Data.Some

An existential type.

The constructor is exported only on GHC-8 and later.

Documentation

data Some tag where

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 ]

You can either use PatternSynonyms (available with GHC >= 8.0)

>>> let x = Some TagInt
>>> x
Some TagInt

>>> case x of { Some TagInt -> "I"; Some TagBool -> "B" } :: String
"I"

or you can use functions

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

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

The implementation of mapSome is safe.

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

but you can also use:

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

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

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

Bundled Patterns

pattern Some :: tag a -> Some tag

Instances

GEq tag => Eq (Some tag)

Defined in Data.Some.Newtype

Methods

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

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

GCompare tag => Ord (Some tag)

Defined in Data.Some.Newtype

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.Some.Newtype

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.Some.Newtype

Methods

showsPrec :: Int -> Some tag -> ShowS

show :: Some tag -> String

showList :: [Some tag] -> ShowS

Applicative m => Semigroup (Some m)

Defined in Data.Some.Newtype

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.Some.Newtype

Methods

mempty :: Some m

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

mconcat :: [Some m] -> Some m

GNFData tag => NFData (Some tag)

Defined in Data.Some.Newtype

Methods

rnf :: Some tag -> ()

mkSome :: tag a -> Some tag

Constructor.

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

Eliminator.

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

Map over argument.

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.