Lazy variant of Hashed?

[mikeplus64]

This would be useful for me for a type (living in a TVar) that:

• updates frequently
• has an expensive hash function
• needs to check hashes infrequently

The implementation should be exactly the same as 'Hashed' without the {-# UNPACK #-} !Int. Maybe have a typeclass:

class Hashable1 h => CacheableHashable h where
  hashed :: Hashable a => a -> h a
  unhashed :: h a -> a

[phadej]

I'm not sure I understand a motivation. Hashed is introduced to memoize expensive-ish hashes, so that is the same. Yet it feels that you use hashable as some kind of checksum, and that is wrong use-case.