Re: GHC support for the new "record" package

On 23/01/15 10:17, Simon Marlow wrote:

On 23/01/2015 04:12, Johan Tibell wrote:

...

On Wed, Jan 21, 2015 at 5:48 PM, Simon Marlow mailto:marlowsd@gmail.com> wrote:

On 21/01/2015 16:01, Johan Tibell wrote:

My thoughts mostly mirror those of Adam and Edward.

1) I want something that is backwards compatible.

Backwards compatible in what sense? Extension flags provide backwards compatibility, because you just don't turn on the extension until you want to use it. That's how all the other extensions work; most of them change syntax in some way or other that breaks existing code.

In this case in the sense of avoiding splitting code into a new-Haskell vs old-Haskell. This means that existing records should work well (and ideally also get the improved name resolution when used in call sites that have the pragma enabled) in the new record system.

I understand that position, but it does impose some pretty big constraints, which may mean the design has to make some compromises. It's probably not worth discussing this tradeoff until there's actually a concrete proposal so that we can quantify how much old code would fail to compile and the cost of any compromises.

In this spirit, I've started to prepare a concrete proposal for a revised OverloadedRecordFields design, based on recent feedback: https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Redesig... This would not necessarily include anonymous records at first, but they do fit nicely as a potential later extension, and it would work well with a slightly amended version of the record library in the meantime. I'd be very interested to hear what you think of this. Also, if someone would be prepared to flesh out a proposal based on the anonymous records idea, that might be a useful point of comparison. Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/