4_a_2: Add solution

1 files changed, 25 insertions, 0 deletions

diff --git a/ch04/4_a_2.hs b/ch04/4_a_2.hs
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+-- 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]]