tightPrimeEmbedding

isprime.hs

import Data.List (find)
import Data.Maybe (fromJust)

isPrime :: Integer -> Bool
isPrime n
    -- negative numbers, 0 and 1 are not prime
    | n < 2          = False
    -- 2 is prime, all other numbers divisible by 2 aren't
    | n == 2         = True
    | n `mod` 2 == 0 = False
    | otherwise      = not $ any ((\t i -> t `mod` i == 0) n) [2 .. maxToTest]
    where
        maxToTest    = (floor . sqrt . fromInteger) n


tightPrimeEmbedding :: Integer -> (Integer, Integer, Integer)
tightPrimeEmbedding n
    | n < 2     = error "need an integer greater than 1"
    | otherwise = (before, n, after)
    where
        before = fromJust $ find (isPrime) [n, (n-1) .. 2]
        after  = fromJust $ find (isPrime) [(n+1), (n+2) ..]