90% of /prog/ cannot write this BASIC PROGRAM

Task: Using your language of choice, implement a program that will a) Prompt the user for 2 numbers, and b) output the number of prime numbers between the two numbers (inclusive.)

Output: A B N

sieve (x:xs) = x : sieve (filter (\a -> mod a x /= 0) xs)


Question: why people tend to use mod as an infix function?