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:set -XHaskell2010
:set -XRankNTypes -XConstraintKinds -XTypeApplications -XTypeOperators
import Data.Typeable
class Deferrable p where deferEither :: proxy p -> (p => r) -> Either String r
instance (Typeable a, Typeable b) => Deferrable (a ~ b) where deferEither _ _ = undefined
:t deferEither @(_ ~ _)
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