a] Source (<\|>) :: [a] -> [a] -> [a] Source some :: [a] -> [[a]] Source many :: [a] -> [[a]] Source
MonadPlus m => Alternative (WrappedMonad m) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods empty :: WrappedMonad m a Source (<\|>) :: WrappedMonad m a -> WrappedMonad m a -> WrappedMonad m a Source some :: WrappedMonad m a -> WrappedMonad m [a] Source many :: WrappedMonad m a -> WrappedMonad m [a] Source
ArrowPlus a => Alternative (ArrowMonad a) Source	Since: base-4.6.0.0
Instance details Defined in Control.Arrow Methods empty :: ArrowMonad a a0 Source (<\|>) :: ArrowMonad a a0 -> ArrowMonad a a0 -> ArrowMonad a a0 Source some :: ArrowMonad a a0 -> ArrowMonad a [a0] Source many :: ArrowMonad a a0 -> ArrowMonad a [a0] Source
Alternative (Proxy :: Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Proxy Methods empty :: Proxy a Source (<\|>) :: Proxy a -> Proxy a -> Proxy a Source some :: Proxy a -> Proxy [a] Source many :: Proxy a -> Proxy [a] Source
Alternative (U1 :: Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods empty :: U1 a Source (<\|>) :: U1 a -> U1 a -> U1 a Source some :: U1 a -> U1 [a] Source many :: U1 a -> U1 [a] Source
(ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods empty :: WrappedArrow a b a0 Source (<\|>) :: WrappedArrow a b a0 -> WrappedArrow a b a0 -> WrappedArrow a b a0 Source some :: WrappedArrow a b a0 -> WrappedArrow a b [a0] Source many :: WrappedArrow a b a0 -> WrappedArrow a b [a0] Source
Alternative m => Alternative (Kleisli m a) Source	Since: base-4.14.0.0
Instance details Defined in Control.Arrow Methods empty :: Kleisli m a a0 Source (<\|>) :: Kleisli m a a0 -> Kleisli m a a0 -> Kleisli m a a0 Source some :: Kleisli m a a0 -> Kleisli m a [a0] Source many :: Kleisli m a a0 -> Kleisli m a [a0] Source
Alternative f => Alternative (Ap f) Source	Since: base-4.12.0.0
Instance details Defined in Data.Monoid Methods empty :: Ap f a Source (<\|>) :: Ap f a -> Ap f a -> Ap f a Source some :: Ap f a -> Ap f [a] Source many :: Ap f a -> Ap f [a] Source
Alternative f => Alternative (Alt f) Source	Since: base-4.8.0.0
Instance details Defined in Data.Semigroup.Internal Methods empty :: Alt f a Source (<\|>) :: Alt f a -> Alt f a -> Alt f a Source some :: Alt f a -> Alt f [a] Source many :: Alt f a -> Alt f [a] Source
(Generic1 f, Alternative (Rep1 f)) => Alternative (Generically1 f) Source	Since: base-4.17.0.0
Instance details Defined in GHC.Generics Methods empty :: Generically1 f a Source (<\|>) :: Generically1 f a -> Generically1 f a -> Generically1 f a Source some :: Generically1 f a -> Generically1 f [a] Source many :: Generically1 f a -> Generically1 f [a] Source
Alternative f => Alternative (Rec1 f) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods empty :: Rec1 f a Source (<\|>) :: Rec1 f a -> Rec1 f a -> Rec1 f a Source some :: Rec1 f a -> Rec1 f [a] Source many :: Rec1 f a -> Rec1 f [a] Source
(Alternative f, Alternative g) => Alternative (Product f g) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Product Methods empty :: Product f g a Source (<\|>) :: Product f g a -> Product f g a -> Product f g a Source some :: Product f g a -> Product f g [a] Source many :: Product f g a -> Product f g [a] Source
(Alternative f, Alternative g) => Alternative (f :*: g) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods empty :: (f :: g) a Source (<\|>) :: (f :: g) a -> (f :: g) a -> (f :: g) a Source some :: (f :: g) a -> (f :: g) [a] Source many :: (f :: g) a -> (f :: g) [a] Source
(Alternative f, Applicative g) => Alternative (Compose f g) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Compose Methods empty :: Compose f g a Source (<\|>) :: Compose f g a -> Compose f g a -> Compose f g a Source some :: Compose f g a -> Compose f g [a] Source many :: Compose f g a -> Compose f g [a] Source
(Alternative f, Applicative g) => Alternative (f :.: g) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods empty :: (f :.: g) a Source (<\|>) :: (f :.: g) a -> (f :.: g) a -> (f :.: g) a Source some :: (f :.: g) a -> (f :.: g) [a] Source many :: (f :.: g) a -> (f :.: g) [a] Source
Alternative f => Alternative (M1 i c f) Source	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods empty :: M1 i c f a Source (<\|>) :: M1 i c f a -> M1 i c f a -> M1 i c f a Source some :: M1 i c f a -> M1 i c f [a] Source many :: M1 i c f a -> M1 i c f [a] Source

Instances

newtype Const a b Source

The Const functor.

