| Stability | experimental |
|---|---|
| Safe Haskell | Safe |
| Language | Haskell2010 |
Control.Monad.Except.CoHas
Description
This module defines a class CoHas intended to be used with the MonadError class
(and similar ones) or Except / ExceptT types.
The problem
Assume there are several types representing the possible errors in different parts of an application:
data DbError = ... data WebUIError = ...
as well as a single sum type containing all of those:
data AppError = AppDbError DbError | AppWebUIError WebUIError
What should be the MonadError constraint of the DB module and web module respectively?
- It could be
MonadError AppError mfor both, introducing unnecessary coupling. - Or it could be
MonadError DbError mfor the DB module andMonadError WebError mfor the web module respectively, but combining them becomes a pain.
Or, it could be MonadError e m, CoHas AppError e for the DB module (and similarly for the web module),
where some appropriately defined CoHas option sum class allows injecting option
creating a value of the sum type.
This approach keeps both modules decoupled, while allowing using them in the same monad stack.
The only downside is that now one has to define the CoHas class
and write tedious instances for the AppError type (and potentially other types in case of, for example, tests).
But why bother doing the work that the machine will happily do for you?
The solution
This module defines the generic CoHas class as well as hides all the boilerplate behind GHC.Generics,
so all you have to do is to add the corresponding deriving-clause:
data AppError = AppDbError DbError | AppWebUIError WebUIError deriving (Generic, CoHas DbError, CoHas WebUIError)
and use throwError . inject instead of throwError (but this is something you'd have to do anyway).
Type safety
What should happen if sum does not have any way to construct it from option at all?
Of course, this means that we cannot inject option into sum, and no CoHas instance can be derived at all.
Indeed, this library will refuse to generate an instance in this case.
On the other hand, what should happen if sum contains multiple values of type option
(like Either option option), perhaps on different levels of nesting?
While technically we could make an arbitrary choice, like taking the first one in breadth-first or depth-first order,
we instead decide that such a choice is inherently ambiguous,
so this library will refuse to generate an instance in this case as well.
Exports
This module also reexports Except along with some functions like throwError or liftEither
with types adjusted for the intended usage of the CoHas class.
Synopsis
- class CoHas option sum where
- inject :: option -> sum
- type SuccessfulSearch option sum path = (Search option (Rep sum) ~ 'Found path, GCoHas path option (Rep sum))
- guard :: Alternative f => Bool -> f ()
- join :: Monad m => m (m a) -> m a
- class Applicative m => Monad (m :: Type -> Type) where
- class Functor (f :: Type -> Type) where
- class Monad m => MonadFix (m :: Type -> Type) where
- mfix :: (a -> m a) -> m a
- class Monad m => MonadFail (m :: Type -> Type) where
- mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b)
- sequence :: (Traversable t, Monad m) => t (m a) -> m (t a)
- class Monad m => MonadIO (m :: Type -> Type) where
- zipWithM_ :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m ()
- zipWithM :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c]
- unless :: Applicative f => Bool -> f () -> f ()
- replicateM_ :: Applicative m => Int -> m a -> m ()
- replicateM :: Applicative m => Int -> m a -> m [a]
- mfilter :: MonadPlus m => (a -> Bool) -> m a -> m a
- mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c])
- forever :: Applicative f => f a -> f b
- foldM_ :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m ()
- foldM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b
- filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a]
- (>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c
- (<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c
- (<$!>) :: Monad m => (a -> b) -> m a -> m b
- forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b)
