add first exercise of lecture 7HEADmaster

1 files changed, 17 insertions, 0 deletions

diff --git a/lec07/JoinList.hs b/lec07/JoinList.hs
new file mode 100644
index 0000000..d04a0c6
--- /dev/null
+++ b/lec07/JoinList.hs
@@ -0,0 +1,17 @@
+module JoinList where
+
+data JoinList m a = Empty
+                  | Single m a
+                  | Append m (JoinList m a) (JoinList m a)
+  deriving (Eq, Show)
+
+-- Exercise 1
+
+tag :: Monoid m => JoinList m a -> m
+tag Empty = mempty
+tag Single m _ = m
+tag Append m _ _ = m
+
+(+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a
+(+++) jl1 jl2 = mappend jl1 jl2
+