[Haskell-cafe] %==%, :==: and type equality in the singletons library

[Paul Brauner]

Hello,

from the output of -ddump-splices I dont think it is the case but I'm
asking anyway: is there any way to deduce a ~ b from a :==: b?

Given

  data T = C1 | ... | Cn

I can easily derive

  data EqT :: T -> T -> * where
     EqT :: a ~ b => EqT a b

  eqT :: ST a -> ST b -> Maybe (EqT a b)
  eqT SC1 SC1 = Just EqT
  ...
  eqT SCn SCn = Just EqT
  eqT _ _ = Nothing

but this kind of replicates the boilerplate generated by the singletons
library for %==%. However I can't see how to leverage %==% to inhabit eqT
since I can't deduce a ~ b from a %==% b == STrue.

Any idea? If there's no way to write eqT using what singletons generates,
wouldn't it make sense for it to generate something that relates :==: and ~
?

Cheers,
Paul
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