Safe Haskell	Safe
Language	Haskell2010

Data.Some

Description

An existential type.

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

Synopsis

data Some tag where
- pattern Some :: tag a -> Some tag
mkSome :: tag a -> Some tag
withSome :: Some tag -> (forall a. tag a -> b) -> b
withSomeM :: Monad m => m (Some tag) -> (forall a. tag a -> m r) -> m r
mapSome :: (forall t. f t -> g t) -> Some f -> Some g
foldSome :: (forall a. tag a -> b) -> Some tag -> b
traverseSome :: Functor m => (forall a. f a -> m (g a)) -> Some f -> m (Some g)

Documentation

data Some tag where Source #

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

>>> data Tag :: Type -> Type 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

Instances details

Applicative m => Monoid (Some m) Source #
Instance details Defined in Data.Some.Newtype Methods mempty :: Some m # mappend :: Some m -> Some m -> Some m # mconcat :: [Some m] -> Some m #
Applicative m => Semigroup (Some m) Source #
Instance details 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 #
GRead f => Read (Some f) Source #
Instance details 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) Source #
Instance details Defined in Data.Some.Newtype Methods showsPrec :: Int -> Some tag -> ShowS # show :: Some tag -> String # showList :: [Some tag] -> ShowS #
GNFData tag => NFData (Some tag) Source #
Instance details Defined in Data.Some.Newtype Methods rnf :: Some tag -> () #
GEq tag => Eq (Some tag) Source #
Instance details Defined in Data.Some.Newtype Methods (==) :: Some tag -> Some tag -> Bool # (/=) :: Some tag -> Some tag -> Bool #
GCompare tag => Ord (Some tag) Source #
Instance details 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 #

mkSome :: tag a -> Some tag Source #

Constructor.

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

Eliminator.

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

Monadic withSome.

Since: 1.0.1

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

Map over argument.

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

flip withSome

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

Traverse over argument.