On Wed, Sep 16, 2009 at 7:12 PM, Ryan Ingram
Here's the difference between these two types:
test1 :: forall a. a -> Int -- The caller of test1 determines the type for test1 test2 :: (forall a. a) -> Int -- The internals of test2 determines what type, or types, to instantiate the argument at
I can easily understand your explanation for test2: the type var a is closed under existential (?) quantification. I can't do the same for test1, even if it seems that a is closed under universal (?) quantification as well.
Or, to put it another way, since there are no non-bottom objects of type (forall a. a):
Why?
test1 converts *anything* to an Int.
Is the only possible implementation of test1 the one ignoring its argument (apart from bottom of course)?
test2 converts *nothing* to an Int
-- type-correct implementation -- instantiates the (forall a. a) argument at Int test2 x = x
-- type error, the caller chose "a" and it is not necessarily Int -- test1 x = x
-- type-correct implementation: ignore the argument test1 _ = 1
Cristiano