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Dave Kahn wrote:
But ITER and LEAVE are really GOTO in a cage. They are extremely
useful for practical programming but they are in a sense dirty tools
that violate the idea of a single exit and entry point.
ANYGHING, on some level, that introduces iteration or choice into
program flow, is (at least in part) "GOTO in a cage."
Niklaus Wirth, for all his genius, had (at least when he invented
Pascal) an incomplete and highly prejudiced understanding of iterative
structures. When he invented Modula-2, his understanding was much
better, and much less biased, because by then he had recognized that
there are plenty of cases in the real world where an iterative structure
shouldn't run to completion unless something goes wrong, and thus he
added "leave" functionality.
Consider this situation: You are running through a table with a hundred
thousand entries, and it is neither indexed nor sorted. Completely
random. By the strict "single entry, single exit" rule, you HAVE to look
at all of the entries, every time, whether the one you're looking for is
the first one or the last one, and if you need values from the matching
entry outside your loop, you need to make working copies. Lots of
unnecessary complexity, a structure that's counter-intuitive to what
you're trying to do, and (on average) half your iterations will be
unnecessary.
But if you go to a "single entry, single exit, with leave" rule, you end
up with a simpler structure that more accurately reflects what it's
actually doing, and NO unnecessary iterations.
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