I think I was the one confused.
I guess I was (falsely) thinking that both
C Int Char T
C Char Int T
could both be instances of class C a b c | c -> a, c -> b but only one
could be an instance of C a b c | c -> a b.
Sorry for adding noise to the discussion.
Ryan Ingram wrote:
I'm still confused about this point:
On 4/16/08, Dan Weston <[EMAIL PROTECTED]> wrote:
class C a b c | c -> a b
Notice that there are multiple (two) parameters in the range of the FD.
It's tempting to convert the above to
class C a b c | c -> a, c -> b
but this yields a weaker (in terms of type improvement) system.
In both cases the statement is that given a type x, the instance C x y
z for some y,z and the constraint C x a b, we unambiguously have a ~
y, b ~ z (where ~ is type equality)
How does the order in (c -> a b) matter?
-- ryan
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