Constructors

Const
Fields getConst :: a

Instances

Instances details

Generic1 (Const a :: k -> Type) Source
Instance details Defined in Data.Functor.Const Associated Types type Rep1 (Const a) :: k -> Type Source Methods from1 :: forall (a0 :: k0). Const a a0 -> Rep1 (Const a) a0 Source to1 :: forall (a0 :: k0). Rep1 (Const a) a0 -> Const a a0 Source
Bifoldable (Const :: Type -> Type -> Type) Source	Since: base-4.10.0.0
Instance details Defined in Data.Bifoldable Methods bifold :: Monoid m => Const m m -> m Source bifoldMap :: Monoid m => (a -> m) -> (b -> m) -> Const a b -> m Source bifoldr :: (a -> c -> c) -> (b -> c -> c) -> c -> Const a b -> c Source bifoldl :: (c -> a -> c) -> (c -> b -> c) -> c -> Const a b -> c Source
Bifunctor (Const :: Type -> Type -> Type) Source	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> Const a c -> Const b d Source first :: (a -> b) -> Const a c -> Const b c Source second :: (b -> c) -> Const a b -> Const a c Source
Bitraversable (Const :: Type -> Type -> Type) Source	Since: base-4.10.0.0
Instance details Defined in Data.Bitraversable Methods bitraverse :: Applicative f => (a -> f c) -> (b -> f d) -> Const a b -> f (Const c d) Source
Eq2 (Const :: Type -> Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftEq2 :: (a -> b -> Bool) -> (c -> d -> Bool) -> Const a c -> Const b d -> Bool Source
Ord2 (Const :: Type -> Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftCompare2 :: (a -> b -> Ordering) -> (c -> d -> Ordering) -> Const a c -> Const b d -> Ordering Source
Read2 (Const :: Type -> Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftReadsPrec2 :: (Int -> ReadS a) -> ReadS [a] -> (Int -> ReadS b) -> ReadS [b] -> Int -> ReadS (Const a b) Source liftReadList2 :: (Int -> ReadS a) -> ReadS [a] -> (Int -> ReadS b) -> ReadS [b] -> ReadS [Const a b] Source liftReadPrec2 :: ReadPrec a -> ReadPrec [a] -> ReadPrec b -> ReadPrec [b] -> ReadPrec (Const a b) Source liftReadListPrec2 :: ReadPrec a -> ReadPrec [a] -> ReadPrec b -> ReadPrec [b] -> ReadPrec [Const a b] Source
Show2 (Const :: Type -> Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftShowsPrec2 :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> (Int -> b -> ShowS) -> ([b] -> ShowS) -> Int -> Const a b -> ShowS Source liftShowList2 :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> (Int -> b -> ShowS) -> ([b] -> ShowS) -> [Const a b] -> ShowS Source
Foldable (Const m :: Type -> Type) Source	Since: base-4.7.0.0
Instance details Defined in Data.Functor.Const Methods fold :: Monoid m0 => Const m m0 -> m0 Source foldMap :: Monoid m0 => (a -> m0) -> Const m a -> m0 Source foldMap' :: Monoid m0 => (a -> m0) -> Const m a -> m0 Source foldr :: (a -> b -> b) -> b -> Const m a -> b Source foldr' :: (a -> b -> b) -> b -> Const m a -> b Source foldl :: (b -> a -> b) -> b -> Const m a -> b Source foldl' :: (b -> a -> b) -> b -> Const m a -> b Source foldr1 :: (a -> a -> a) -> Const m a -> a Source foldl1 :: (a -> a -> a) -> Const m a -> a Source toList :: Const m a -> [a] Source null :: Const m a -> Bool Source length :: Const m a -> Int Source elem :: Eq a => a -> Const m a -> Bool Source maximum :: Ord a => Const m a -> a Source minimum :: Ord a => Const m a -> a Source sum :: Num a => Const m a -> a Source product :: Num a => Const m a -> a Source
Eq a => Eq1 (Const a :: Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftEq :: (a0 -> b -> Bool) -> Const a a0 -> Const a b -> Bool Source
Ord a => Ord1 (Const a :: Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftCompare :: (a0 -> b -> Ordering) -> Const a a0 -> Const a b -> Ordering Source
Read a => Read1 (Const a :: Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftReadsPrec :: (Int -> ReadS a0) -> ReadS [a0] -> Int -> ReadS (Const a a0) Source liftReadList :: (Int -> ReadS a0) -> ReadS [a0] -> ReadS [Const a a0] Source liftReadPrec :: ReadPrec a0 -> ReadPrec [a0] -> ReadPrec (Const a a0) Source liftReadListPrec :: ReadPrec a0 -> ReadPrec [a0] -> ReadPrec [Const a a0] Source
Show a => Show1 (Const a :: Type -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftShowsPrec :: (Int -> a0 -> ShowS) -> ([a0] -> ShowS) -> Int -> Const a a0 -> ShowS Source liftShowList :: (Int -> a0 -> ShowS) -> ([a0] -> ShowS) -> [Const a a0] -> ShowS Source
Contravariant (Const a :: Type -> Type) Source
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a0) -> Const a a0 -> Const a a' Source (>$) :: b -> Const a b -> Const a a0 Source
Traversable (Const m :: Type -> Type) Source	Since: base-4.7.0.0
Instance details Defined in Data.Traversable Methods traverse :: Applicative f => (a -> f b) -> Const m a -> f (Const m b) Source sequenceA :: Applicative f => Const m (f a) -> f (Const m a) Source mapM :: Monad m0 => (a -> m0 b) -> Const m a -> m0 (Const m b) Source sequence :: Monad m0 => Const m (m0 a) -> m0 (Const m a) Source
Monoid m => Applicative (Const m :: Type -> Type) Source	Since: base-2.0.1
Instance details Defined in Data.Functor.Const Methods pure :: a -> Const m a Source (<>) :: Const m (a -> b) -> Const m a -> Const m b Source liftA2 :: (a -> b -> c) -> Const m a -> Const m b -> Const m c Source (>) :: Const m a -> Const m b -> Const m b Source (<*) :: Const m a -> Const m b -> Const m a Source
