Try {-# LANGUAGE RankNTypes #-}? "forall" does denote a universal quantifier, but because the 'implies' of the function arrow, in logic, includes negation, you can use it to emulate existential quantifiers.
data Existential = forall a. Ex a
The type of the constructor Ex: Ex :: forall a. a -> Existential Pattern matching on Ex brings "a" back into scope (with no information about it, so this type isn't that useful on its own):
use (Ex x) = 0 -- can't recover any useful information about x sample = Ex "sample"
However, you can use existential types to encode additional information about the included data; for example:
-- Ex2 :: forall a. Show a => a -> Exist2 data Exist2 = forall a. Show a => Ex2 a
Now, pattern matching on Ex2 brings the Show instance into scope as well:
sample2 = Ex2 "sample2" use2 (Ex2 x) = show x
You can also use higher rank polymorphism to encode existential types:
-- Ex3 :: (forall a. (forall b. Show b => b -> a) -> a) -> Exist3 -- note the rank-3 type on Ex3! newtype Exist3 = Ex3 (forall a. (forall b. Show b => b -> a) -> a)
sample3 = Ex3 (\k -> k "sample3") use3 (Ex3 f) = f (\x -> show x)
-- ryan
2008/7/8 Galchin, Vasili
Hello,
It seems to me by its name that "forall" denotes a logical universal quantifier. In any case, hsql-1.7/Database/HSQL/Types.hs uses "forall" at line #134. I got a nasty build so I added {-# LANGUAGE ExistentialQuantification #-} at the top of the module. Now I get the following a coupleof lines up:
Database/HSQL/Types.hs:131:5: Illegal polymorphic or qualified type: forall a. Int -> FieldDef -> FieldDef -> CString -> Int -> IO a -> IO a In the definition of data constructor `Statement' In the data type declaration for `Statement'
If seems that GHC doesn't like "a". Why?
Kind regards, Vasili
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