base-compat-0.11.0: A compatibility layer for base

Synopsis

Documentation

guard :: Alternative f => Bool -> f () #

Conditional failure of Alternative computations. Defined by

guard True  = pure ()
guard False = empty


Examples

Expand

Common uses of guard include conditionally signaling an error in an error monad and conditionally rejecting the current choice in an Alternative-based parser.

As an example of signaling an error in the error monad Maybe, consider a safe division function safeDiv x y that returns Nothing when the denominator y is zero and Just (x div y) otherwise. For example:

>>> safeDiv 4 0
Nothing
>>> safeDiv 4 2
Just 2


A definition of safeDiv using guards, but not guard:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y | y /= 0    = Just (x div y)
| otherwise = Nothing


A definition of safeDiv using guard and Monad do-notation:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y = do
guard (y /= 0)
return (x div y)


join :: Monad m => m (m a) -> m a #

The join function is the conventional monad join operator. It is used to remove one level of monadic structure, projecting its bound argument into the outer level.

Examples

Expand

A common use of join is to run an IO computation returned from an STM transaction, since STM transactions can't perform IO directly. Recall that

atomically :: STM a -> IO a


is used to run STM transactions atomically. So, by specializing the types of atomically and join to

atomically :: STM (IO b) -> IO (IO b)
join       :: IO (IO b)  -> IO b


we can compose them as

join . atomically :: STM (IO b) -> IO b


to run an STM transaction and the IO action it returns.

class Applicative m => Monad (m :: Type -> Type) where #

The Monad class defines the basic operations over a monad, a concept from a branch of mathematics known as category theory. From the perspective of a Haskell programmer, however, it is best to think of a monad as an abstract datatype of actions. Haskell's do expressions provide a convenient syntax for writing monadic expressions.

Instances of Monad should satisfy the following laws:

• return a >>= k  =  k a
• m >>= return  =  m
• m >>= (\x -> k x >>= h)  =  (m >>= k) >>= h

Furthermore, the Monad and Applicative operations should relate as follows:

• pure = return
• (<*>) = ap

The above laws imply:

• fmap f xs  =  xs >>= return . f
• (>>) = (*>)

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

The instances of Monad for lists, Maybe and IO defined in the Prelude satisfy these laws.

Minimal complete definition

(>>=)

Methods

(>>=) :: m a -> (a -> m b) -> m b infixl 1 #

Sequentially compose two actions, passing any value produced by the first as an argument to the second.

(>>) :: m a -> m b -> m b infixl 1 #

Sequentially compose two actions, discarding any value produced by the first, like sequencing operators (such as the semicolon) in imperative languages.

return :: a -> m a #

Inject a value into the monadic type.

Instances

class Functor (f :: Type -> Type) where #

The Functor class is used for types that can be mapped over. Instances of Functor should satisfy the following laws:

fmap id  ==  id
fmap (f . g)  ==  fmap f . fmap g

The instances of Functor for lists, Maybe and IO satisfy these laws.

Methods

fmap :: (a -> b) -> f a -> f b #

Instances

Strict version of <$>. Since: base-4.8.0.0 unless :: Applicative f => Bool -> f () -> f () # The reverse of when. replicateM_ :: Applicative m => Int -> m a -> m () # Like replicateM, but discards the result. replicateM :: Applicative m => Int -> m a -> m [a] # replicateM n act performs the action n times, gathering the results. foldM_ :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m () # Like foldM, but discards the result. foldM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b # The foldM function is analogous to foldl, except that its result is encapsulated in a monad. Note that foldM works from left-to-right over the list arguments. This could be an issue where (>>) and the folded function' are not commutative. foldM f a1 [x1, x2, ..., xm] == do a2 <- f a1 x1 a3 <- f a2 x2 ... f am xm If right-to-left evaluation is required, the input list should be reversed. Note: foldM is the same as foldlM zipWithM_ :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m () # zipWithM_ is the extension of zipWithM which ignores the final result. zipWithM :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c] # The zipWithM function generalizes zipWith to arbitrary applicative functors. mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c]) # The mapAndUnzipM function maps its first argument over a list, returning the result as a pair of lists. This function is mainly used with complicated data structures or a state-transforming monad. forever :: Applicative f => f a -> f b # Repeat an action indefinitely. Examples Expand A common use of forever is to process input from network sockets, Handles, and channels (e.g. MVar and Chan). For example, here is how we might implement an echo server, using forever both to listen for client connections on a network socket and to echo client input on client connection handles: echoServer :: Socket -> IO () echoServer socket = forever$ do
client <- accept socket
forkFinally (echo client) (\_ -> hClose client)
where
echo :: Handle -> IO ()
echo client = forever $hGetLine client >>= hPutStrLn client  (<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c infixr 1 # Right-to-left composition of Kleisli arrows. (>=>), with the arguments flipped. Note how this operator resembles function composition (.): (.) :: (b -> c) -> (a -> b) -> a -> c (<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c (>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c infixr 1 # Left-to-right composition of Kleisli arrows. filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a] # This generalizes the list-based filter function. forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b) # forM is mapM with its arguments flipped. For a version that ignores the results see forM_. msum :: (Foldable t, MonadPlus m) => t (m a) -> m a # The sum of a collection of actions, generalizing concat. As of base 4.8.0.0, msum is just asum, specialized to MonadPlus. sequence_ :: (Foldable t, Monad m) => t (m a) -> m () # Evaluate each monadic action in the structure from left to right, and ignore the results. For a version that doesn't ignore the results see sequence. As of base 4.8.0.0, sequence_ is just sequenceA_, specialized to Monad. forM_ :: (Foldable t, Monad m) => t a -> (a -> m b) -> m () # forM_ is mapM_ with its arguments flipped. For a version that doesn't ignore the results see forM. As of base 4.8.0.0, forM_ is just for_, specialized to Monad. mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m () # Map each element of a structure to a monadic action, evaluate these actions from left to right, and ignore the results. For a version that doesn't ignore the results see mapM. As of base 4.8.0.0, mapM_ is just traverse_, specialized to Monad. void :: Functor f => f a -> f () # void value discards or ignores the result of evaluation, such as the return value of an IO action. Examples Expand Replace the contents of a Maybe Int with unit: >>> void Nothing Nothing >>> void (Just 3) Just ()  Replace the contents of an Either Int Int with unit, resulting in an Either Int '()': >>> void (Left 8675309) Left 8675309 >>> void (Right 8675309) Right ()  Replace every element of a list with unit: >>> void [1,2,3] [(),(),()]  Replace the second element of a pair with unit: >>> void (1,2) (1,())  Discard the result of an IO action: >>> mapM print [1,2] 1 2 [(),()] >>> void$ mapM print [1,2]
1
2


