fermat.hs

import Text.Read

asInt v = readEither "You must enter a number." :: Either String Int
atLeast2 v
    | v <= 2 = Left "The value must be greater than 2."
    | otherwise = Right v

checkVal [] v = Right v
checkVal (p:ps) v =
    p v >>= checkVal ps

getInput :: String -> [String -> Either String a] -> IO (Either b String)
getInput p cs = do
    putStr p
    e <- checkVal cs <$> getLine
    case e of
        Left msg -> getInput p cs
        Right v -> return $ Right 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)