[Haskell-cafe] Request for feedback: HaskellDB + HList

Brian Bloniarz phunge0 at hotmail.com
Sat May 16 18:08:07 EDT 2009


It's come time to share something that I've been playing around with recently:
a branch of HaskellDB which replaces the home-grown Record code with HList
records. It's definitely not ready for primetime, but I thought it'd be a good
time to post the code and solicit some feedback from the community.

HaskellDB the concept is very promising, but IMHO the code still falls short
of that promise. Hopefully this is a small step in the right direction -- the
advantages of using HList:
* Shared implementation of extensible records
* Additional features from HList
  * Better error messages for record misuse
  * "Lacks" predicates 
* Simpler code

As an example of how this can be better, a DB insert looks like so:
> insert db table $ constantRecord $
>     film .=. "Munchie" .*.
>     director .=. Just "Jim Wynorski" .*.
>     emptyRecord
The columns need not appear in the same order as in the database. If you forget
a column, you'll get "error: No instance for (Fail (FieldNotFound (Proxy Director)))"
rather than an opaque error. Using the new "insertOpt" function, Maybe columns
will default to Nothing rather than needing to be specified.

The details:

I haven't updated everything, but there's enough to run test/TestCases.hs
under Postgresql. TestCases is probably the best place to look for examples of
the new syntax for now.

HList had name conflicts with HaskellDB's SQL expression language
((.*.), (.++.), etc.) My temporary band-aid is to move the expression functions
to Database.HaskellDB.SqlExpr, and require people to import qualified.

The Attr type is gone, columns labels are untyped now. I also replaced a 
few instances of primitive type-level recursion with HMap/HMapOut. This makes
the code simpler, and the type signatures more complex -- type families would
help a lot here, I think.

Feedback welcome! You can find my darcs tree at:
It also requires minor changes to HList, available at:
I'll talk to the HList people about getting those merged.


Brian Bloniarz

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