On Wednesday 19 January 2005 14:31, Keean Schupke wrote:
I may have got this wrong, but I think you can do named instances without any extensions, by using datatypes and fundeps:
data Instance0 data Instance1
instance0 :: Instance0 instance0 = undefined
instance1 :: Instance1 instance1 = undefined
class Named a b | a -> b test :: a -> b -> b instance Named Instance0 Int test _ a = a + a instance Named Instance1 Float test _ a = a * a
test instance0 1 test instance1 1.5
Sure you can. However, the extension presented in the paper goes way beyond this (while still being conservative). For instance, named instances can be defined for standard Haskell98 classes, they don't need special class definitions as in your example. For a given class, standard unnamed instances may be used together with additional named instances. Another benefit of the proposal is that it slves a number of problems regarding multi-parameter classes quite elegantly. But the reason why I mentioned the paper was that it offers a lot of insight into exactly what Ashley Yakeley was thinking about. Citing again:
I have been musing on the connection between data-types, modules, classes, and implicit parameters, and wondering if there might be some grand scheme to tie it all together.
Neither I nor the authors claim that their proposal is the ultimate "grand scheme", yet. Still I think there are very interesting ideas in there that should be considered for experimental implementation or further research. Ben