- sequence_ :: (Foldable t, Monad m) => t (m a) -> m ()
- msum :: (Foldable t, MonadPlus m) => t (m a) -> m a
- mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
- forM_ :: (Foldable t, Monad m) => t a -> (a -> m b) -> m ()
- fix :: (a -> a) -> a
- void :: Functor f => f a -> f ()
- class (Alternative m, Monad m) => MonadPlus (m :: Type -> Type) where
- when :: Applicative f => Bool -> f () -> f ()
- liftM5 :: Monad m => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
- liftM4 :: Monad m => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
- liftM3 :: Monad m => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
- liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
- liftM :: Monad m => (a1 -> r) -> m a1 -> m r
- ap :: Monad m => m (a -> b) -> m a -> m b
- (=<<) :: Monad m => (a -> m b) -> m a -> m b
- class MonadTrans (t :: (Type -> Type) -> Type -> Type) where
- class Monad m => MonadError e (m :: Type -> Type) | m -> e where
- catchError :: m a -> (e -> m a) -> m a
- mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' b
- mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b
- runExcept :: Except e a -> Either e a
- runExceptT :: ExceptT e m a -> m (Either e a)
- withExcept :: (e -> e') -> Except e a -> Except e' a
- withExceptT :: forall (m :: Type -> Type) e e' a. Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a
- type Except e = ExceptT e Identity
- newtype ExceptT e (m :: Type -> Type) a = ExceptT (m (Either e a))
- throwError :: (MonadError error m, CoHas option error) => option -> m a
- liftEither :: (MonadError error m, CoHas option error) => Either option a -> m a
- liftMaybe :: (MonadError error m, CoHas option error) => option -> Maybe a -> m a
Documentation
class CoHas option sum where Source #
The CoHas option sum class is used for sum types that could be created from a value of type option.
Minimal complete definition
Nothing
Methods
inject :: option -> sum Source #
Inject an option into the sum type.
The default implementation searches sum for some constructor
that's compatible with option and creates sum using that constructor.
The default implementation typechecks iff there is a single matching constructor.
default inject :: forall path. (Generic sum, SuccessfulSearch option sum path) => option -> sum Source #
Instances
| CoHas sum sum Source # | Each type can be injected into itself (and that is an |
Defined in Control.Monad.Except.CoHas | |
| SuccessfulSearch a (Either l r) path => CoHas a (Either l r) Source # | |
Defined in Control.Monad.Except.CoHas | |
type SuccessfulSearch option sum path = (Search option (Rep sum) ~ 'Found path, GCoHas path option (Rep sum)) Source #
Type alias representing that the search of option in sum has been successful.
The path is used to guide the default generic implementation of CoHas.
guard :: Alternative f => Bool -> f () #
Conditional failure of Alternative computations. Defined by
guard True =pure() guard False =empty
Examples
Common uses of guard include conditionally signaling an error in
an error monad and conditionally rejecting the current choice in an
Alternative-based parser.
As an example of signaling an error in the error monad Maybe,
consider a safe division function safeDiv x y that returns
Nothing when the denominator y is zero and Just (x `div`
y)
>>>safeDiv 4 0Nothing
>>>safeDiv 4 2Just 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.
'join bssdo expression
do bs <- bss bs
Examples
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
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:
- Left identity
returna>>=k = k a- Right identity
m>>=return= m- Associativity
m>>=(\x -> k x>>=h) = (m>>=k)>>=h
Furthermore, the Monad and Applicative operations should relate as follows:
The above laws imply:
and that pure and (<*>) satisfy the applicative functor laws.
The instances of Monad for lists, Maybe and IO
defined in the Prelude satisfy these laws.
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.
'as ' can be understood as the >>= bsdo expression
do a <- as bs a
(>>) :: 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.
'as ' can be understood as the >> bsdo expression
do as bs
Inject a value into the monadic type.
