Control.Monad.Trans.Maybe · rerebase-1.21.2

valueliftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b

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Lift a callCC operation to the new monad.

valueexceptToMaybeT :: Functor m => ExceptT e m a -> MaybeT m a

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Convert a ExceptT computation to MaybeT, discarding the value of any exception.

valuemapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b

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Transform the computation inside a MaybeT.

runMaybeT (mapMaybeT f m) = f (runMaybeT m)

valuemaybeToExceptT :: Functor m => e -> MaybeT m a -> ExceptT e m a

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Convert a MaybeT computation to ExceptT, with a default exception value.

newtypenewtype MaybeT (m :: Type -> Type) a

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The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.

Computations are actions that may produce a value or exit.

The return function yields a computation that produces that value, while >>= sequences two subcomputations, exiting if either computation does.

Constructors

MaybeT
- runMaybeT :: m (Maybe a)

Instances52MonadTrans, MonadRWS, Generic1, MonadAccum, MonadError, MonadReader, …

MonadTrans MaybeTDefined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
MonadRWS r w s m => MonadRWS r w s (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.RWS.Class
Functor m => Generic1 (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
MonadAccum w m => MonadAccum w (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Accum
The accumulated value 'survives' an error: even if the computation fails to deliver a result, we still have an accumulated value.
MonadError e m => MonadError e (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Error.Class
MonadReader r m => MonadReader r (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Reader.Class
MonadState s m => MonadState s (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.State.Class
MonadWriter w m => MonadWriter w (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadSelect r m => MonadSelect r (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Select
'Extends' the possibilities considered by m to include Nothing; this means that Nothing gains a 'rank' (namely, a value of r), and the potential result could also be Nothing.
MonadBase b m => MonadBase b (MaybeT m)Defined in transformers-base-0.4.6 · Control.Monad.Base
(Functor f, MonadFree f m) => MonadFree f (MaybeT m)Defined in free-5.2 · Control.Monad.Free.Class
Monad m => Monad (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Functor m => Functor (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
MonadFix m => MonadFix (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Monad m => MonadFail (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
(Functor m, Monad m) => Applicative (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Foldable f => Foldable (MaybeT f)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Traversable f => Traversable (MaybeT f)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
(Functor m, Monad m) => Alternative (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Monad m => MonadPlus (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
MonadIO m => MonadIO (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
MonadZip m => MonadZip (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Eq1 m => Eq1 (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Ord1 m => Ord1 (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Read1 m => Read1 (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Show1 m => Show1 (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Contravariant m => Contravariant (MaybeT m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
MonadCont m => MonadCont (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Cont.Class
MonadCatch m => MonadCatch (MaybeT m)Defined in exceptions-0.10.9 · Control.Monad.Catch
Catches exceptions from the base monad.
MonadMask m => MonadMask (MaybeT m)Defined in exceptions-0.10.9 · Control.Monad.Catch
MonadThrow m => MonadThrow (MaybeT m)Defined in exceptions-0.10.9 · Control.Monad.Catch
Throws exceptions into the base monad.
PrimMonad m => PrimMonad (MaybeT m)Defined in primitive-0.9.1.0 · Control.Monad.Primitive
Divisible m => Decidable (MaybeT m)Defined in contravariant-1.5.5 · Data.Functor.Contravariant.Divisible
Divisible m => Divisible (MaybeT m)Defined in contravariant-1.5.5 · Data.Functor.Contravariant.Divisible
(Functor f, Monad f) => Alt (MaybeT f)Defined in semigroupoids-6.0.1 · Data.Functor.Alt
(Functor m, Monad m) => Apply (MaybeT m)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Class
(Functor m, Monad m) => Bind (MaybeT m)Defined in semigroupoids-6.0.1 · Data.Functor.Bind.Class
(Divisible m, Divise m) => Conclude (MaybeT m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Conclude
This instance is only available if the +contravariant cabal flag is enabled.
Divise m => Decide (MaybeT m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Decide
Divise m => Divise (MaybeT m)Defined in semigroupoids-6.0.1 · Data.Functor.Contravariant.Divise
(Functor f, Monad f) => Plus (MaybeT f)Defined in semigroupoids-6.0.1 · Data.Functor.Plus
Invariant m => Invariant (MaybeT m)Defined in invariant-0.6.4 · Data.Functor.Invariant
from the transformers package
Monad m => Selective (MaybeT m)Defined in selective-0.7.0.1 · Control.Selective
Pointed m => Pointed (MaybeT m)Defined in pointed-5.0.4 · Data.Pointed
(Eq1 m, Eq a) => Eq (MaybeT m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
(Ord1 m, Ord a) => Ord (MaybeT m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
(Read1 m, Read a) => Read (MaybeT m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
(Show1 m, Show a) => Show (MaybeT m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
Generic (MaybeT m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
type Rep (MaybeT m a) = D1 ('MetaData "MaybeT" "Control.Monad.Trans.Maybe" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "MaybeT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runMaybeT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (m (Maybe a)))))Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
type Rep1 (MaybeT m) = D1 ('MetaData "MaybeT" "Control.Monad.Trans.Maybe" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "MaybeT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runMaybeT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (m :.: Rec1 Maybe)))Defined in transformers-0.6.1.1 · Control.Monad.Trans.Maybe
type PrimState (MaybeT m) = PrimState mDefined in primitive-0.9.1.0 · Control.Monad.Primitive

valueliftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a

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Lift a listen operation to the new monad.

valueliftPass :: Monad m => Pass w m (Maybe a) -> Pass w (MaybeT m) a

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Lift a pass operation to the new monad.

valuehoistMaybe :: Applicative m => Maybe b -> MaybeT m b

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Convert a Maybe computation to MaybeT.

valueliftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a

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Lift a catchE operation to the new monad.