>>47
よーしパパそれほどHaskellerじゃないけど調子に乗って習作書いちゃうぞー

nthElementIs :: [a] -> Int -> a
nthElementIs (x:xs) num | num == 1 = x
. | num > 1 = nthElementIs xs (num - 1)

これじゃいかんの ?

不適なデータが来ても何とかするなら

nthElementIs :: [a] -> Int -> Maybe a

nthElementIs [] _ = Nothing
nthElementIs (x:xs) num | num < 1 = Nothing
. | num == 1 = Just x
. | num > 1 = nthElementIs xs (num - 1)

とか