For review: PEP 308 - If-then-else expression

[Bengt Richter]

On Tue, 11 Feb 2003 00:04:11 +1300, Paul Foley <see at below.invalid> wrote:

>On 10 Feb 2003 09:31:42 GMT, Bengt Richter wrote:
>
>>     log.info(ok ? "failure.", "success!" )
>>     print "auto colorizing turned", (self.allow_colorizing ? "off", "on")
>
>> is pretty clear IMO, once you get used to the order, which I would put that
>> way so as to reserve
>
>That'd confuse the hell out of C programmers, though :-)
>
>
>I like
>
>   log.info([ok -> "success!"; True -> "failure."])
>   print "auto colorizing turned", [self.allow_colorizing -> "on";
>                                    True -> "off"]
>
>(of course "True" could be any true expression)
>
>+ it reads right-to-left
>
>+ the "test evaluates True" expression comes first
>
>+ it's easy to read
>
>+ it generalizes to multiple test/result pairs
>
>+ it doesn't use an ugly mix of some symbol (like "?") and a word ("else")
>
>+ it doesn't use comma (you can write [x -> a,b,c; y -> x,y,z] to get
>  one or the other tuple)
>
>+ it's the original conditional expression syntax proposed by John
>  McCarthy :-)
>
>- it uses [] for a non-list, but Python already uses other characters
>  for several different things, and that doesn't seem too awful, to me.
>  [Don't want to have to write foo((...)) to use it in functions]
>
>* it could be an error to "fall off the end" or could return None in
>  that case.  If it's going to be an error, a trailing ";" could be
>  used to imply `True->None'.
>
>+ you can easily program Emacs to turn "->" into "→" (Unicode U+2192
>  RIGHTWARDS ARROW) for display, which looks better :-)
>
Apologies, Paul (Foley!). I took this post and tweaked the idea and misattributed
the source to Paul Moore, I now realize. D'oh. I knew you were the lisp guy,
somehow I just transposed the last names *<;-/

BTW, the tweak is to drop the brackets and substitute "><" in place of ';'
as in:
    log.info(ok -> "success!" >< True -> "failure.")
    print "auto colorizing turned", self.allow_colorizing -> "on" >< True -> "off"

then also allowing a trailing ">< True" to be dropped so it looks like:
    log.info(ok -> "success!" -> "failure.")
    print "auto colorizing turned", self.allow_colorizing -> "on" -> "off"

Now also extended with Holger Krekel's idea of catching exceptions:

    cond1 -> val1 >< cond2 -> val2 >< c3->v3 >!< E1 -> ev1 >!< E2 -> ev2

I.e., if E1 or E2 is raised during processing of the first part, the exception
is caught and the associated value becomes the whole expression value.

Regards,
Bengt Richter