Lennart Augustsson writes: [...]
Slightly more interesting might be data Foo = forall a . Foo a (a -> Int)
Now you can at least apply the function to the value after pattern matching. You don't have to carry any types around, because the type system ensures that you don't misuse the value.
Hi. In that particular example, I'd be more inclined to use the existential properties of lazy evaluation: packFoo x f = Foo x f -- The actual type of x is buried in the existential data Bar = Bar Int packBar x f = Bar (f x) -- The actual type of x is buried in the (f x) suspension Of course, when there are more usable things which can be retrieved from the type, an explicit existential is more appealing: data Foo' = forall a . Ord a => Foo' [a] packFoo' xs = Foo' xs -- The actual type of xs is buried in the existential data Bar' = Bar' [[Ordering]] packBar' xs = Bar' oss where oss = [[compare x1 x2 | x2 <- xs] | x1 <- xs] -- The actual type of xs is buried in the oss suspension, which -- is rather bloated Regards, Tom