fermat.hs

import Text.Read
import Control.Monad

readEither' :: Read a => String -> String -> Either String a
readEither' e i =
    case readEither i of
        Left x -> Left e
        Right y -> Right y

asInt :: String -> Either String Integer
asInt = readEither' "You must enter a number."

atLeast2 :: (Ord b, Num b) => b -> Either [Char] b
atLeast2 v
    | v <= 2 = Left "The value must be greater than 2."
    | otherwise = Right v

getInput :: String -> (String -> Either String a) -> IO a
getInput prompt check = do
    putStr prompt
    input <- getLine
    case check input of
        Left msg -> do
            print msg
            getInput prompt check
        Right v -> return v

main = do
    a <- getInput "a: "   asInt
    b <- getInput "b: "   asInt
    c <- getInput "c: "   asInt
    n <- getInput "n: " $ asInt >=> atLeast2
    print $ (a ^ n) + (b ^ n) == (c ^ n)