4_a_2: Add solution

2 files changed, 26 insertions, 1 deletions

diff --git a/README.md b/README.md
index e330d0f..0cc741e 100644
--- a/README.md
+++ b/README.md
@@ -69,7 +69,7 @@ more visible in the list the first exercise of a group is in bold italics.
 | 3_b_11         | yes, in 3_b_9 |      | |
 | 3_b_12         | yes, in 3_b_9 |      | |
 | **_4_a_1_**    | yes    | 84   | 4. Functional programming |
-| 4_a_2          |        |      | |
+| 4_a_2          | yes    |      | |
 | 4_a_3          |        |      | |
 | 4_a_4          |        |      | |
 | **_4_b_1_**    |        | 97   | |
diff --git a/ch04/4_a_2.hs b/ch04/4_a_2.hs
new file mode 100644
index 0000000..9a14c34
--- /dev/null
+++ b/ch04/4_a_2.hs
@@ -0,0 +1,25 @@
+-- Write a function splitWith that acts similarly to words, but takes a
+-- predicate and a list of any type, and splits its input list on every element
+-- for which the predicate returns False.
+--
+--   -- file: ch04/ch04.exercises.hs
+--   splitWith :: (a -> Bool) -> [a] -> [[a]]
+
+safeTail :: [a] -> [a]
+safeTail [] = []
+safeTail xs = tail xs
+
+splitWith :: (a -> Bool) -> [a] -> [[a]]
+splitWith f [] = []
+splitWith f xs = if not (null first)
+                 then first:splitRest
+                 else splitRest
+  where (first, suffix) = span f xs
+        rest = safeTail suffix
+        splitRest = splitWith f rest
+
+-- ghci> :l 4_a_2.hs
+-- [1 of 1] Compiling Main             ( 4_a_2.hs, interpreted )
+-- Ok, one module loaded.
+-- ghci> splitWith even [2, 1, 4, 4, 3, 3, 6, 6, 6]
+-- [[2],[4,4],[6,6,6]]