On Wednesday 24 April 2013, 14:19:35, gs wrote:
So as far as removing redundant code is concerned, this can't really be done?
That depends. Your example is not well-suited for that. In other situations, you can get rid of contexts well.
The instance declarations still need the redundant context,
Well, the context is not redundant, that's the crux. You can specify the type `Source v a` even when there is no `Variable v` instance [whether you use DatatypeContexts - which, again, are pretty useless because they don't do what one would expect - or GADTs]. And such a type is inhabited by bottom, you just can't create non-bottom values of such a type. Hence the `Variable v` context gives additional information that is needed for the instance. If you had a `v a` as argument in all methods of `class Variable v`, you could write the instance Variable (Source v) where ... without the `Variable v` context by pattern-matching on `Source` to make the `v` instance available (when using GADTs), then you wouldn't need the `Variable v` context on the `Bindable` instance either (similarly for BindingList). But to have a usable `newVar`, you need the context, and that propagates.
and function definitions need a redundant pattern-matching instead of a redundant context.
The pattern-matching isn't redundant either. But all in all, basically you have the choice between adding a context or pattern-matching for stuff like that.