Documentation

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

Constructor.

Map over argument.

Monadic withSome.

Since: 1.0.1

flip withSome

Traverse over argument.