At the risk of doing someone's homework...
A naive solution is to do trial division by all integers from 2 up to sqrt n.

{-
isPrime :: Integer -> Bool
isPrime n 
 | n < 2     = False
 | otherwise = f 2 n
 where f k n 
  = if k > isqrt 
     then True
     else undefined -- exercise for the reader
-}

and where 
isqrt n returns floor (sqrt n)

Here, f is the helper function, and is only in scope inside the definition of isPrime. This is a common haskell idiom- a helper function that is not quite general purpose enough to be made a standalone function can be defined "on the fly" and doesn't need a name or type signature. 

You could fancy this up to make it more efficient. I've also seen probabilistic functions that can handle huge numbers, but I can't remember if they are recursive.