Possibly Pythonic Tail Call Optimization (TCO/TRE)
tjreedy at udel.edu
Wed Jul 15 02:41:06 CEST 2015
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 <marko at 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)
and your traceback has been reduced to
In modc: line 1
Terry Jan Reedy
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