[Python-Dev] PEP 315 - do while

Mon Oct 2 08:41:53 CEST 2006

Hi Nick,

Yep, PEP 315. Sorry about that.

Now, about your suggestion 
     do:
         <setup code>
         while <condition>
         <loop body>
     else:
         <loop completion code>

This is pythonic, but not logical. The 'do' will execute at least once, so
the else clause is not needed, nor is the <loop completion code>.  The <loop
body> should go before the while terminator.

I'm bound to reiterate my proposal:

	<before loop code>
	do:
		<loop body>
		<setup code>
		while <condition>
	<after loop code>

Example (if you know there will be at least one val).
	source.open()
	do:
		val = source.read(1)
		process(val)
		while val != lastitem
	source.close()

The c syntax is:
do
{
     block of code
} while (condition is satisfied);

The VB syntax is:
do
	block
loop while <cond>

Cheers & thanks for your reply,
Hans Polak.

-----Original Message-----
From: Nick Coghlan [mailto:ncoghlan at gmail.com] 
Sent: domingo, 01 de octubre de 2006 6:18
To: Hans Polak
Cc: python-dev at python.org
Subject: Re: [Python-Dev] PEP 351 - do while

Hans Polak wrote:
> Hi,
> 
>  
> 
> Just an opinion, but many uses of the 'while true loop' are instances of 
> a 'do loop'. I appreciate the language layout question, so I'll give you 
> an alternative:
> 
>  
> 
> do:
> 
>             <body>
> 
>             <setup code>
> 
>             while <condition>
> 

I believe you meant to write PEP 315 in the subject line :)

To fully account for loop else clauses, this suggestion would probably need
to 
be modified to look something like this:

Basic while loop:

     <setup code>
     while <condition>:
         <loop body>
         <setup code>
     else:
         <loop completion code>

Using break to avoid code duplication:

     while True:
         <setup code>
         if not <condition>:
             <loop completion code>
             break
         <loop body>

Current version of PEP 315:

     do:
         <setup code>
     while <condition>:
         <loop body>
     else:
         <loop completion code>

This suggestion:

     do:
         <setup code>
         while <condition>
         <loop body>
     else:
         <loop completion code>

I personally like that style, and if the compiler can dig through a function

looking for yield statements to identify generators, it should be able to
dig 
through a do-loop looking for the termination condition.

As I recall, the main objection to this style was that it could hide the
loop 
termination condition, but that isn't actually mentioned in the PEP (and in 
the typical do-while case, the loop condition will still be clearly visible
at 
the end of the loop body).

Regards,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
---------------------------------------------------------------
             http://www.boredomandlaziness.org

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