I’ve now cleaned up (in befb0f3cca0c212e368497e86f030aa96355be18) the Reader and Writer interfaces and added it to Statistics.GModeling.Gibbs. I’ve removed references to Support and simply parameterized using a key type k and value type v.

> data Reader k v = Reader
>   {
>     -- | Number of available indices
>     size :: Int
>     -- | Read the value at the given key
>   , readn :: Int -> k -> v
>     -- | Create a copy for writing only
>   , copy :: IO (Writer k v)
>   }
> 
> data Writer k v = Writer
>   {
>     -- | Write the value at the given key
>     writen :: Int -> k -> v -> IO ()
>     -- | Create a read-only copy
>   , readOnly :: IO (Reader k v)
>   }

I’ve also simplified the type of Indexed and added an implementation of Reader and Writer for HMM in Statistics.GModeling.Models.HMM.