ap :: Monad m => m (a -> b) -> m a -> m b #

In many situations, the liftM operations can be replaced by uses of ap, which promotes function application.

return f ap x1 ap ... ap xn

is equivalent to

liftMn f x1 x2 ... xn

liftM5 :: Monad m => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

liftM4 :: Monad m => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

liftM3 :: Monad m => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right. For example,

liftM2 (+) [0,1] [0,2] = [0,2,1,3]
liftM2 (+) (Just 1) Nothing = Nothing

liftM :: Monad m => (a1 -> r) -> m a1 -> m r #

Promote a function to a monad.

when :: Applicative f => Bool -> f () -> f () #

Conditional execution of Applicative expressions. For example,

when debug (putStrLn "Debugging")

will output the string Debugging if the Boolean value debug is True, and otherwise do nothing.

(=<<) :: Monad m => (a -> m b) -> m a -> m b infixr 1 #

Same as >>=, but with the arguments interchanged.

class (Alternative m, Monad m) => MonadPlus (m :: Type -> Type) where #

Monads that also support choice and failure.

Minimal complete definition

Nothing

Methods

mzero :: m a #

The identity of mplus. It should also satisfy the equations

mzero >>= f  =  mzero
v >> mzero   =  mzero

The default definition is

mzero = empty


mplus :: m a -> m a -> m a #

An associative operation. The default definition is

mplus = (<|>)

Instances

class Applicative m => Monad (m :: Type -> Type) #

The Monad class defines the basic operations over a monad, a concept from a branch of mathematics known as category theory. From the perspective of a Haskell programmer, however, it is best to think of a monad as an abstract datatype of actions. Haskell's do expressions provide a convenient syntax for writing monadic expressions.

Instances of Monad should satisfy the following laws:

• return a >>= k  =  k a
• m >>= return  =  m
• m >>= (\x -> k x >>= h)  =  (m >>= k) >>= h

Furthermore, the Monad and Applicative operations should relate as follows:

• pure = return
• (<*>) = ap

The above laws imply:

• fmap f xs  =  xs >>= return . f
• (>>) = (*>)

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

The instances of Monad for lists, Maybe and IO defined in the Prelude satisfy these laws.

Minimal complete definition

(>>=)

Instances

When a value is bound in do-notation, the pattern on the left hand side of <- might not match. In this case, this class provides a function to recover.

A Monad without a MonadFail instance may only be used in conjunction with pattern that always match, such as newtypes, tuples, data types with only a single data constructor, and irrefutable patterns (~pat).

Instances of MonadFail should satisfy the following law: fail s should be a left zero for >>=,

fail s >>= f  =  fail s


If your Monad is also MonadPlus, a popular definition is

fail _ = mzero


Since: base-4.9.0.0

Minimal complete definition

fail

Instances

fail :: MonadFail m => String -> m a #

class (Alternative m, Monad m) => MonadPlus (m :: Type -> Type) where #

Monads that also support choice and failure.

Minimal complete definition

Nothing

Methods

mzero :: m a #

The identity of mplus. It should also satisfy the equations

mzero >>= f  =  mzero
v >> mzero   =  mzero

The default definition is

mzero = empty


mplus :: m a -> m a -> m a #

An associative operation. The default definition is

mplus = (<|>)
`
Instances