instance Lift (Dict cls) about constraints HOT 2 OPEN

Sounds reasonable to me. Care to make a pull request?

Can it be given for a :- b?

Sure, you could do this:

instance Lift (a :- b) where
  lift :: a :- b -> Q Exp
  lift d = [| Sub (Dict \\ d) |]

  liftTyped :: a :- b -> Q (TExp (a :- b))
  liftTyped d = [|| Sub (Dict \\ d) ||]

from constraints.

On the other hand, @mpickering reminds me that if these instances were to be defined, they would have to be used with care, as today's typed Template Haskell is unsound. For instance, here is an example of unsoundness that can result from splicing in Dict:

f :: a => Q (TExp (Dict a))
f = liftTyped Dict

g :: a => Dict a
g = $$f

error:
    • Could not deduce: a arising from a use of ‘f’
    • In the expression: f
      In the Template Haskell splice $$f
      In the expression: $$f
    • Relevant bindings include g :: Dict a
   |
   | g = $$f
   |       ^

from constraints.