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

[gs]

Daniel Fischer <daniel.is.fischer <at> googlemail.com> writes:

> 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.

I'm not following - perhaps you could give a practical example? I'm trying
to understand the flaw in this:

1) The compiler sees that a Source v a can only be created when v is a variable.
2) Whenever Source v a is used, the compiler should understand that v is a
variable without me having to remind it.