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Hello! I would love to see type level as-patterns in Haskell. They would be very helpfull when writing complex type families. Here is a good example:

type family XOut m1 m2 where
    XOut (ValueS vbase vs) (MonadCtx2Dummy' m2 s2 env set m s) = MonadCtx2Dummy' m2 s2 (XEnv env (ValueS vbase vs)) (XSet set (ValueS vbase vs)) m (XSafety s (ValueS vbase vs))

could then be written as:

type family XOut m1 m2 where
    XOut a@(ValueS vbase vs) (MonadCtx2Dummy' m2 s2 env set m s) = MonadCtx2Dummy' m2 s2 (XEnv env a) (XSet set a) m (XSafety s a)

which is much cleaner, less verbose and lot safer than the first one. (Safer in terms of accidental typos when refactoring etc).

Cheers, Wojciech