| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Control.Effect.Internal.Derive
Description
Module exporting typical type classes that are newtype-derived by Carriers
Synopsis
- class Applicative f => Alternative (f :: Type -> Type)
- class (Alternative m, Monad m) => MonadPlus (m :: Type -> Type)
- class Monad m => MonadFix (m :: Type -> Type)
- class Monad m => MonadFail (m :: Type -> Type)
- class Monad m => MonadIO (m :: Type -> Type)
- class Monad m => MonadThrow (m :: Type -> Type)
- class MonadThrow m => MonadCatch (m :: Type -> Type)
- class MonadCatch m => MonadMask (m :: Type -> Type)
- class (Applicative b, Applicative m, Monad b, Monad m) => MonadBase (b :: Type -> Type) (m :: Type -> Type) | m -> b
- class MonadBase b m => MonadBaseControl (b :: Type -> Type) (m :: Type -> Type) | m -> b
- class MonadTrans (t :: (Type -> Type) -> Type -> Type)
- class MonadTrans t => MonadTransControl (t :: (Type -> Type) -> Type -> Type)
- data IdentityT (f :: k -> Type) (a :: k)
Documentation
class Applicative f => Alternative (f :: Type -> Type) #
A monoid on applicative functors.
If defined, some and many should be the least solutions
of the equations:
Instances
class (Alternative m, Monad m) => MonadPlus (m :: Type -> Type) #
Monads that also support choice and failure.
Instances
class Monad m => MonadFix (m :: Type -> Type) #
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.
Minimal complete definition
Instances
class Monad m => MonadFail (m :: Type -> Type) #
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
Instances
class Monad m => MonadIO (m :: Type -> Type) #
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:
Minimal complete definition
Instances
class Monad m => MonadThrow (m :: Type -> Type) #
A class for monads in which exceptions may be thrown.
Instances should obey the following law:
throwM e >> x = throwM e
In other words, throwing an exception short-circuits the rest of the monadic computation.
Minimal complete definition
Instances
class MonadThrow m => MonadCatch (m :: Type -> Type) #
A class for monads which allow exceptions to be caught, in particular
exceptions which were thrown by throwM.
Instances should obey the following law:
catch (throwM e) f = f e
Note that the ability to catch an exception does not guarantee that we can
deal with all possible exit points from a computation. Some monads, such as
continuation-based stacks, allow for more than just a success/failure
strategy, and therefore catch cannot be used by those monads to properly
implement a function such as finally. For more information, see
MonadMask.
Minimal complete definition
Instances
class MonadCatch m => MonadMask (m :: Type -> Type) #
A class for monads which provide for the ability to account for all possible exit points from a computation, and to mask asynchronous exceptions. Continuation-based monads are invalid instances of this class.
Instances should ensure that, in the following code:
fg = f `finally` g
The action g is called regardless of what occurs within f, including
async exceptions. Some monads allow f to abort the computation via other
effects than throwing an exception. For simplicity, we will consider aborting
and throwing an exception to be two forms of "throwing an error".
If f and g both throw an error, the error thrown by fg depends on which
errors we're talking about. In a monad transformer stack, the deeper layers
override the effects of the inner layers; for example, ExceptT e1 (Except
e2) a represents a value of type Either e2 (Either e1 a), so throwing both
an e1 and an e2 will result in Left e2. If f and g both throw an
error from the same layer, instances should ensure that the error from g
wins.
Effects other than throwing an error are also overriden by the deeper layers.
For example, StateT s Maybe a represents a value of type s -> Maybe (a,
s), so if an error thrown from f causes this function to return Nothing,
any changes to the state which f also performed will be erased. As a
result, g will see the state as it was before f. Once g completes,
f's error will be rethrown, so g' state changes will be erased as well.
This is the normal interaction between effects in a monad transformer stack.
By contrast, lifted-base's
version of finally always discards all of g's non-IO effects, and g
never sees any of f's non-IO effects, regardless of the layer ordering and
regardless of whether f throws an error. This is not the result of
interacting effects, but a consequence of MonadBaseControl's approach.
Minimal complete definition
Instances
class (Applicative b, Applicative m, Monad b, Monad m) => MonadBase (b :: Type -> Type) (m :: Type -> Type) | m -> b #
Minimal complete definition
Instances
class MonadBase b m => MonadBaseControl (b :: Type -> Type) (m :: Type -> Type) | m -> b #
Writing instances
The usual way to write a MonadBaseControlB is to write an instance MonadBaseControl B B
for the base monad, and MonadTransControl T instances for every transformer
T. Instances for MonadBaseControlComposeStdefaultLiftBaseWithdefaultRestoreM
Minimal complete definition
Instances
class MonadTrans (t :: (Type -> Type) -> Type -> Type) #
The class of monad transformers. Instances should satisfy the
following laws, which state that lift is a monad transformation:
Minimal complete definition
Instances
class MonadTrans t => MonadTransControl (t :: (Type -> Type) -> Type -> Type) #
The MonadTransControl type class is a stronger version of MonadTrans
Instances of MonadTranslift
MonadTransControl
This allows to lift functions that have a monad transformer in both positive and negative position. Take, for example, the function
withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
MonadTranswithFile
function:
withFileLifted :: MonadTrans t => FilePath -> IOMode -> (Handle -> IO r) -> t IO r withFileLifted file mode action = lift (withFile file mode action)
However, MonadTranswithFileLifted
accept a function that returns t IO. The reason is that we need to take
away the transformer layer in order to pass the function to withFileMonadTransControl
withFileLifted' :: (Monad (t IO), MonadTransControl t) => FilePath -> IOMode -> (Handle -> t IO r) -> t IO r withFileLifted' file mode action = liftWith (\run -> withFile file mode (run . action)) >>= restoreT . return
Instances
data IdentityT (f :: k -> Type) (a :: k) #
The trivial monad transformer, which maps a monad to an equivalent monad.