[Python-ideas] Rehabilating reduce (as "fold")

[Nick Coghlan]

The strange contortions of the "fast sum for lists" discussions got me
wondering about whether it was possible to rehabilitate reduce with a less
error-prone API. It was banished to functools in 3.0 because it was so
frequently used incorrectly, but now its disfavour seems to be causing
people to propose ridiculous things.

The 2.x reduce is modelled on map and filter: it accepts the combinator as
the first argument, and then the iterable, and finally an optional initial
value. The most common error was failing to handle the empty iterable case
sensibly by leaving out the initial value, so you got a TypeError instead
of returning a result.

So, what if we instead added a new alternative API based on Haskell's
"fold" [1] where the initial value is *mandatory*:

    def fold(op, start, iterable):
        ...

Efficiently merging a collection of iterables into a list would then just
be:

    data = fold(operator.iadd, [], iterables)

I'd personally be in favour of the notion of also allowing strings as the
first argument, so you could instead write:

    data = fold("+=", [], iterables)


This could also be introduced as an alternative API in functools.

(Independent of this idea, it would actually be nice if the operator module
had a dictionary mapping from op symbols to names, like
operator.by_symbol["+="] giving operator.iadd)

Cheers,
Nick.

[1] https://en.wikipedia.org/wiki/Fold_%28higher-order_function%29


-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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