Records (was Re: [Haskell] Improvements to GHC)

[David Roundy]

On Sun, Nov 27, 2005 at 08:47:54PM +0000, Rob Ennals wrote:
> On 11/23/05, David Roundy <droundy at abridgegame.org> wrote:
> > > 3. "Safe" getters for multi-constructor data types: ditto
> >
> > I think either you misunderstood my meaning by "safe", or I misunderstood
> > your paper.  I meant that if I write
> >
> > data FooBar = Foo { foo :: String } | Bar { bar :: String }
> >
> > there shouldn't be accessors of type
> >
> > foo :: FooBar -> String
> > bar :: FooBar -> String
> 
> I did indeed misunderstand what you meant by "safe". Bottom is indeed
> a nasty thing.
> 
> Perhaps such definitions should generate a warning? (banning them
> outright would cause compatability issues)

Yeah, issuing a warning (which can become an error with -Werr) is a nice
option.  The other option would be some sort of syntax to declare that a
particular record is unordered.  Or I suppose to just give up on backward
compatibility.  Any of these three alternatives would be fine with me.

> > > 7. Unordered records: yep (if I understand the problem correctly)
> >
> > I don't think you understood correctly.
> 
> I was thinking along the same lines as Wolfgang : don't export the
> internal representation of the type, but do expose the field
> manipulator functions.
> 
> This needn't prevent the use of pattern matching, provided the
> desugaring of patterns is consistent with the rest of the system.
> 
> E.g. I was assuming that
> 
> case e of { x = 3, y = 4} -> ...
> 
> would desugar to
> 
> case e of _ | x z = 3 && y z = 4 -> ...
>
> Note that this pattern matching syntax will continue to work, even if
> 'x' and 'y' are reimplemented as normal functions, rather than fields.

Indeed, it hadn't occurred to me to make pattern matching work this way.
It actually sounds a lot like pattern guards, since you're suggesting this
sugar could be applied to any sort of object?  So your desugarer would
allow a function like

islong :: [a] -> Bool
islong {length = l} = l > 10
-- 
David Roundy
http://www.darcs.net