Haskell中的模式匹配错误

Question

Hello I have been attempting to answer a question that deals with checking if a number is a prime number. I came up with this code below:

isitprime :: Int->Bool
isitprime n 
   | n<=1 = False
   | otherwise = isitprime2 n (n-1)

isitprime2 :: Int->Int->Bool
isitprime2 x y 
  | y > 1 && x `mod` y == 0 = False 
  | y == 1 && x `mod` y == 0 = True

When run in winhugs, it returns an error saying "pattern matching failure: isitprime2 a a-1 " for any value greater than 2.

However it returns false for any values which are directly multiplied by 2, eg isitprime2 2 1 returns false, isitprime2 4 2, isitprime2 6 3, isitprime2 10 5 etc. all return false.

What is going wrong and why?

Thanks :D

Answer 1

The problem with the function isitprime2 is that it doesn't handle all cases. Like what happens when:

• y > 1 && x mod y != 0
• y == 1 && x mod y != 0

Handle other cases and your code will work fine. You can also use otherwise clause for handling all the edge case:

isitprime2 :: Int->Int->Bool
isitprime2 x y 
  | y > 1 && x `mod` y == 0 = False 
  | y == 1 && x `mod` y == 0 = True
  | otherwise = ??? -- handle all edge case