A little of coordination for Rosettacode

[bearophile]

ixid:

> The prime factorization of 1 is an empty set, so surely to be 
> correct it should return [] when given 1 and not throw an 
> exception.

The Task says that the input can't be 1, so the input 1 needs to 
be a pre-condition violation:

>Write a function which returns an array or collection which 
>contains the prime decomposition of a given number, n, greater 
>than 1<


> This also suggests a possible modification to [].reduce!"a * b" 
> as mathematically the product of the empty set is defined as 1.

reduce() is a general function, so it's not supposed to know that.

Python reduce does the same:


>>> reduce(lambda a, b: a * b, [])
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: reduce() of empty sequence with no initial value


If you want that, then you have to use:
reduce!"a * b"(1, items)

And some time from now:
items.reduce!"a * b"(1)


If we add a product() function to Phobos similar to sum() 
(http://d.puremagic.com/issues/show_bug.cgi?id=4725 ) then I 
agree that for empty ranges it will need to return the 
multiplicative identity element.

Bye,
bearophile