PEP 308: Discussion Summary
vze4rx4y at verizon.net
Wed Feb 19 03:05:27 CET 2003
Here is a recap of the discussion over the last few days:
* Someone re-proposed c ? a else b. The rationale is that the ?
is a better marker than "then" and is suggestive enough to not
need a leading "if". It doesn't require a new keyword and does
not overload the colon. No one shot this down but it didn't
generate strong support either.
* There was an interesting, but fruitless discussion on how
"c then a else b" could be implemented as a pair of binary
operators which could also have standalone semantics.
* There was a rapidly shot down idea to use bool.choose(a, b) where
choose() was a special method that could somehow implement
lazy evaluation of its arguments.
* A recent proposal is: (? <cond1>: <expr1>, <cond2>: <expr2>, <default>
There have only been a few hours for comment. Though the
visual cues are there, my first thought is that the colon usage is
likely to confuse anyone already exposed to the c?a:b form in
other languages where the colon separates the alteratives rather
than the condition and alternative.
Here is a recap of the results of everyone's ad-hoc, non-scientific
usability tests on their friends, relatives, and employees:
* In general, all of the forms were decipherable by people who already
* The c?a:b form was understandable to people familiar
with other languages. With a single example, others were able
to understand the form and found it easy to use. The surprise
was that the n-ary form (using parens for sub-expressions)
managed to confuse even non-beginners. A possible
cause is that the grouping and precedence cues for parentheses
do not mix well with the concepts of grouping by ternary operator
and of short-circuit evaluation.
* The (if c: a else: b) form stumped some of the respondants.
In some experiments, this was especially surprising because
they had just seen one of the other forms and had some expectation
of what it was supposed to do. The root problem appears to be
the blurring of the line between statements and expressions where
the keyword and colon cues proved misleading. On the plus side,
everyone who understood the ternary version had no problem with
the n-ary version.
* The "c then a else b" form was understood by most though a few
thought it looked unnatural in longer expressions. There were some
who understood it but could not make the jump to the n-ary form
(using the elif keyword). It appears that the colons help to visually
parse the n-ary form.
Since thread activity has dropped considerably, voting will
take place soon but not earlier than Friday.
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