4_b_5 and 4_b_6: Add solutions

2 files changed, 32 insertions, 2 deletions

diff --git a/README.md b/README.md
index fee3989..2caaa7f 100644
--- a/README.md
+++ b/README.md
@@ -76,8 +76,8 @@ more visible in the list the first exercise of a group is in bold italics.
 | 4_b_2          | yes    | 98   | |
 | 4_b_3          | yes    |      | |
 | 4_b_4          | yes    |      | |
-| 4_b_5          |        |      | |
-| 4_b_6          |        |      | |
+| 4_b_5          | yes, in 4_b_6 |      | |
+| 4_b_6          | yes    |      | |
 | 4_b_7          |        |      | |
 | 4_b_8          |        |      | |
 | 4_b_9          |        |      | |
diff --git a/ch04/4_b_6.hs b/ch04/4_b_6.hs
new file mode 100644
index 0000000..91604d4
--- /dev/null
+++ b/ch04/4_b_6.hs
@@ -0,0 +1,30 @@
+-- 1. The Prelude function concat concatenates a list of lists into a single list
+--    and has the following type:
+--
+--      -- file: ch04/ch04.exercises.hs
+--      concat :: [[a]] -> [a]
+--
+-- 2. Write your own definition of concat using foldr.
+
+myConcat :: [[a]] -> [a]
+myConcat xs = foldr (++) [] xs
+
+-- ghci> :l 4_b_6.hs
+-- [1 of 1] Compiling Main             ( 4_b_6.hs, interpreted )
+-- Ok, one module loaded.
+-- ghci> concat []
+-- []
+-- ghci> concat [[]]
+-- []
+-- ghci> concat [[1]]
+-- [1]
+-- ghci> concat [[1], [2,3], [4,5,6]]
+-- [1,2,3,4,5,6]
+-- ghci> myConcat []
+-- []
+-- ghci> myConcat [[]]
+-- []
+-- ghci> myConcat [[1]]
+-- [1]
+-- ghci> myConcat [[1], [2,3], [4,5,6]]
+-- [1,2,3,4,5,6]