[Haskell-cafe] Re: functional database queries

[Henning Thielemann]

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.