Instances
| Monad Down | Since: base-4.11.0.0 |
| Monad Par1 | Since: base-4.9.0.0 |
| Monad P | Since: base-2.1 |
| Monad ReadP | Since: base-2.1 |
| Monad IO | Since: base-2.1 |
| Monad NonEmpty | Since: base-4.9.0.0 |
| Monad Maybe | Since: base-2.1 |
| Monad Solo | Since: base-4.15 |
| Monad [] | Since: base-2.1 |
| Monad (Either e) | Since: base-4.4.0.0 |
| Monad (Proxy :: Type -> Type) | Since: base-4.7.0.0 |
| Monad (U1 :: Type -> Type) | Since: base-4.9.0.0 |
| Monad m => Monad (ListT m) | |
| Monad m => Monad (MaybeT m) | |
| Monoid a => Monad ((,) a) | Since: base-4.9.0.0 |
| Monad f => Monad (Rec1 f) | Since: base-4.9.0.0 |
| (Monad m, Error e) => Monad (ErrorT e m) | |
| Monad m => Monad (ExceptT e m) | |
| Monad m => Monad (IdentityT m) | |
| Monad m => Monad (ReaderT r m) | |
| Monad m => Monad (StateT s m) | |
| Monad m => Monad (StateT s m) | |
| (Monoid w, Monad m) => Monad (WriterT w m) | |
| (Monoid w, Monad m) => Monad (WriterT w m) | |
| (Monoid a, Monoid b) => Monad ((,,) a b) | Since: base-4.14.0.0 |
| (Monad f, Monad g) => Monad (f :*: g) | Since: base-4.9.0.0 |
| Monad (ContT r m) | |
| (Monoid a, Monoid b, Monoid c) => Monad ((,,,) a b c) | Since: base-4.14.0.0 |
| Monad ((->) r) | Since: base-2.1 |
| Monad f => Monad (M1 i c f) | Since: base-4.9.0.0 |
| (Monoid w, Monad m) => Monad (RWST r w s m) | |
| (Monoid w, Monad m) => Monad (RWST r w s m) | |
class Functor (f :: Type -> Type) where #
A type f is a Functor if it provides a function fmap which, given any types a and b
lets you apply any function from (a -> b) to turn an f a into an f b, preserving the
structure of f. Furthermore f needs to adhere to the following:
Note, that the second law follows from the free theorem of the type fmap and
the first law, so you need only check that the former condition holds.
Minimal complete definition
Methods
fmap :: (a -> b) -> f a -> f b #
fmap is used to apply a function of type (a -> b) to a value of type f a,
where f is a functor, to produce a value of type f b.
Note that for any type constructor with more than one parameter (e.g., Either),
only the last type parameter can be modified with fmap (e.g., b in `Either a b`).
Some type constructors with two parameters or more have a Bifunctor
Examples
Convert from a Maybe IntMaybe String
using show:
>>>fmap show NothingNothing>>>fmap show (Just 3)Just "3"
Convert from an Either Int IntEither Int String using show:
>>>fmap show (Left 17)Left 17>>>fmap show (Right 17)Right "17"
Double each element of a list:
>>>fmap (*2) [1,2,3][2,4,6]
Apply even to the second element of a pair:
>>>fmap even (2,2)(2,True)
It may seem surprising that the function is only applied to the last element of the tuple
compared to the list example above which applies it to every element in the list.
To understand, remember that tuples are type constructors with multiple type parameters:
a tuple of 3 elements (a,b,c) can also be written (,,) a b c and its Functor instance
is defined for Functor ((,,) a b) (i.e., only the third parameter is free to be mapped over
with fmap).
