On 7/14/2015 10:02 AM, Antoon Pardon wrote:
On 07/14/2015 03:43 PM, Chris Angelico wrote:
On Tue, Jul 14, 2015 at 11:38 PM, Marko Rauhamaa <ma...@pacujo.net> wrote:
No, tail call optimization doesn't change the behavior of the program,
for the worse anyway.
It does, because you lose traceback records. That's pretty significant
when you come to try to debug something.
I doubt it would be really significant. Not compared to writing it iteratively.
When you write it iteratively, you don't get to keep a traceback record per time
you go throught the loop. So the traceback records you loose in tale call
elimination
would be the traceback records you wouldn't have anyway in the iterative
version.
To repeat: loosing tracebacks is probably fine when the function has a
single *recursive* tail call. This are precise the cases when it is
simple and perhaps preferable to use a loop. But *recursive* tail calls
are only a small fraction of all tail calls. So most of the time, the
loss *would* be significant. Consider
In moda: def f(a): return modb.g(a-3)
In modb: def g(b): return modc.h(b*(b-1))
In modc: def h(c): return 1.0/c
from moda import f
... (500 lines later)
f(3)
and your traceback has been reduced to
In modc: line 1
return 1.0/c
ZeroDivisionError: ...
???
--
Terry Jan Reedy
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