At 2001-08-08 23:00, Hal Daume III wrote:
however, if you try something like:
instance TypeName a where typeName T = "??"
it will complain because there must be one non-type variable in the instance head.
Try using "-fglasgow-exts -fallow-undecidable-instances".
moreover, even if this restriction is removed (anyone know what it's there for?), there will be the problem of overlapping instances with, say TypeName Char and TypeName a.
Correct. And the instances really would overlap, it wouldn't just be GHC complaining that they _might_ overlap but not being able to figure out that they don't.
so i'm wondering if there is a way to specify something like:
instance Not (TypeName a) => TypeName a where ...
No.
that is, this type variable a matches with all a such that a is not an instance of TypeName...
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? -- Ashley Yakeley, Seattle WA