[Haskell-beginners] How to avoid repeating a type restriction from a data constructor

[Daniel Fischer]

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.