control structures (was "Re: Sins")

[skaller]

Skip Montanaro wrote:
> 
>     Emile> Doesn't this do that now? or is there some objection to this usage?
> 
>     [ ... snip try/except used to implement state machine ... ]
> 
> I find nothing wrong with it.  Some people object to using try/except for
> anything but handling errors however.

My objection is twofold. First, the syntax is unweildy:

	try:
		for i in range(3):
			if ... : raise X
	except X:

requires at least one extra level of indentation.

[I'm interested in a 'generic' way to combine
python statements which avoids this (like 'elif' does)
Any ideas?]

	My second objection is more serious:
try .. raise .. except is too dynamic (for this usage): 
there is no way to match up the exceptions actually 
thrown with the handlers.

	The constructions under consideration
(variants on a construction proposed by Knuth)
would support compile time verification that
every loop exit had a corresponding handler
lexically following the loop.

	In particular, 'falling through'
such a loop is impossible: all exits must be explicit.

	Current looping constructions are so bad,
that the simplest of uses cannot be expressed
using them : searching a list for an element
cannot be written correctly:

	for i in X: 
		if X[i]=e: goto Found
	print "Not Found"
	goto next
	Found: ...
	next

YUKKKKKK!!! There is no natural way to do this.
We fall through the loop on failure, instead
of success. (using a flag, or testing anything
afterwards, is not acceptable as a solution).

Using exceptions:

	try:
		for i in X:
			if X[i]==e: raise Found
		raise NotFound
	except Found: ..
	except NotFound: ..

is much cleaner, but exceptions are excessively
dynamic for what is essential a static control
flow problem. A solution will resemble the
above exception handling implementation .. but
with the matching up of exits and handlers enforced
at compile time.

-- 
John (Max) Skaller, mailto:skaller at maxtal.com.au
10/1 Toxteth Rd Glebe NSW 2037 Australia
homepage: http://www.maxtal.com.au/~skaller
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