Data.Some.Church

Documentation

newtype Some tag

Existential. This is type is useful to hide GADTs' parameters.

>>> data Tag :: * -> * where TagInt :: Tag Int; TagBool :: Tag Bool
>>> instance GShow Tag where gshowsPrec _ TagInt = showString "TagInt"; gshowsPrec _ TagBool = showString "TagBool"
>>> classify s = case s of "TagInt" -> [mkGReadResult TagInt]; "TagBool" -> [mkGReadResult TagBool]; _ -> []
>>> instance GRead Tag where greadsPrec _ s = [ (r, rest) | (con, rest) <-  lex s, r <- classify con ]

With Church-encoding youcan only use a functions:

>>> let y = mkSome TagBool
>>> y
mkSome TagBool

>>> withSome y $ \y' -> case y' of { TagInt -> "I"; TagBool -> "B" } :: String
"B"

or explicitly work with S

>>> let x = S $ \f -> f TagInt
>>> x
mkSome TagInt

>>> case x of S f -> f $ \x' -> case x' of { TagInt -> "I"; TagBool -> "B" } :: String
"I"

The implementation of mapSome is safe.

>>> let f :: Tag a -> Tag a; f TagInt = TagInt; f TagBool = TagBool
>>> mapSome f y
mkSome TagBool

but you can also use:

>>> withSome y (mkSome . f)
mkSome TagBool

>>> read "Some TagBool" :: Some Tag
mkSome TagBool

>>> read "mkSome TagInt" :: Some Tag
mkSome TagInt

Constructors

S

Fields

• withSome :: forall r. (forall a. tag a -> r) -> r

  Eliminator.

Instances

GEq tag => Eq (Some tag)

Defined in Data.GADT.Internal

Methods

(==) :: Some tag -> Some tag -> Bool

(/=) :: Some tag -> Some tag -> Bool

GCompare tag => Ord (Some tag)

Defined in Data.GADT.Internal

Methods

compare :: Some tag -> Some tag -> Ordering

(<) :: Some tag -> Some tag -> Bool

(<=) :: Some tag -> Some tag -> Bool

(>) :: Some tag -> Some tag -> Bool

(>=) :: Some tag -> Some tag -> Bool

max :: Some tag -> Some tag -> Some tag

min :: Some tag -> Some tag -> Some tag

GRead f => Read (Some f)

Defined in Data.GADT.Internal

Methods

readsPrec :: Int -> ReadS (Some f)

readList :: ReadS [Some f]

readPrec :: ReadPrec (Some f)

readListPrec :: ReadPrec [Some f]

GShow tag => Show (Some tag)

Defined in Data.GADT.Internal

Methods

showsPrec :: Int -> Some tag -> ShowS

show :: Some tag -> String

showList :: [Some tag] -> ShowS

Applicative m => Semigroup (Some m)

Defined in Data.GADT.Internal

Methods

(<>) :: Some m -> Some m -> Some m

sconcat :: NonEmpty (Some m) -> Some m

stimes :: Integral b => b -> Some m -> Some m

Applicative m => Monoid (Some m)

Defined in Data.GADT.Internal

Methods

mempty :: Some m

mappend :: Some m -> Some m -> Some m

mconcat :: [Some m] -> Some m

mkSome :: tag a -> Some tag

Constructor.

mapSome :: (forall x. f x -> g x) -> Some f -> Some g

Map over argument.

withSomeM :: Monad m => m (Some tag) -> (forall a. tag a -> m r) -> m r

Monadic withSome.

Since: 1.0.1

foldSome :: (forall a. tag a -> b) -> Some tag -> b

flip withSome

traverseSome :: Functor m => (forall a. f a -> m (g a)) -> Some f -> m (Some g)

Traverse over argument.