Functor (Const m :: Type -> Type) Source	Since: base-2.1
Instance details Defined in Data.Functor.Const Methods fmap :: (a -> b) -> Const m a -> Const m b Source (<$) :: a -> Const m b -> Const m a Source
(Typeable k, Data a, Typeable b) => Data (Const a b) Source	Since: base-4.10.0.0
Instance details Defined in Data.Data Methods gfoldl :: (forall d b0. Data d => c (d -> b0) -> d -> c b0) -> (forall g. g -> c g) -> Const a b -> c (Const a b) Source gunfold :: (forall b0 r. Data b0 => c (b0 -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Const a b) Source toConstr :: Const a b -> Constr Source dataTypeOf :: Const a b -> DataType Source dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Const a b)) Source dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Const a b)) Source gmapT :: (forall b0. Data b0 => b0 -> b0) -> Const a b -> Const a b Source gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Const a b -> r Source gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Const a b -> r Source gmapQ :: (forall d. Data d => d -> u) -> Const a b -> [u] Source gmapQi :: Int -> (forall d. Data d => d -> u) -> Const a b -> u Source gmapM :: Monad m => (forall d. Data d => d -> m d) -> Const a b -> m (Const a b) Source gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Const a b -> m (Const a b) Source gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Const a b -> m (Const a b) Source
IsString a => IsString (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.String Methods fromString :: String -> Const a b Source
Storable a => Storable (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods sizeOf :: Const a b -> Int Source alignment :: Const a b -> Int Source peekElemOff :: Ptr (Const a b) -> Int -> IO (Const a b) Source pokeElemOff :: Ptr (Const a b) -> Int -> Const a b -> IO () Source peekByteOff :: Ptr b0 -> Int -> IO (Const a b) Source pokeByteOff :: Ptr b0 -> Int -> Const a b -> IO () Source peek :: Ptr (Const a b) -> IO (Const a b) Source poke :: Ptr (Const a b) -> Const a b -> IO () Source
Monoid a => Monoid (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods mempty :: Const a b Source mappend :: Const a b -> Const a b -> Const a b Source mconcat :: [Const a b] -> Const a b Source
Semigroup a => Semigroup (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods (<>) :: Const a b -> Const a b -> Const a b Source sconcat :: NonEmpty (Const a b) -> Const a b Source stimes :: Integral b0 => b0 -> Const a b -> Const a b Source
Bits a => Bits (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods (.&.) :: Const a b -> Const a b -> Const a b Source (.\|.) :: Const a b -> Const a b -> Const a b Source xor :: Const a b -> Const a b -> Const a b Source complement :: Const a b -> Const a b Source shift :: Const a b -> Int -> Const a b Source rotate :: Const a b -> Int -> Const a b Source zeroBits :: Const a b Source bit :: Int -> Const a b Source setBit :: Const a b -> Int -> Const a b Source clearBit :: Const a b -> Int -> Const a b Source complementBit :: Const a b -> Int -> Const a b Source testBit :: Const a b -> Int -> Bool Source bitSizeMaybe :: Const a b -> Maybe Int Source bitSize :: Const a b -> Int Source isSigned :: Const a b -> Bool Source shiftL :: Const a b -> Int -> Const a b Source unsafeShiftL :: Const a b -> Int -> Const a b Source shiftR :: Const a b -> Int -> Const a b Source unsafeShiftR :: Const a b -> Int -> Const a b Source rotateL :: Const a b -> Int -> Const a b Source rotateR :: Const a b -> Int -> Const a b Source popCount :: Const a b -> Int Source
FiniteBits a => FiniteBits (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods finiteBitSize :: Const a b -> Int Source countLeadingZeros :: Const a b -> Int Source countTrailingZeros :: Const a b -> Int Source
Bounded a => Bounded (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods minBound :: Const a b Source maxBound :: Const a b Source
Enum a => Enum (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods succ :: Const a b -> Const a b Source pred :: Const a b -> Const a b Source toEnum :: Int -> Const a b Source fromEnum :: Const a b -> Int Source enumFrom :: Const a b -> [Const a b] Source enumFromThen :: Const a b -> Const a b -> [Const a b] Source enumFromTo :: Const a b -> Const a b -> [Const a b] Source enumFromThenTo :: Const a b -> Const a b -> Const a b -> [Const a b] Source
Floating a => Floating (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods pi :: Const a b Source exp :: Const a b -> Const a b Source log :: Const a b -> Const a b Source sqrt :: Const a b -> Const a b Source (**) :: Const a b -> Const a b -> Const a b Source logBase :: Const a b -> Const a b -> Const a b Source sin :: Const a b -> Const a b Source cos :: Const a b -> Const a b Source tan :: Const a b -> Const a b Source asin :: Const a b -> Const a b Source acos :: Const a b -> Const a b Source atan :: Const a b -> Const a b Source sinh :: Const a b -> Const a b Source cosh :: Const a b -> Const a b Source tanh :: Const a b -> Const a b Source asinh :: Const a b -> Const a b Source acosh :: Const a b -> Const a b Source atanh :: Const a b -> Const a b Source log1p :: Const a b -> Const a b Source expm1 :: Const a b -> Const a b Source log1pexp :: Const a b -> Const a b Source log1mexp :: Const a b -> Const a b Source
