On Feb 21, 2007, at 20:47 , [EMAIL PROTECTED] wrote:
Albert Y. C. Lai wrote:
[EMAIL PROTECTED] 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. In other words, 'Query a' just assembles a valid SQL-string,
it does not query or execute anything.
Regards,
apfelmus
This is correct, the Query monad is just used to construct the query.
Running the query is done in IO.
If we look in the source code (http://darcs.haskell.org/haskelldb/src/
Database/HaskellDB/Query.hs), we see that the Query monad is a state
monad, whose state is the current query and an Int used to generate
fresh field names. It would certainly possible to do this without a
monad, though it would probably require reworking the PrimQuery type.
/Björn_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
