Dear Dominic, Simon,
Funnily enough, I'm currently ruminating on the possibility of adding extensible records to GHC. My current thought is: much like TRex, except with 'has' predicates instead of 'lacks' predicates.
I seem to remember that I have discussed "has" predicates once with Mark Shields and there seem to be some problems with it. Maybe it was just the polymorphic extensions but I am not entirely sure about that. Maybe you should contact Mark about this and ask his (expert) opinion.
Question: does anyone really care about polymorphic extension.
The extensibility of records is not a necessary feature for haskell/DB. The only reason haskellDB uses TREX is to share label names across different tables. However, during the implementation of haskellDB, I also used a technique where records are nested tuples. Labels can than be modelled as overloaded functions in multiple parameter type classes. Unfortunately, this lead to a lot of 'unresolved overloading' messages. However, I believe that functional dependencies can be used nowadays to remedy this problem -- and thus use haskellDB on most Haskell systems. (I actually have a functional pearl in the works about modelling records with type classes and functional dependencies.) Anyway: bottom line: your proposal is good enough for Haskell/DB and seems a *very* worthwile addition to Haskell.
Sorry that this message is a bit brief; I'm just back from a week away. I can elaborate if anyone would be interested.
Yes! I would love to hear more about it. :-) All the best, Daan.
Simon
| -----Original Message----- | From: Dominic Steinitz [mailto:dominic.steinitz@blueyonder.co.uk] | Sent: 26 January 2003 10:19 | To: libraries@haskell.org | Cc: daan@cs.uu.nl | Subject: HaskellDB | | Having used HToolkit successfully to extract information from a MySQL | database, I am painfully aware of the limitations of sending embedded SQL | strings and finding out at runtime that something didn't quite work (in an | often subtle and difficult to track down way). | | I'd love to be able to put HaskellDB on top of HToolkit. There are two | approaches: i) add extensible records to ghc or ii) modify HaskellDB so it | uses Haskell records not extensible records (I'd have modify HaskellDB | anyway to use HToolkit rather than talk to ODBC directly). The latter sounds | as though it might be more feasible that the former. Having read the paper | and briefly looked at the code, I assume the extensibility is required if | you have two tables each with the same column name and therefore you want to | use the same label in two different records. Obviously you can't do this | with Haskell records as the labels become top level functions. However, if I | rename labels then I won't have this problem. For example, if I have two | tables, passwd and notify each containing a column called email then I could | have labels notifyEmail and passwdEmail and remember that they really | referred to email. | | Does anyone have any views on this? | | Dominic Steinitz | | _______________________________________________ | Libraries mailing list | Libraries@haskell.org | http://www.haskell.org/mailman/listinfo/libraries