RealFloat a => RealFloat (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods floatRadix :: Const a b -> Integer Source floatDigits :: Const a b -> Int Source floatRange :: Const a b -> (Int, Int) Source decodeFloat :: Const a b -> (Integer, Int) Source encodeFloat :: Integer -> Int -> Const a b Source exponent :: Const a b -> Int Source significand :: Const a b -> Const a b Source scaleFloat :: Int -> Const a b -> Const a b Source isNaN :: Const a b -> Bool Source isInfinite :: Const a b -> Bool Source isDenormalized :: Const a b -> Bool Source isNegativeZero :: Const a b -> Bool Source isIEEE :: Const a b -> Bool Source atan2 :: Const a b -> Const a b -> Const a b Source
Generic (Const a b) Source
Instance details Defined in Data.Functor.Const Associated Types type Rep (Const a b) :: Type -> Type Source Methods from :: Const a b -> Rep (Const a b) x Source to :: Rep (Const a b) x -> Const a b Source
Ix a => Ix (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods range :: (Const a b, Const a b) -> [Const a b] Source index :: (Const a b, Const a b) -> Const a b -> Int Source unsafeIndex :: (Const a b, Const a b) -> Const a b -> Int Source inRange :: (Const a b, Const a b) -> Const a b -> Bool Source rangeSize :: (Const a b, Const a b) -> Int Source unsafeRangeSize :: (Const a b, Const a b) -> Int Source
Num a => Num (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods (+) :: Const a b -> Const a b -> Const a b Source (-) :: Const a b -> Const a b -> Const a b Source (*) :: Const a b -> Const a b -> Const a b Source negate :: Const a b -> Const a b Source abs :: Const a b -> Const a b Source signum :: Const a b -> Const a b Source fromInteger :: Integer -> Const a b Source
Read a => Read (Const a b) Source	This instance would be equivalent to the derived instances of the `Const` newtype if the `getConst` field were removed Since: base-4.8.0.0
Instance details Defined in Data.Functor.Const Methods readsPrec :: Int -> ReadS (Const a b) Source readList :: ReadS [Const a b] Source readPrec :: ReadPrec (Const a b) Source readListPrec :: ReadPrec [Const a b] Source
Fractional a => Fractional (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods (/) :: Const a b -> Const a b -> Const a b Source recip :: Const a b -> Const a b Source fromRational :: Rational -> Const a b Source
Integral a => Integral (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods quot :: Const a b -> Const a b -> Const a b Source rem :: Const a b -> Const a b -> Const a b Source div :: Const a b -> Const a b -> Const a b Source mod :: Const a b -> Const a b -> Const a b Source quotRem :: Const a b -> Const a b -> (Const a b, Const a b) Source divMod :: Const a b -> Const a b -> (Const a b, Const a b) Source toInteger :: Const a b -> Integer Source
Real a => Real (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods toRational :: Const a b -> Rational Source
RealFrac a => RealFrac (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods properFraction :: Integral b0 => Const a b -> (b0, Const a b) Source truncate :: Integral b0 => Const a b -> b0 Source round :: Integral b0 => Const a b -> b0 Source ceiling :: Integral b0 => Const a b -> b0 Source floor :: Integral b0 => Const a b -> b0 Source
Show a => Show (Const a b) Source	This instance would be equivalent to the derived instances of the `Const` newtype if the `getConst` field were removed Since: base-4.8.0.0
Instance details Defined in Data.Functor.Const Methods showsPrec :: Int -> Const a b -> ShowS Source show :: Const a b -> String Source showList :: [Const a b] -> ShowS Source
Eq a => Eq (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods (==) :: Const a b -> Const a b -> Bool Source (/=) :: Const a b -> Const a b -> Bool Source
Ord a => Ord (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const Methods compare :: Const a b -> Const a b -> Ordering Source (<) :: Const a b -> Const a b -> Bool Source (<=) :: Const a b -> Const a b -> Bool Source (>) :: Const a b -> Const a b -> Bool Source (>=) :: Const a b -> Const a b -> Bool Source max :: Const a b -> Const a b -> Const a b Source min :: Const a b -> Const a b -> Const a b Source
type Rep1 (Const a :: k -> Type) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const type Rep1 (Const a :: k -> Type) = D1 ('MetaData "Const" "Data.Functor.Const" "base" 'True) (C1 ('MetaCons "Const" 'PrefixI 'True) (S1 ('MetaSel ('Just "getConst") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)))
type Rep (Const a b) Source	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Const type Rep (Const a b) = D1 ('MetaData "Const" "Data.Functor.Const" "base" 'True) (C1 ('MetaCons "Const" 'PrefixI 'True) (S1 ('MetaSel ('Just "getConst") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)))

