[racket-dev] else clauses: possible change to match?

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Sat May 4 11:06:56 EDT 2013

At Sat, 4 May 2013 09:15:22 -0500, Robby Findler wrote:
> On Sat, May 4, 2013 at 9:07 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> > At Fri, 3 May 2013 17:29:52 -0400, Eli Barzilay wrote:
> > > A few minutes ago, Robby Findler wrote:
> > > >
> > > > FWIW, this was the bug in redex that prompted me to send this
> > > > message (it was there for some time since it wasn't a syntax error
> > > > .... it was similar in spirit to the code I posted; things broke
> > > > when #f was an argument)
> > >
> > > [I think that it's good to have a much more relaxed policy about
> > > breaking compatibility in cases like this: so far there was no real
> > > code found that uses the feature, but there is one instance of code
> > > that would get fixed by the change...]
> >
> > Well, Ian provided an example from real code, right? Ian is willing to
> > change his code, but the code sounds real.
> >
> > There's also the use in `unparse-pattern' in Redex. Maybe that's the
> > troublesome one that Robby has in mind changing (or he would be happy
> > to change it, obviously), but it's another real example.
> >
> >
> No, that was not the example. The code I sent at the beginning of the
> thread was an adjusted version of the bug that hid in Redex for, roughly,
> months. It was a real bug and caused real problems and we knew something
> was wrong but didn't find it for some time.
> In other words, this isn't some made-up, code cleanliness-based request.

Yes, I understand that you faced a real bug. I hedged above on
`unparse-pattern' not to suggest that your actual bug was
uninteresting, but to suggest that I might misunderstand the
relationship between the bug and the current state of our repository.

All else being equal, I'm definitely in favor of a change to a sensible
`else' for `match'. The "else" that isn't equal, however, is backward
compatibility, and I think we're at the right point in our development
cycle to defer backward incompatibilities to the next language ---
hence my vote to defer.

Posted on the dev mailing list.