On 07/10/2013 18:57, Antoon Pardon wrote:
Op 07-10-13 19:15, Alain Ketterlin schreef:
I want to consider here what it would mean to concretely
implement the abstract notion 'disallow rebinding of function
names' and show what would be behind calling the idea 'not
feasible'.
Again, I'm more concerned about the function than about the name.
And the fact that "disallow rebinding of function names" is not
feasible in Python is a design choice, not an essential
characteristic of every programming language.
That's fine. My point was: you can't at the same time have full
dynamicity *and* procedural optimizations (like tail call opt).
Everybody should be clear about the trade-off.
Your wrong. Full dynamics is not in contradiction with tail call
optimisation. Scheme has already done it for years. You can rebind
names to other functions in scheme and scheme still has working tail
call optimisatiosn.
Consider this code:
def fact(n, acc=1):
if n <= 1:
return acc
return fact(n-1, n*acc)
It compiles to this:
>>> dis.dis(fact)
2 0 LOAD_FAST 0 (n)
3 LOAD_CONST 1 (1)
6 COMPARE_OP 1 (<=)
9 POP_JUMP_IF_FALSE 16
3 12 LOAD_FAST 1 (acc)
15 RETURN_VALUE
4 >> 16 LOAD_GLOBAL 0 (fact)
19 LOAD_FAST 0 (n)
22 LOAD_CONST 1 (1)
25 BINARY_SUBTRACT
26 LOAD_FAST 0 (n)
29 LOAD_FAST 1 (acc)
32 BINARY_MULTIPLY
33 CALL_FUNCTION 2 (2 positional, 0 keyword pair)
36 RETURN_VALUE
I think that CALL_FUNCTION immediately followed by RETURN_VALUE could
be tail-call optimised by dropping the caller's stack frame provided
that (like in this case) the callee doesn't refer to any name in the
callers stack frame (nonlocal).
You could also consider replacing the caller's stack frame with a
smaller pseudo-frame, perhaps compressing multiple pseudo-frames or
repeated sequences of pseudo-frames into a single pseudo-frame, so that
it could still generate the same traceback as before (or an compressed
traceback like what was discussed on python-ideas in the threads
"Compressing excepthook output" and "Idea: Compressing the stack on the
fly").
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