On 23/10/2007, Jules Bean
I will also repeat the non-justified assertion that others have made, and that I've made myself in the other thread, that you don't need existentials as often in haskell as you do in OO languages, and they certainly don't always need to be type-class quantified ones.
I also find this, but I worry that it's the old blub paradox again. Haskell makes it a tiny bit clumsy to do value-based dispatch, so you tend to solve problems in ways which doesn't require it. I'm not saying that's the case, but I just can't say that the reason I find myself not using existentials is because I don't need them (whereas they're immensely useful in other languages). I think I've suggested this before, but some sort of syntactical gizmo for expressing "the data type which wraps values in class C, and is also instantiated in C itself" and just automatically construct the apropriate existential whenever someone uses it, might be worthwhile. Perhaps something like enclosing the class name in angle brackets or something. There needs to be an equivalent way of wrapping the value itself (constructing the wrapper data type), of course (angle brackets again for consistency?). Or automatic type conversion could work, though that seems a bit "un-Haskell" to me. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862