GHC 9.10.1 is now available!

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It’s a nice trick. A lower technology approach would be to define a second function outside of the class:

class Storable a where
  sizeOfImpl :: Int

sizeOf :: forall a -> Storable a => Int
sizeOf t = sizeOfImpl @t

That way it’s also marginally easier to define instances (you don’t have to explicitly ignore the type variable).

instance Storable Int where
  sizeOfImpl = 8

EDIT: Oh, and I used something similar to the trick in withST in TypeAbstractions in GHC 9.10 - #5 by tomjaguarpaw

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