4_b_7: Add solution

2 files changed, 41 insertions, 1 deletions

diff --git a/README.md b/README.md
index 2caaa7f..c734f3d 100644
--- a/README.md
+++ b/README.md
@@ -78,7 +78,7 @@ more visible in the list the first exercise of a group is in bold italics.
 | 4_b_4          | yes    |      | |
 | 4_b_5          | yes, in 4_b_6 |      | |
 | 4_b_6          | yes    |      | |
-| 4_b_7          |        |      | |
+| 4_b_7          | yes    |      | |
 | 4_b_8          |        |      | |
 | 4_b_9          |        |      | |
 | 4_b_10         |        |      | |
diff --git a/ch04/4_b_7.hs b/ch04/4_b_7.hs
new file mode 100644
index 0000000..7cfe383
--- /dev/null
+++ b/ch04/4_b_7.hs
@@ -0,0 +1,40 @@
+-- Write your own definition of the standard takeWhile function, first using
+-- explicit recursion, and then foldr.
+
+myTakeWhileRecursive :: (a -> Bool) -> [a] -> [a]
+myTakeWhileRecursive f (x:xs) = if f x
+                                then x:(myTakeWhileRecursive f xs)
+                                else []
+myTakeWhileRecursive _ [] = []
+
+
+myTakeWhileFoldr :: (a -> Bool) -> [a] -> [a]
+myTakeWhileFoldr f xs = foldr op [] xs
+  where op x acc = if f x
+                   then x:acc
+                   else [] -- Folding from the right. When function f is false for x,
+                           -- the accumulated list is thrown away, thus eventually
+                           -- keeping only the leftmost sublist for which the f is
+                           -- true for all its items.
+
+-- ghci> :l 4_b_7.hs
+-- [1 of 1] Compiling Main             ( 4_b_7.hs, interpreted )
+-- Ok, one module loaded.
+-- ghci> takeWhile odd []
+-- []
+-- ghci> takeWhile odd [2, 1]
+-- []
+-- ghci> takeWhile odd [1, 3, 2, 7]
+-- [1,3]
+-- ghci> myTakeWhileRecursive odd []
+-- []
+-- ghci> myTakeWhileRecursive odd [2, 1]
+-- []
+-- ghci> myTakeWhileRecursive odd [1, 3, 2, 7]
+-- [1,3]
+-- ghci> myTakeWhileFoldr odd []
+-- []
+-- ghci> myTakeWhileFoldr odd [2, 1]
+-- []
+-- ghci> myTakeWhileFoldr odd [1, 3, 2, 7]
+-- [1,3]