It explains why fmap can be used with tuples containing values of different types as in the
following example:
>>>fmap even ("hello", 1.0, 4)("hello",1.0,True)
Instances
| Functor Handler | Since: base-4.6.0.0 |
| Functor Down | Since: base-4.11.0.0 |
| Functor Par1 | Since: base-4.9.0.0 |
| Functor P | Since: base-4.8.0.0 |
Defined in Text.ParserCombinators.ReadP | |
| Functor ReadP | Since: base-2.1 |
| Functor IO | Since: base-2.1 |
| Functor NonEmpty | Since: base-4.9.0.0 |
| Functor Maybe | Since: base-2.1 |
| Functor Solo | Since: base-4.15 |
| Functor [] | Since: base-2.1 |
| Functor (Either a) | Since: base-3.0 |
| Functor (Proxy :: Type -> Type) | Since: base-4.7.0.0 |
| Functor (U1 :: Type -> Type) | Since: base-4.9.0.0 |
| Functor (V1 :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor m => Functor (ListT m) | |
| Functor m => Functor (MaybeT m) | |
| Functor ((,) a) | Since: base-2.1 |
| Functor f => Functor (Rec1 f) | Since: base-4.9.0.0 |
| Functor (URec (Ptr ()) :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor (URec Char :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor (URec Double :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor (URec Float :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor (URec Int :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor (URec Word :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor m => Functor (ErrorT e m) | |
| Functor m => Functor (ExceptT e m) | |
| Functor m => Functor (IdentityT m) | |
| Functor m => Functor (ReaderT r m) | |
| Functor m => Functor (StateT s m) | |
| Functor m => Functor (StateT s m) | |
| Functor m => Functor (WriterT w m) | |
| Functor m => Functor (WriterT w m) | |
| Functor ((,,) a b) | Since: base-4.14.0.0 |
| (Functor f, Functor g) => Functor (f :*: g) | Since: base-4.9.0.0 |
| (Functor f, Functor g) => Functor (f :+: g) | Since: base-4.9.0.0 |
| Functor (K1 i c :: TYPE LiftedRep -> Type) | Since: base-4.9.0.0 |
| Functor (ContT r m) | |
| Functor ((,,,) a b c) | Since: base-4.14.0.0 |
| Functor ((->) r) | Since: base-2.1 |
| (Functor f, Functor g) => Functor (f :.: g) | Since: base-4.9.0.0 |
| Functor f => Functor (M1 i c f) | Since: base-4.9.0.0 |
| Functor m => Functor (RWST r w s m) | |
| Functor m => Functor (RWST r w s m) | |
class Monad m => MonadFix (m :: Type -> Type) where #
Monads having fixed points with a 'knot-tying' semantics.
Instances of MonadFix should satisfy the following laws:
- Purity
mfix(return. h) =return(fixh)- Left shrinking (or Tightening)
mfix(\x -> a >>= \y -> f x y) = a >>= \y ->mfix(\x -> f x y)- Sliding
mfix(liftMh . f) =liftMh (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.
Methods
Instances
class Monad m => MonadFail (m :: Type -> Type) where #
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
Instances
mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) #
Map each element of a structure to a monadic action, evaluate
these actions from left to right, and collect the results. For
a version that ignores the results see mapM_.
Examples
sequence :: (Traversable t, Monad m) => t (m a) -> m (t a) #
Evaluate each monadic action in the structure from left to
right, and collect the results. For a version that ignores the
results see sequence_.
Examples
Basic usage:
The first two examples are instances where the input and
and output of sequence are isomorphic.
>>>sequence $ Right [1,2,3,4][Right 1,Right 2,Right 3,Right 4]
>>>sequence $ [Right 1,Right 2,Right 3,Right 4]Right [1,2,3,4]
The following examples demonstrate short circuit behavior
for sequence.
>>>sequence $ Left [1,2,3,4]Left [1,2,3,4]
>>>sequence $ [Left 0, Right 1,Right 2,Right 3,Right 4]Left 0
class Monad m => MonadIO (m :: Type -> Type) where #
Monads in which IO computations may be embedded.
Any monad built by applying a sequence of monad transformers to the
IO monad will be an instance of this class.
Instances should satisfy the following laws, which state that liftIO
is a transformer of monads:
Methods
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 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 IO a(m a): liftIO
> evalStateT printState "hello" "hello" > evalStateT printState 3 3
Instances
zipWithM_ :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m () #
zipWithM :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c] #
unless :: Applicative f => Bool -> f () -> f () #
The reverse of when.
replicateM_ :: Applicative m => Int -> m a -> m () #
replicateM :: Applicative m => Int -> m a -> m [a] #
replicateM n actact n times,
and then returns the list of results:
Examples
>>>import Control.Monad.State>>>runState (replicateM 3 $ state $ \s -> (s, s + 1)) 1([1,2,3],4)
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 monad.
forever :: Applicative f => f a -> f b #
Repeat an action indefinitely.