newtype WrappedMonad m a Source

Constructors

WrapMonad
Fields unwrapMonad :: m a

Instances

Instances details

Generic1 (WrappedMonad m :: Type -> Type) Source
Instance details Defined in Control.Applicative Associated Types type Rep1 (WrappedMonad m) :: k -> Type Source Methods from1 :: forall (a :: k). WrappedMonad m a -> Rep1 (WrappedMonad m) a Source to1 :: forall (a :: k). Rep1 (WrappedMonad m) a -> WrappedMonad m a Source
MonadPlus m => Alternative (WrappedMonad m) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods empty :: WrappedMonad m a Source (<\|>) :: WrappedMonad m a -> WrappedMonad m a -> WrappedMonad m a Source some :: WrappedMonad m a -> WrappedMonad m [a] Source many :: WrappedMonad m a -> WrappedMonad m [a] Source
Monad m => Applicative (WrappedMonad m) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods 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
Monad m => Functor (WrappedMonad m) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods fmap :: (a -> b) -> WrappedMonad m a -> WrappedMonad m b Source (<$) :: a -> WrappedMonad m b -> WrappedMonad m a Source
Monad m => Monad (WrappedMonad m) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative Methods (>>=) :: WrappedMonad m a -> (a -> WrappedMonad m b) -> WrappedMonad m b Source (>>) :: WrappedMonad m a -> WrappedMonad m b -> WrappedMonad m b Source return :: a -> WrappedMonad m a Source
(Typeable m, Typeable a, Data (m a)) => Data (WrappedMonad m a) Source	Since: base-4.14.0.0
Instance details Defined in Data.Data Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> WrappedMonad m a -> c (WrappedMonad m a) Source gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (WrappedMonad m a) Source toConstr :: WrappedMonad m a -> Constr Source dataTypeOf :: WrappedMonad m a -> DataType Source dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (WrappedMonad m a)) Source dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (WrappedMonad m a)) Source gmapT :: (forall b. Data b => b -> b) -> WrappedMonad m a -> WrappedMonad m a Source gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> WrappedMonad m a -> r Source gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> WrappedMonad m a -> r Source gmapQ :: (forall d. Data d => d -> u) -> WrappedMonad m a -> [u] Source gmapQi :: Int -> (forall d. Data d => d -> u) -> WrappedMonad m a -> u Source gmapM :: Monad m0 => (forall d. Data d => d -> m0 d) -> WrappedMonad m a -> m0 (WrappedMonad m a) Source gmapMp :: MonadPlus m0 => (forall d. Data d => d -> m0 d) -> WrappedMonad m a -> m0 (WrappedMonad m a) Source gmapMo :: MonadPlus m0 => (forall d. Data d => d -> m0 d) -> WrappedMonad m a -> m0 (WrappedMonad m a) Source
Generic (WrappedMonad m a) Source
Instance details Defined in Control.Applicative Associated Types type Rep (WrappedMonad m a) :: Type -> Type Source Methods from :: WrappedMonad m a -> Rep (WrappedMonad m a) x Source to :: Rep (WrappedMonad m a) x -> WrappedMonad m a Source
type Rep1 (WrappedMonad m :: Type -> Type) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative type Rep1 (WrappedMonad m :: Type -> Type) = D1 ('MetaData "WrappedMonad" "Control.Applicative" "base" 'True) (C1 ('MetaCons "WrapMonad" 'PrefixI 'True) (S1 ('MetaSel ('Just "unwrapMonad") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec1 m)))
type Rep (WrappedMonad m a) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative type Rep (WrappedMonad m a) = D1 ('MetaData "WrappedMonad" "Control.Applicative" "base" 'True) (C1 ('MetaCons "WrapMonad" 'PrefixI 'True) (S1 ('MetaSel ('Just "unwrapMonad") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (m a))))

