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

[gs]

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

> It's an implementation issue, yes. But DatatypeContexts were specified so
that 
> an implementation wasn't allowed to make the dictionary available without 
> context.

Thank you for your patience in explaining everything. It's a pity that
DatatypeContexts don't do this - it would make my code much cleaner.