4_a_1: Add solution

1 files changed, 64 insertions, 0 deletions

diff --git a/ch04/4_a_1.hs b/ch04/4_a_1.hs
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+-- Write your own "safe" definitions of the standard partial list functions, but
+-- make sure that yours never fail. As a hint, you might want to consider using
+-- the following types.
+--
+--   -- file: ch04/ch04.exercises.hs
+--   safeHead :: [a] -> Maybe a
+--   safeTail :: [a] -> Maybe [a]
+--   safeLast :: [a] -> Maybe a
+--   safeInit :: [a] -> Maybe [a]
+
+safeHead :: [a] -> Maybe a
+safeHead [] = Nothing
+safeHead xs = Just (head xs)
+
+-- ghci> :l 4_a_1.hs
+-- [1 of 1] Compiling Main             ( 4_a_1.hs, interpreted )
+-- Ok, one module loaded.
+-- ghci> head [1, 2, 3]
+-- 1
+-- ghci> head []
+-- *** Exception: Prelude.head: empty list
+-- ghci> safeHead [1, 2, 3]
+-- Just 1
+-- ghci> safeHead []
+-- Nothing
+
+safeTail :: [a] -> Maybe [a]
+safeTail [] = Nothing
+safeTail xs = Just (tail xs)
+
+-- ghci> tail [1, 2, 3]
+-- [2,3]
+-- ghci> tail []
+-- *** Exception: Prelude.tail: empty list
+-- ghci> safeTail [1, 2, 3]
+-- Just [2,3]
+-- ghci> safeTail []
+-- Nothing
+
+safeLast :: [a] -> Maybe a
+safeLast [] = Nothing
+safeLast xs = Just (last xs)
+
+-- ghci> last [1, 2, 3]
+-- 3
+-- ghci> last []
+-- *** Exception: Prelude.last: empty list
+-- ghci> safeLast [1, 2, 3]
+-- Just 3
+-- ghci> safeLast []
+-- Nothing
+
+safeInit :: [a] -> Maybe [a]
+safeInit [] = Nothing
+safeInit xs = Just (init xs)
+
+-- ghci> init [1, 2, 3]
+-- [1,2]
+-- ghci> init []
+-- *** Exception: Prelude.init: empty list
+-- ghci> safeInit [1, 2, 3]
+-- Just [1,2]
+-- ghci> safeInit []
+-- Nothing