Data.Functor.Trans.Tagged

Tagged values

newtype TaggedT s m b

A Tagged s b value is a value b with an attached phantom type s. This can be used in place of the more traditional but less safe idiom of passing in an undefined value with the type, because unlike an (s -> b), a Tagged s b can't try to use the argument s as a real value.

Moreover, you don't have to rely on the compiler to inline away the extra argument, because the newtype is "free"

Constructors

TagT

Fields

untagT :: m b

Instances

MonadReader r m => MonadReader r (TaggedT s m)

MonadWriter w m => MonadWriter w (TaggedT s m)

MonadState t m => MonadState t (TaggedT s m)

ComonadHoist (TaggedT s)

ComonadTrans (TaggedT s)

MonadTrans (TaggedT s)

Monad m => Monad (TaggedT s m)

Functor m => Functor (TaggedT s m)

MonadFix m => MonadFix (TaggedT s m)

MonadPlus m => MonadPlus (TaggedT s m)

Applicative m => Applicative (TaggedT s m)

Foldable f => Foldable (TaggedT s f)

Traversable f => Traversable (TaggedT s f)

Alternative m => Alternative (TaggedT s m)

Comonad w => Comonad (TaggedT s w)

Contravariant m => Contravariant (TaggedT s m)

Distributive f => Distributive (TaggedT s f)

MonadCatch m => MonadCatch (TaggedT s m)

Plus m => Plus (TaggedT s m)

Alt m => Alt (TaggedT s m)

Apply m => Apply (TaggedT s m)

Bind m => Bind (TaggedT s m)

Extend f => Extend (TaggedT s f)

MonadIO m => MonadIO (TaggedT s m)

MonadCont m => MonadCont (TaggedT s m)

Eq (m b) => Eq (TaggedT s m b)

Ord (m b) => Ord (TaggedT s m b)

Read (m b) => Read (TaggedT s m b)

Show (m b) => Show (TaggedT s m b)

tag :: m b -> TaggedT s m b

Easier to type alias for TagT

untag :: TaggedT s m b -> m b

Easier to type alias for untagT

retag :: TaggedT s m b -> TaggedT t m b

Some times you need to change the tag you have lying around. Idiomatic usage is to make a new combinator for the relationship between the tags that you want to enforce, and define that combinator using retag.

 data Succ n
 retagSucc :: Tagged n a -> Tagged (Succ n) a
 retagSucc = retag

mapTaggedT :: (m a -> n b) -> TaggedT s m a -> TaggedT s n b

Lift an operation on underlying monad

reflected :: forall s m a. (Applicative m, Reifies s a) => TaggedT s m a

Reflect reified value back in Applicative context

reflectedM :: forall s m a. (Monad m, Reifies s a) => TaggedT s m a

Reflect reified value back in Monad context

asTaggedTypeOf :: s -> TaggedT s m b -> s

asTaggedTypeOf is a type-restricted version of const. It is usually used as an infix operator, and its typing forces its first argument (which is usually overloaded) to have the same type as the tag of the second.