On 07/16/2015 12:45 PM, Terry Reedy wrote:
On 7/16/2015 2:02 PM, Ethan Furman wrote:
On 07/16/2015 06:35 AM, Antoon Pardon wrote:
Here is one way to do the odd, even example.
def even(n):
return odd_even('even', n)
def odd(n):
return odd_even('odd', n)
def odd_even(fn, n):
while fn is not None:
if fn == 'even':
fn, n = (None, True) if n == 0 else ('odd', n - 1)
elif fn == 'odd':
fn, n = (None, False) if n == 0 else ('even', n - 1)
else:
raise ValueError("Unknown state: %s" % fn)
return n
Wow -- definitely uglier and less easy to understand than the original
(even if the original had had the error-checking).
What you call ugly is an example of the core of the proof that alternation and
indefinite
while looping are enough for turing completeness, and that no recursive syntax
is needed.
Another way to put it is that while loops perform recursion in the operational
sense, as
opposed to the syntactical sense.
And we add new constructs and new language features to make certain ways of thinking/programming easier -- async and await come to mind. ;) After all, we don't really even need while loops as we
could get by with if and goto.
--
~Ethan~
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