Exposing newtype coercions to Haskell

[Simon Peyton-Jones]

POPLing today.  Let's discuss on Monday

| -----Original Message-----
| From: ghc-devs-bounces at haskell.org [mailto:ghc-devs-bounces at haskell.org]
| On Behalf Of Joachim Breitner
| Sent: 11 July 2013 15:25
| To: ghc-devs at haskell.org
| Subject: Re: Exposing newtype coercions to Haskell
| 
| Hi,
| 
| despite my issues with recursive data types, I continued with the
| implementation.
| 
| I now added the check if the data type arguments can safely be coerced.
| I do not scan what is in scope for NT values to use, but rather expect
| the progammer to promise the required witness when he uses "deriving"
| and the plug the functions together by himself:
| https://github.com/nomeata/nt-coerce/blob/ec9b5/tests/LiftAbstract.hs
| 
| I also added a test suite to show what works already:
| https://github.com/nomeata/nt-coerce/tree/master/tests
| what does rightfully not work, due to abstraction:
| https://github.com/nomeata/nt-coerce/tree/master/tests/failing
| and what does not yet work, although it conceivably should:
| https://github.com/nomeata/nt-coerce/tree/master/tests/todo
| 
| The test suite contains expected output in *.out and *.err, so you can
| check out what happens without having to run it. The error messages are
| not yet as nice as they should be.
| 
| How to proceed from here Is this approach and design good enough to to
| into GHC, despite not being able to handle _all_ cases where it
| conceivably could, and the slight unwieldiness when coerctions for data
| types based on abstract data types?
| 
| BTW, I find it quite nice to be able to develop such a feature without
| touching GHC’s source. If that is possible for more contributions, the
| entry barrier to GHC would be noticeably lowered.
| 
| Thanks,
| Joachim
| 
| 
| --
| Joachim “nomeata” Breitner
|   mail at joachim-breitner.de • http://www.joachim-breitner.de/
|   Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0x4743206C
|   Debian Developer: nomeata at debian.org