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

[Haoyi Li]

I'd be all for increasing usage of fold and reduce, and higher order
combinators in general, but wasn't it always a somewhat philosophical issue
that kept lambdas intentionally verbose/crippled and discouraged usage of
Higher-Order-Functions when direct imperative code (i.e. loops) works? I've
always felt reduce() being banished was but a small facet of this overall
philosophy, and not so much because it was individually difficult to use.

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

Seems like a terrible hack to me =( it brings back memories of my PHP days
where "first class functions" meant you passed in the functions name as a
string which got concatted-around and eval-ed. We all laughed at how badly
they designed the language to have it end up like that. Naturally, it is
the path of least resistance, since it could be implemented with existing
language features (i.e. `eval`, which can implement anything really) but it
would leave a sour taste in my mouth every time i use it.

I would much prefer the somewhat-more-difficult route of modifying the
parser to let `a += b` be an expression, and then you could write

data = fold(lambda a, b: a += b, [], iterables)

or even groovy/scala/mathematica style

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

Which is a lot further (implementation wise) from where we are now, and 2
characters more verbose, but it would be far more generally usable than a
one-off "let's pass in operators as strings and concat/eval them" rule.

-Haoyi



On Fri, Jul 12, 2013 at 6:11 PM, Paul Moore <p.f.moore at gmail.com> wrote:

> On 12 July 2013 09:03, Andrew Barnert <abarnert at yahoo.com> wrote:
>
>> Meanwhile, there are an awful lot of people who just don't like
>> reduce/fold in any situation. The quote "Inside every reduce is a loop
>> trying to get out" appears quite frequently, on this list and elsewhere.
>
>
> And yet we keep getting cases like the sum discussion which is a fold in
> essence, but people reject suggestions of "just use a loop". So it doesn't
> look like the loop is trying very hard to get out :-)
>
> Whether "inside every specialised function there is a fold trying to get
> out" is any more likely to gain traction, I don't know...
>
> Paul
>
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