[Haskell-cafe] Inverse of HaskellDB

[Felipe Lessa]

On Wed, Sep 29, 2010 at 7:21 AM, Michael Snoyman <michael at snoyman.com> wrote:
> I think this approach is not possible without involving some fairly
> ugly unsafeInterleaveIO/unsafePerformIO calls. A simple example using
> a common web programming example: support I have a multi-user blog
> site, where each user can have multiple entries. I would model this
> using standard Haskell datatypes as:
>
> data Entry = Entry { title :: String, content :: String }
> data Blogger = Blogger { name :: String, entries :: [Entry] }
>
> Obviously we'll need some kind of blogger loading function:
>
> getBloggerByName :: String -> IO Blogger
>
> Either this will load up all entries (a potentially incredibly costly
> operation) or use unsafe IO down the road. Especially when using
> database connections, this can be incredibly bad: the connection could
> be closed, the SQL statement could be reused by another request, etc.

It may be possible to tag those data fields that are not to be
loaded on the spot.  For example,

> data Entry = Entry { title :: String, content :: String }
> data Blogger db = Blogger { name :: String, entries :: OnDB db [Entry] }
>
> class Monad db => Database db where
>   data OnDB db :: * -> *
>   fetch :: OnDB db a -> db a
>   fetchSome :: Criteria a -> OnDB db [a] -> db [a]
>
> newtype InMemory a = InMemory a
> instance Database InMemory where
>   newtype OnDB db a = OnDBMem a
>   fetch (OnDBMem x) = return x
>   fetchSome = ...
>
> instance Database SQL where
>   ...

Cheers,

--
Felipe.