Examples
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 socketforkFinally(echo client) (\_ -> hClose client) where echo :: Handle -> IO () echo client =forever$ hGetLine client >>= hPutStrLn client
Note that "forever" isn't necessarily non-terminating.
If the action is in a MonadPlusforevermzero, effectively short-circuiting its caller.
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.
filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a] #
This generalizes the list-based filter function.
(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c infixr 1 #
Left-to-right composition of Kleisli arrows.
'(bs ' can be understood as the >=> cs) ado expression
do b <- bs a cs b
forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b) #
sequence_ :: (Foldable t, Monad m) => t (m a) -> m () #
Evaluate each monadic action in the structure from left to right,
and ignore the results. For a version that doesn't ignore the
results see sequence.
sequence_ is just like sequenceA_, but specialised to monadic
actions.
fix ff,
i.e. the least defined x such that f x = x.
For example, we can write the factorial function using direct recursion as
>>>let fac n = if n <= 1 then 1 else n * fac (n-1) in fac 5120
This uses the fact that Haskell’s let introduces recursive bindings. We can
rewrite this definition using fix,
>>>fix (\rec n -> if n <= 1 then 1 else n * rec (n-1)) 5120
Instead of making a recursive call, we introduce a dummy parameter rec;
when used within fix, this parameter then refers to fix’s argument, hence
the recursion is reintroduced.
void :: Functor f => f a -> f () #
void valueIO action.
Examples
Replace the contents of a Maybe Int
>>>void NothingNothing>>>void (Just 3)Just ()
Replace the contents of an Either Int IntEither 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
class (Alternative m, Monad m) => MonadPlus (m :: Type -> Type) where #
Monads that also support choice and failure.
Minimal complete definition
Nothing
Methods
The identity of mplus. It should also satisfy the equations
mzero >>= f = mzero v >> mzero = mzero
The default definition is
mzero = empty
An associative operation. The default definition is
mplus = (<|>)
Instances
| MonadPlus P | Since: base-2.1 |
Defined in Text.ParserCombinators.ReadP | |
| MonadPlus ReadP | Since: base-2.1 |
| MonadPlus IO | Since: base-4.9.0.0 |
| MonadPlus Maybe | Since: base-2.1 |
| MonadPlus [] | Since: base-2.1 |
| MonadPlus (Proxy :: Type -> Type) | Since: base-4.9.0.0 |
| MonadPlus (U1 :: Type -> Type) | Since: base-4.9.0.0 |
| Monad m => MonadPlus (ListT m) | |
| Monad m => MonadPlus (MaybeT m) | |
| MonadPlus f => MonadPlus (Rec1 f) | Since: base-4.9.0.0 |
| (Monad m, Error e) => MonadPlus (ErrorT e m) | |
| (Monad m, Monoid e) => MonadPlus (ExceptT e m) | |
| MonadPlus m => MonadPlus (IdentityT m) | |
| MonadPlus m => MonadPlus (ReaderT r m) | |
| MonadPlus m => MonadPlus (StateT s m) | |
| MonadPlus m => MonadPlus (StateT s m) | |
| (Monoid w, MonadPlus m) => MonadPlus (WriterT w m) | |
| (Monoid w, MonadPlus m) => MonadPlus (WriterT w m) | |
| (MonadPlus f, MonadPlus g) => MonadPlus (f :*: g) | Since: base-4.9.0.0 |
| MonadPlus f => MonadPlus (M1 i c f) | Since: base-4.9.0.0 |
| (Monoid w, MonadPlus m) => MonadPlus (RWST r w s m) | |
| (Monoid w, MonadPlus m) => MonadPlus (RWST r w s m) | |
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.
