From: Jacob Fugal <lukfugl@...>
Date: 2009-03-07T04:42:24+09:00
Subject: [ruby-core:22704] Re: [Bug #1243] 1 is prime

I have a hard time imagining a situation where asking if a negative
number is prime would be necessary. Code the cares about prime numbers
almost by design implies that it expects positive integers. In that
mindset, I call YAGNI.

There is no compelling argument (to me) to have the Ruby standard
library definition of prime be inconsistent with the mathematical
definition. A prime number -- not a prime factor or a prime element of
the ring of integers or a prime element of any other arbitrary ring --
is a natural number (that is, a positive -- or non-negative, it
doesn't matter here -- integer) which has exactly two distinct natural
number divisors: 1 and itself. 1 is not prime and negative integers
are not prime. Rationals or Floats not coinciding with an integer are
not prime. That should be the definition of "prime?".

In the rare case where someone does need to check if the absolute
value of a negative integer x is prime, x.abs.prime? is not a horrible
burden, and it reveals the intent that negative numbers are acceptable
input to boot.

Jacob Fugal