TypeLits and type families wrt. equality

[Richard Eisenberg]

On Jul 27, 2015, at 10:36 AM, Gabor Greif <ggreif at gmail.com> wrote:

> When saying "Big Deal" did you mean
>  - highly desirable and somebody should go for it
>  - or a terrible amount of work, and who tackles it is probably a
> fool on a hubris trip?
> 
> (I still have my problems deciphering British irony.)

My use of Big Deal here is more your second option, but without so much negativity. :) I'm sure the person who tackled this problem would learn a ton and contribute to programming language research. It's just that we haven't yet found a great motivation for doing so.

> 
> I guess a full-blown solution is too much work for now (also considering
> previous comments from Richard). But what about a simpler construct
> 
> decideRefl :: Proxy (a :: Symbol) -> Proxy b
>               -> Proxy (Equal a b :~: 'False)
>               -> Either (Equal a b :~: 'False) (a :~: b)

I believe that this function could be written only with the help of unsafeCoerce. Indeed, the `singletons` package provides an `SDecide` instance for the type-lits (which accomplishes a similar, though not identical, goal) via unsafeCoerce.

Richard