On Thu, 9 Aug 2001, Ashley Yakeley wrote:
At 2001-08-08 23:00, Hal Daume III wrote: If there were such a thing, it would be incoherent to say: "if a is not TypeName, then a is TypeName with this typeName".
there are other places where i would find such a thing useful/interesting beyond this TypeName foo, but this seemed like a good, simple way to explain it...
Can you think of a use for it that isn't semantically incoherent?
So say you wanted to define a sort of "equality" on pairs (this isn't made up -- I wanted to do just this, recently). Say you have: data MyPair a b = MyPair a b instance (Eq a, Eq b) => Eq (MyPair a b) where (MyPair a b) == (MyPair a' b') = (a == a') && (b == b') this is obvious. but supposing you wanted a sort of relaxed equality wherein, if you chould check for equality on the second element, you would, otherwise you would just check the first element. i would want to say something like: instance (Eq a, Not (Eq b)) => Eq (MyPair a b) where (MyPair a _) == (MyPair a' _) = a == a' But from what you're telling me there's no way to have both of these instances, correct? - Hal ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Hal Daume III hal@cmu.edu "arrest this man, he talks in maths" www.andrew.cmu.edu/~hcd ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~