Mark P Jones wrote:
| I am finding functional dependencies confusing. (I suspect I am | not alone.) Should the following code work? | | class HasConverter a b | a -> b where | convert :: a -> b | | instance (HasConverter a b,Show b) => Show a where | show value = show (convert value)
It's a separate issue. There's no reason why a system using functional dependencies *should* support this. But it is an attractive and useful extension that such a system would probably *want* to include. (i.e., it's a desirable feature, not a requirement.)
I typed your example into Hugs on my machine and it seemed to accept the syntax (which suggests that the implementation is intended to allow this kind of thing). But then it choked on the definition with a curious error message that I suspect is an indication of a bug in Hugs' treatment of functional dependencies. And, for that reason, I'm crossposting this message to hugs-bugs. [Let me take the opportunity to remind good readers of these lists that it is now more than a year since I retired from the joys of maintaining Hugs, so I'm not planning to try and track down the source of this bug myself!] Of course, it could be that my version of Hugs is out of date!
Could you share the error message please. Boy, even old maintainers don't know how to submit a bug report... ;-) Actually, I suspect whatever the problem was has been fixed, because (with -98 +o), it compiles fine. (I'm using the bleeding edge, out-of-the-repository version.) --Jeff