[Python-ideas] Updating PEP 315: do-while loops
Ron Adam
rrr at ronadam.com
Mon Apr 27 07:09:35 CEST 2009
Arnaud Delobelle wrote:
>
> On 26 Apr 2009, at 21:20, Larry Hastings wrote:
>
>>
>> Anyway, this is one reason why I like the "do:" statement I proposed.
>> (The other one--the one where I also proposed "continue if" and "break
>> if". But given your reaction to those let's ignore 'em for now.)
>> This isn't *pure* sugar--it's a mildly novel flow control construct
>> for structured programming. Certainly Python doesn't have anything
>> like it right now; "while True:" isn't an exact match (and "while
>> False:" is equivalent to "if 0:"). My "do:" would allow spelling
>> do/while as:
>>
>> do:
>> something()
>> if condition: continue
>>
>> I think that's a reasonable do/while, and so economical: we only added
>> one new keyword. This form of "do:" has other uses too, as per my
>> previous email.
>>
>> Not that I expect this syntax to take the world by storm.
>
> It's funny because I had something like this in mind when I myself
> suggested 'break if ...' and 'continue if ...' a little while ago
> (although I would go for a 'loop:' that doesn't need to be continue'd
> explicitely). I had come to the point of view that the restrictions of
> the looping construct had in fact somehow liberated it and made it more
> versatile by making
>
> if condition: break
>
> and
>
> if condition: continue
>
> common control flow constructs inside a while loop, that in particular
> encompass the do ... while / repeat ... until constructs, etc seen in
> other languages (I may have expressed myself better in the original
> thread). My motivation for 'break/continue if ...' was that it would
> make those constructs stand out more (and could be hilighted clearly as
> such in editors) - I was a bit surprised when I was told it would make
> code less readable and that it was a slippery slope, but then I have
> lots of ideas, most of which don't turn out to be very good :)
I think it would help it to stand out.
while 1:
...
if a == 2: b = finished
if b == 3: breakit()
if c == 4: break
if d == 5: baker = name
if e == 6: steak = done
while 1:
...
if a == 2: b = finished
if b == 3: breakit()
break if c == 4
if d == 5: baker = name
if e == 6: steak = done
Ron
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