[Haskell-cafe] Re: Small displeasure with associated type synonyms
dave at zednenem.com
Thu Mar 6 22:26:30 EST 2008
On Thu, Mar 6, 2008 at 3:57 PM, ChrisK <haskell at list.mightyreason.com> 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?
class C a where
data T a
val :: T a
Dave Menendez <dave at zednenem.com>
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