[Python-ideas] Add nullifier argument to functools.reduce?

[Steven D'Aprano]

On Sat, Aug 23, 2014 at 10:25:06AM -0700, David Mertz wrote:
> This "nullifier" is mathematically called an "absorbing element", but
> saying an "attractor" might be a little more general.  I.e. think of a
> local optimization problem, where multiple local min/max points might
> occur.  If you reach one, further iteration won't budge from that point,
> even if it's not the "global absorbing element."

Yes. Note that arbitrary systems may have more than one attractors, 
including cycles (a -> b -> c -> a) and even "strange attractors" of 
infinite complexity. It's probably too much to expect reduce to deal 
with cycles, but a really general solution should at least deal with 
multiple attractors.

> Given that one can easily write one's own three line wrapper
> 'reduce_with_attractor()' for this special semantics 

I don't think you can. Although it's 3:30am here and it's past my bed 
time, so perhaps I'm wrong. The problem is that the wrapper cannot see 
the reduced value until reduce() returns, and we want to short-circuit 
the call once the reduced value is the attractor.

Still, easy or not, I think the semantics are too specialised to 
justify in the standard library, especially given that Guido doesn't 
like reduce and it almost got removed. A better solution, I think, would 
be to stick this reduce_with_attractor() in some third-party functional 
tool library.


-- 
Steven