Henning Thielemann wrote:
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.
Yes, though this is most probably infeasible: unrestricted lambda-calculus queries are disallowed for reasons of computability and efficiency. Of course, you can still conceive a DSEL offering 'map' and 'filter', it's just that these will not be the known Haskell functions then. As far as I see, the main point for 'Query a' being a monad is that you need 'concatMap' == '>>=' to express table joins. After all, accessing databases just means general list comprehensions. Regards, apfelmus