--safetail :: [a] -> [a] -- tail(cdr)のように振る舞うが、違いは、空リストを与えた時に空リストを返すようにすること。 -- 実装方法は、それぞれ、a. 条件式、 b. ガード付きの等式、 c. パターンマッチ でやること。

programing_in_Haskell_4_2.hs

-- a. 条件式
safetail_A :: [a] -> [a]
safetail_A xs = if null xs then [] else tail xs

-- b. ガード付きの等式
safetail_B :: [a] -> [a]
safetail_B xs | null xs = []
              | otherwise = tail xs

-- c. パターンマッチ
safetail_C :: [a] -> [a]
safetail_C [] = []
safetail_C xs = tail xs