>Kamil Kukura <kamk at volny.cz> writes:
>
>> Is it possible in loops such as "true-or-false whileTrue: [ block ]" do
>> the iteration repeat or break out similiar to 'break' and 'continue'
>> keywords in C?

Martin Drautzburg wrote some good advice about breaking methods up into 
smaller ones, and looking at the collection iteration methods 
(collect/select/detect).

But since Smalltalk control flow constructs are written in Smalltalk, it 
is definitely possible to create a breakable/continuable while loop 
control structure.

The following examples aren't recommended for actual use, just as 
examples of how you can extend Squeak's control flow constructs directly 
in Squeak.

One way is to create return and continue blocks which are passed in to 
the loop as arguments.
For example, adding the following two methods to the BlockContext class:

----

breakableWhileTrue: aBlock 
	| returnBlock |
	returnBlock := [:result | ^ result].
	self whileTrue: [self continuableWhileTrue: aBlock returnBlock: 
returnBlock]


continuableWhileTrue: aBlock returnBlock: returnBlock
	| continueBlock |
	continueBlock := [^ self].
	^ aBlock value: returnBlock value: continueBlock

----

allows you to write loops like:

n := 0.
m := 0.
[n := n + 1. n < 50] breakableWhileTrue: [:return :continue |
	n > 10 ifTrue: [return value: m].
	n odd ifTrue: [continue value].
	m := m + 1].


Another way is to piggyback on the powerful exception handling 
capabilities exisiting in Squeak.  You can define two new Exception 
subclasses: LoopExit and LoopContinue.  Then add a new loop method to 
BlockContext:

breakableWhileTrue: aBlock
self whileTrue:
	[[[aBlock value: LoopExit value: LoopContinue]
		on: LoopExit do: [:ex | ^ ex messageText]]
			on: LoopContinue do: []]

Then your loop would look like:

n := 0.
m := 0.
[n := n + 1. n < 50] breakableWhileTrue: [:return :continue |
	n > 10 ifTrue: [return signal: m].
	n odd ifTrue: [continue signal].
	m := m + 1]. 



     -- tim