newtype WrappedArrow a b c Source

Constructors

WrapArrow
Fields unwrapArrow :: a b c

Instances

Instances details

Generic1 (WrappedArrow a b :: Type -> Type) Source
Instance details Defined in Control.Applicative Associated Types type Rep1 (WrappedArrow a b) :: k -> Type Source Methods from1 :: forall (a0 :: k). WrappedArrow a b a0 -> Rep1 (WrappedArrow a b) a0 Source to1 :: forall (a0 :: k). Rep1 (WrappedArrow a b) a0 -> WrappedArrow a b a0 Source
(ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods empty :: WrappedArrow a b a0 Source (<\|>) :: WrappedArrow a b a0 -> WrappedArrow a b a0 -> WrappedArrow a b a0 Source some :: WrappedArrow a b a0 -> WrappedArrow a b [a0] Source many :: WrappedArrow a b a0 -> WrappedArrow a b [a0] Source
Arrow a => Applicative (WrappedArrow a b) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods 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
Arrow a => Functor (WrappedArrow a b) Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods fmap :: (a0 -> b0) -> WrappedArrow a b a0 -> WrappedArrow a b b0 Source (<$) :: a0 -> WrappedArrow a b b0 -> WrappedArrow a b a0 Source
(Typeable a, Typeable b, Typeable c, Data (a b c)) => Data (WrappedArrow a b c) Source	Since: base-4.14.0.0
Instance details Defined in Data.Data Methods gfoldl :: (forall d b0. Data d => c0 (d -> b0) -> d -> c0 b0) -> (forall g. g -> c0 g) -> WrappedArrow a b c -> c0 (WrappedArrow a b c) Source gunfold :: (forall b0 r. Data b0 => c0 (b0 -> r) -> c0 r) -> (forall r. r -> c0 r) -> Constr -> c0 (WrappedArrow a b c) Source toConstr :: WrappedArrow a b c -> Constr Source dataTypeOf :: WrappedArrow a b c -> DataType Source dataCast1 :: Typeable t => (forall d. Data d => c0 (t d)) -> Maybe (c0 (WrappedArrow a b c)) Source dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c0 (t d e)) -> Maybe (c0 (WrappedArrow a b c)) Source gmapT :: (forall b0. Data b0 => b0 -> b0) -> WrappedArrow a b c -> WrappedArrow a b c Source gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> WrappedArrow a b c -> r Source gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> WrappedArrow a b c -> r Source gmapQ :: (forall d. Data d => d -> u) -> WrappedArrow a b c -> [u] Source gmapQi :: Int -> (forall d. Data d => d -> u) -> WrappedArrow a b c -> u Source gmapM :: Monad m => (forall d. Data d => d -> m d) -> WrappedArrow a b c -> m (WrappedArrow a b c) Source gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> WrappedArrow a b c -> m (WrappedArrow a b c) Source gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> WrappedArrow a b c -> m (WrappedArrow a b c) Source
Generic (WrappedArrow a b c) Source
Instance details Defined in Control.Applicative Associated Types type Rep (WrappedArrow a b c) :: Type -> Type Source Methods from :: WrappedArrow a b c -> Rep (WrappedArrow a b c) x Source to :: Rep (WrappedArrow a b c) x -> WrappedArrow a b c Source
type Rep1 (WrappedArrow a b :: Type -> Type) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative type Rep1 (WrappedArrow a b :: Type -> Type) = D1 ('MetaData "WrappedArrow" "Control.Applicative" "base" 'True) (C1 ('MetaCons "WrapArrow" 'PrefixI 'True) (S1 ('MetaSel ('Just "unwrapArrow") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec1 (a b))))
type Rep (WrappedArrow a b c) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative type Rep (WrappedArrow a b c) = D1 ('MetaData "WrappedArrow" "Control.Applicative" "base" 'True) (C1 ('MetaCons "WrapArrow" 'PrefixI 'True) (S1 ('MetaSel ('Just "unwrapArrow") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (a b c))))

