[Haskell-cafe] Request for feedback: HaskellDB + HList
phunge0 at hotmail.com
Tue May 19 10:23:24 EDT 2009
I updated my changes to apply against that repo, thanks for
the pointer. Cool to see new changes to haskelldb, especially
all the new unit tests!
You can find my updated repo at:
Re-reading your email now, I see you asked for a patch, but seeing
patch-tag for the first time I got excited and uploaded a whole new
Anyway, I had to do some minor surgery to update to the new
version -- everything compiles, but I haven't tested much beyond that
yet. Let me know if you have any questions.
> I like the direction you are going. I looked into using HList a year
> or so ago and I wasn't quite up to it. The latest (unreleased)
> version of HaskellDB is on patch-tag at
> Would you mind creating a patch file against that for easier review? I
> won't commit it until you say its ready but I'd like to see what
> changes you have made.
> No announcement has been made but I took over maintainership from
> Bjorn a few months ago. I hope to get a 1.0 release of HaskellDB out
> this summer, and having something new like this in it would be pretty
> On Sat, May 16, 2009 at 3:08 PM, Brian Bloniarz <phunge0 at hotmail.com> wrote:
> > Hi,
> > 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:
> > http://mysite.verizon.net/vzewxzuh/sitebuildercontent/sitebuilderfiles/haskelldb-hlist-20090516.tar.gz
> > It also requires minor changes to HList, available at:
> > http://mysite.verizon.net/vzewxzuh/sitebuildercontent/sitebuilderfiles/hlist-20090516.tar.gz
> > I'll talk to the HList people about getting those merged.
> > Thanks!
> > Brian Bloniarz
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