functors

1 files changed, 14 insertions, 0 deletions

diff --git a/functor/h5.hs b/functor/h5.hs
new file mode 100644
index 0000000..88bc79a
--- /dev/null
+++ b/functor/h5.hs
@@ -0,0 +1,14 @@
+-- The composition of two Functors is also a Functor.
+
+import Data.Functor
+
+newtype Comp f g a = Comp { unComp :: f (g a) }
+
+compose :: f (g a) -> Comp f g a
+compose = Comp
+
+decompose :: Comp f g a -> f (g a)
+decompose = unComp
+
+instance (Functor f, Functor g) => Functor (Comp f g) where
+  fmap f = compose . fmap (fmap f) . decompose