newtype ZipList a Source

Lists, but with an Applicative functor based on zipping.

Constructors

ZipList
Fields getZipList :: [a]

Instances

Instances details

Foldable ZipList Source	Since: base-4.9.0.0
Instance details Defined in Control.Applicative Methods fold :: Monoid m => ZipList m -> m Source foldMap :: Monoid m => (a -> m) -> ZipList a -> m Source foldMap' :: Monoid m => (a -> m) -> ZipList a -> m Source foldr :: (a -> b -> b) -> b -> ZipList a -> b Source foldr' :: (a -> b -> b) -> b -> ZipList a -> b Source foldl :: (b -> a -> b) -> b -> ZipList a -> b Source foldl' :: (b -> a -> b) -> b -> ZipList a -> b Source foldr1 :: (a -> a -> a) -> ZipList a -> a Source foldl1 :: (a -> a -> a) -> ZipList a -> a Source toList :: ZipList a -> [a] Source null :: ZipList a -> Bool Source length :: ZipList a -> Int Source elem :: Eq a => a -> ZipList a -> Bool Source maximum :: Ord a => ZipList a -> a Source minimum :: Ord a => ZipList a -> a Source sum :: Num a => ZipList a -> a Source product :: Num a => ZipList a -> a Source
Traversable ZipList Source	Since: base-4.9.0.0
Instance details Defined in Data.Traversable Methods traverse :: Applicative f => (a -> f b) -> ZipList a -> f (ZipList b) Source sequenceA :: Applicative f => ZipList (f a) -> f (ZipList a) Source mapM :: Monad m => (a -> m b) -> ZipList a -> m (ZipList b) Source sequence :: Monad m => ZipList (m a) -> m (ZipList a) Source
Alternative ZipList Source	Since: base-4.11.0.0
Instance details Defined in Control.Applicative Methods empty :: ZipList a Source (<\|>) :: ZipList a -> ZipList a -> ZipList a Source some :: ZipList a -> ZipList [a] Source many :: ZipList a -> ZipList [a] Source
Applicative ZipList Source	`f <$> ZipList xs1 <> ... <> ZipList xsN = ZipList (zipWithN f xs1 ... xsN)` where `zipWithN` refers to the `zipWith` function of the appropriate arity (`zipWith`, `zipWith3`, `zipWith4`, ...). For example: `(\a b c -> stimes c [a, b]) <$> 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
Instance details Defined in Control.Applicative Methods pure :: a -> ZipList a Source (<>) :: ZipList (a -> b) -> ZipList a -> ZipList b Source liftA2 :: (a -> b -> c) -> ZipList a -> ZipList b -> ZipList c Source (>) :: ZipList a -> ZipList b -> ZipList b Source (<*) :: ZipList a -> ZipList b -> ZipList a Source
Functor ZipList Source	Since: base-2.1
Instance details Defined in Control.Applicative Methods fmap :: (a -> b) -> ZipList a -> ZipList b Source (<$) :: a -> ZipList b -> ZipList a Source
Generic1 ZipList Source
Instance details Defined in Control.Applicative Associated Types type Rep1 ZipList :: k -> Type Source Methods from1 :: forall (a :: k). ZipList a -> Rep1 ZipList a Source to1 :: forall (a :: k). Rep1 ZipList a -> ZipList a Source
Data a => Data (ZipList a) Source	Since: base-4.14.0.0
Instance details Defined in Data.Data Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> ZipList a -> c (ZipList a) Source gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (ZipList a) Source toConstr :: ZipList a -> Constr Source dataTypeOf :: ZipList a -> DataType Source dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (ZipList a)) Source dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (ZipList a)) Source gmapT :: (forall b. Data b => b -> b) -> ZipList a -> ZipList a Source gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> ZipList a -> r Source gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> ZipList a -> r Source gmapQ :: (forall d. Data d => d -> u) -> ZipList a -> [u] Source gmapQi :: Int -> (forall d. Data d => d -> u) -> ZipList a -> u Source gmapM :: Monad m => (forall d. Data d => d -> m d) -> ZipList a -> m (ZipList a) Source gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> ZipList a -> m (ZipList a) Source gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> ZipList a -> m (ZipList a) Source
Generic (ZipList a) Source
Instance details Defined in Control.Applicative Associated Types type Rep (ZipList a) :: Type -> Type Source Methods from :: ZipList a -> Rep (ZipList a) x Source to :: Rep (ZipList a) x -> ZipList a Source
IsList (ZipList a) Source	Since: base-4.15.0.0
Instance details Defined in GHC.IsList Associated Types type Item (ZipList a) Source Methods fromList :: [Item (ZipList a)] -> ZipList a Source fromListN :: Int -> [Item (ZipList a)] -> ZipList a Source toList :: ZipList a -> [Item (ZipList a)] Source
Read a => Read (ZipList a) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative Methods readsPrec :: Int -> ReadS (ZipList a) Source readList :: ReadS [ZipList a] Source readPrec :: ReadPrec (ZipList a) Source readListPrec :: ReadPrec [ZipList a] Source
Show a => Show (ZipList a) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative Methods showsPrec :: Int -> ZipList a -> ShowS Source show :: ZipList a -> String Source showList :: [ZipList a] -> ShowS Source
Eq a => Eq (ZipList a) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative Methods (==) :: ZipList a -> ZipList a -> Bool Source (/=) :: ZipList a -> ZipList a -> Bool Source
Ord a => Ord (ZipList a) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative Methods compare :: ZipList a -> ZipList a -> Ordering Source (<) :: ZipList a -> ZipList a -> Bool Source (<=) :: ZipList a -> ZipList a -> Bool Source (>) :: ZipList a -> ZipList a -> Bool Source (>=) :: ZipList a -> ZipList a -> Bool Source max :: ZipList a -> ZipList a -> ZipList a Source min :: ZipList a -> ZipList a -> ZipList a Source
type Rep1 ZipList Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative type Rep1 ZipList = D1 ('MetaData "ZipList" "Control.Applicative" "base" 'True) (C1 ('MetaCons "ZipList" 'PrefixI 'True) (S1 ('MetaSel ('Just "getZipList") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec1 [])))
type Rep (ZipList a) Source	Since: base-4.7.0.0
Instance details Defined in Control.Applicative type Rep (ZipList a) = D1 ('MetaData "ZipList" "Control.Applicative" "base" 'True) (C1 ('MetaCons "ZipList" 'PrefixI 'True) (S1 ('MetaSel ('Just "getZipList") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 [a])))
type Item (ZipList a) Source
Instance details Defined in GHC.IsList type Item (ZipList a) = a

