Deborah Pickett wrote:

You are going to see empty lists more often than you think in expressions like
  $product = [*] @array;
and having to write that as
  $product = [*] 1, @array;
just to protect against a common case doesn't exactly flaunt Perl's DWIMmery to me. I *have* to write 1 there, or otherwise the reduce meta-operator isn't calculating the product when there are items in @array. This hardly makes sense from a Huffman perspective. Someone please convince me otherwise.

The problem is that writing 1 there is still wrong in the "no arguments" case. The product of zero numbers cannot possibly be one in any common sense interpretation. (And, yes, I'm perfectly well aware of the mathematical interpretations in which it does make sense...that's not the point.)

What you want is:

        $product = ([*] @values err 0);


        $factorial = ([*] 1..$n err 1);

So what you want is not an identity value as default (which isn't even possible for many operators, as Luke pointed out), but a predictable failure value as default, so you can intercept that failure and choose your own outcome in the edge case.


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