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

As a generalization, maybe Racket2 could keep the invariant "`#:else' is
the absorbing (default) case of any multi-arm form" ?

On Sun, May 5, 2013 at 4:30 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> More generally, the role of `else' in `cond' is to select a particular
> production in the grammar of `cond' clauses, and keywords are normally
> the right way to do that in Racket. Keywords are normally right because
> they are syntactically distinct from expressions --- and so using a
> keyword avoids various potential ambiguities and sources of confusion.
>
> At Sat, 4 May 2013 10:36:14 -0500, Robby Findler wrote:
> > I think the bad property is the shadowing of the "else" identifier and
> > Matthew's point is that one way to avoid that is to not use an identifier
> > at all.
> >
> > The racket2 wiki currently says "try this out" so I guess it isn't
> > something people believe will definitely be better, but something to
> > explore.
> >
> > Robby
> >
> >
> > On Sat, May 4, 2013 at 10:33 AM, Laurent <laurent.orseau at gmail.com>
> wrote:
> >
> > > (that was assuming Ryan's assertion that "[...]Matthew say that he
> would
> > > have used a keyword for `else` in `cond` if he had it to do over
> again",
> > > which seem to mean that even in Racket2 Matthew would prefer `#:else'
> over
> > > `[else ...]' ?)
> > >
> > >
> > > On Sat, May 4, 2013 at 5:14 PM, Laurent <laurent.orseau at gmail.com>
> wrote:
> > >
> > >> Matthew,
> > >> Out of curiosity, could you explain why you'd prefer #:else everywhere
> > >> instead of [else ...] ?
> > >> Would such an #:else allow for multi-line bodies?
> > >>
> > >>
> > >> On Sat, May 4, 2013 at 5:06 PM, Matthew Flatt <mflatt at cs.utah.edu>
> wrote:
> > >>
> > >>> 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.
> > >>>
> > >>> _________________________
> > >>>   Racket Developers list:
> > >>>   http://lists.racket-lang.org/dev
> > >>>
> > >>
> > >>
> > >
>
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