[Haskell-cafe] ANN: unordered-containers - a new, faster hashing-based containers library

[Emil Axelsson]

2011-02-23 13:56, Victor Nazarov skrev:
> Also I think that value of hash functions is obviously a Monoid and it
> will be convenient to have Monoid instance
>
>      newtype Hash = Hash Int
>      instance Monoid Hash where
>        mempty = Hash 0
>        Hash a `mappend` Hash b = Hash (a `combine` b)
>
>      class Eq a =>  Hashable a where
>          hash :: a ->  Hash
>          hashWithSalt :: Hash ->  a ->  Hash
>
>          hashWithSalt salt x = salt `mappend` hash x

Monoid would be a good idea if combine was associative :)

   Prelude Data.Hashable> let a = hash "a"
   Prelude Data.Hashable> let b = hash "b"
   Prelude Data.Hashable> let c = hash "c"
   Prelude Data.Hashable> (a `combine` b) `combine` c
   198573605
   Prelude Data.Hashable> a `combine` (b `combine` c)
   177445

/ Emil