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

[Daniel Fischer]

On Wednesday 24 April 2013, 16:45:35, gs wrote:
> 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.

No, that is the catch:

undefined :: Source NotAnInstanceOfVariable Int

is possible.

> 2) Whenever Source v a is used, the compiler should understand
> that v is a variable without me having to remind it.

But when you use the value constructor,

foo (Source list var) = ...

you know that the `v` parameter of the type must have a Variable instance.

When you use a GADT, the compiler knows that too and can use that fact, when 
you use DatatypeContexts, the compiler can't use that knowledge (even though 
it has the knowledge).

The point is that the type class dictionary is required to use the class 
methods, and with a GADT, the dictionary is bundled with the constructor, so 
pattern-matching makes the dictionary available.

Not so with DatatypeContexts, hence the dictionary must be explicitly passed 
with the context on the function.

Did I already mention that DatatypeContexts are fairly useless and have 
therefore been removed from the language?