On Thu, Mar 6, 2008 at 3:57 PM, ChrisK <[EMAIL PROTECTED]> wrote:
> Okay, I get the difference.
>
> The "T a" annotation in "val :: T a)"and "val :: T a" does not help choose
> the
> "C a" dictionary.
> But the "val :: a-> T a" and "val (undefined :: a)" allows "a" to
> successfully
> choose the "C a" dictionary.
>
> val :: T a fixes "T a" but does not imply "C a".
> (undefined :: a) fixes "a" and does imply "C a".
> I now see how the functional dependency works here (which I should have
> tried to
> do in the first place -- I should have thought more and relied on the mailing
> list less).
>
> "class C a b | a -> b" is here "class C a where type T a = b".
> So only knowing "T a" or "b" does not allow "a" to be determined.
Am I correct in thinking this would have worked if it were an
associated type instead of an associated type synonym?
ie,
class C a where
data T a
val :: T a
--
Dave Menendez <[EMAIL PROTECTED]>
<http://www.eyrie.org/~zednenem/>
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