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
(=<<) :: Monad m => (a -> m b) -> m a -> m b infixr 1 #
Same as >>=, but with the arguments interchanged.
class MonadTrans (t :: (Type -> Type) -> Type -> Type) where #
The class of monad transformers. Instances should satisfy the
following laws, which state that lift is a monad transformation:
Methods
lift :: Monad m => m a -> t m a #
Lift a computation from the argument monad to the constructed monad.
Instances
| MonadTrans ListT | |
Defined in Control.Monad.Trans.List | |
| MonadTrans MaybeT | |
Defined in Control.Monad.Trans.Maybe | |
| MonadTrans (ErrorT e) | |
Defined in Control.Monad.Trans.Error | |
| MonadTrans (ExceptT e) | |
Defined in Control.Monad.Trans.Except | |
| MonadTrans (IdentityT :: (Type -> Type) -> Type -> Type) | |
Defined in Control.Monad.Trans.Identity | |
| MonadTrans (ReaderT r) | |
Defined in Control.Monad.Trans.Reader | |
| MonadTrans (StateT s) | |
Defined in Control.Monad.Trans.State.Lazy | |
| MonadTrans (StateT s) | |
Defined in Control.Monad.Trans.State.Strict | |
| Monoid w => MonadTrans (WriterT w) | |
Defined in Control.Monad.Trans.Writer.Lazy | |
| Monoid w => MonadTrans (WriterT w) | |
Defined in Control.Monad.Trans.Writer.Strict | |
| MonadTrans (ContT r) | |
Defined in Control.Monad.Trans.Cont | |
| Monoid w => MonadTrans (RWST r w s) | |
Defined in Control.Monad.Trans.RWS.Lazy | |
| Monoid w => MonadTrans (RWST r w s) | |
Defined in Control.Monad.Trans.RWS.Strict | |
class Monad m => MonadError e (m :: Type -> Type) | m -> e where #
The strategy of combining computations that can throw exceptions by bypassing bound functions from the point an exception is thrown to the point that it is handled.
Is parameterized over the type of error information and
the monad type constructor.
It is common to use Either StringMonadError class.
You can also define your own error type and/or use a monad type constructor
other than Either StringEither IOErrorMonadError
class.
(If you are using the deprecated Control.Monad.Error or
Control.Monad.Trans.Error, you may also have to define an Error instance.)
Minimal complete definition
Methods
catchError :: m a -> (e -> m a) -> m a #
A handler function to handle previous errors and return to normal execution. A common idiom is:
do { action1; action2; action3 } `catchError` handlerwhere the action functions can call throwError.
Note that handler and the do-block must have the same return type.
Instances
mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b #
Map the unwrapped computation using the given function.
runExceptT(mapExceptTf m) = f (runExceptTm)
runExcept :: Except e a -> Either e a #
Extractor for computations in the exception monad.
(The inverse of except).
runExceptT :: ExceptT e m a -> m (Either e a) #
The inverse of ExceptT.
withExcept :: (e -> e') -> Except e a -> Except e' a #
Transform any exceptions thrown by the computation using the given
function (a specialization of withExceptT).
withExceptT :: forall (m :: Type -> Type) e e' a. Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a #
Transform any exceptions thrown by the computation using the given function.
newtype ExceptT e (m :: Type -> Type) a #
A monad transformer that adds exceptions to other monads.
ExceptT constructs a monad parameterized over two things:
- e - The exception type.
- m - The inner monad.
The return function yields a computation that produces the given
value, while >>= sequences two subcomputations, exiting on the
first exception.
Instances
throwError :: (MonadError error m, CoHas option error) => option -> m a Source #
Begin error processing for the error of type option.
This is Control.Monad.Except's throwError
with the type adjusted for better compatibility with CoHas.
liftEither :: (MonadError error m, CoHas option error) => Either option a -> m a Source #
Lifts an Either option into any MonadError error where option can be injected into error.
This is Control.Monad.Except's liftEither
with the type adjusted for better compatibility with CoHas.