Utility functions

(<$>) :: 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

Expand

Convert from a Maybe Int to a Maybe String using show:

>>> show <$> Nothing
Nothing
>>> show <$> Just 3
Just "3"

Convert from an Either Int Int to an 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)

(<$) :: Functor f => a -> f b -> f a infixl 4 Source

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.

(<**>) :: Applicative f => f a -> f (a -> b) -> f b infixl 4 Source

A variant of <*> with the arguments reversed.

liftA :: Applicative f => (a -> b) -> f a -> f b Source

Lift a function to actions. Equivalent to Functor's fmap but implemented using only Applicative's methods: `liftA f a = pure f * a`

As such this function may be used to implement a Functor instance from an Applicative one.

liftA3 :: Applicative f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d Source

Lift a ternary function to actions.

optional :: Alternative f => f a -> f (Maybe a) Source

One or none.

It is useful for modelling any computation that is allowed to fail.

Examples

Expand

Using the Alternative instance of Control.Monad.Except, the following functions:

>>> import Control.Monad.Except

>>> canFail = throwError "it failed" :: Except String Int
>>> final = return 42                :: Except String Int

Can be combined by allowing the first function to fail:

>>> runExcept $ canFail *> final
Left "it failed"
>>> runExcept $ optional canFail *> final
Right 42

asum :: (Foldable t, Alternative f) => t (f a) -> f a Source

The sum of a collection of actions, generalizing concat.

asum is just like msum, but generalised to Alternative.

Examples

Expand

Basic usage:

>>> asum [Just "Hello", Nothing, Just "World"]
Just "Hello"

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/9.4.2/docs/libraries/base-4.17.0.0/Control-Applicative.html

Control.Applicative

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Applicative functors

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