[Haskell-cafe] Re: functional database queries

[Bjorn Bringert]

On Feb 22, 2007, at 14:56 , Henning Thielemann wrote:

>
> On Wed, 21 Feb 2007 apfelmus at quantentunnel.de wrote:
>
>> Albert Y. C. Lai wrote:
>>> apfelmus at quantentunnel.de wrote:
>>>> Albert Y. C. Lai wrote:
>>>>
>>>>> If and only if the database is a purely functional immutable data
>>>>> structure, this can be done. [...]
>>>>> Many interesting databases are not purely functional immutable;  
>>>>> most
>>>>> reside in the external world and can spontaneously change  
>>>>> behind your
>>>>> program's back.
>>>>>
>>>> I don't think this is the problem because SQL requests are emitted
>>>> atomically anyway. The (Query a) monad here has nothing to do with
>>>> mutability of the data base.
>>>
>>> The same clock read twice, each reading atomic, can give two  
>>> different
>>> results. (Cf. the monadic type signature of
>>> Data.Time.Clock.getCurrentTime.)
>>>
>>> The same SELECT to the same database issued twice, each time  
>>> atomic, can
>>> give two different results.
>>
>> Yeah, of course. That's why the function that executes the query  
>> is in
>> the IO-monad:
>>
>>     query :: GetRec er vr =>
>>         Database -> Query (Rel er) -> IO [Record vr]
>>
>> Hennings' question is whether the query type 'Query (Rel el)'  
>> really has
>> to be a monad, not whether the function 'query' has to be in the
>> IO-monad.
>
> Right.
>
>> In other words, 'Query a' just assembles a valid SQL-string,
>> it does not query or execute anything.
>
> Of course, instead of the DSEL approach "don't execute anything, only
> construct a program in a foreign language which does that" it would be
> nice to have a database where Haskell is the native query language,
> allowing to access the database content with 'map', 'filter' and so  
> on.

Have you seen CoddFish (http://wiki.di.uminho.pt/twiki/bin/view/ 
Research/PURe/CoddFish)?

/Björn