Problem Statement

Write a boolean function that tells whether a given integer is prime using Wilson's theorem. By Wilson's theorem, a number p is prime if and only if p divides (p - 1)! + 1. Remember that 1 and all non-positive integers are not prime.

Let's start with the solution:

factorial: function [x]-> product 1..x

wprime?: function [n][
    if n < 2 -> return false
    zero? mod add factorial sub n 1 1 n
]

print "Primes below 20 via Wilson's theorem:"
print select 1..20 => wprime?