[Haskell-cafe] Inverse of HaskellDB

[Michael Snoyman]

On Thu, Sep 30, 2010 at 4:35 PM, Felipe Lessa <felipe.lessa at gmail.com> wrote:
> 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
>>   ...

I wasn't claiming my approach was the *only* approach, just stating
that it doesn't seem feasible to use the "simple" Haskell data type
declarations.

Michael