Add Semigroup instance for NonEmpty

CHANGELOG.md

@@ -7,6 +7,7 @@ Notable changes to this project are documented in this file. The format is based
 Breaking changes:
 
 New features:
+- Added semigroup instance (#18 by @jmatsushita)
 
 Bugfixes:
 

src/Data/NonEmpty.purs

@@ -167,3 +167,11 @@ instance foldable1NonEmpty :: Foldable f => Foldable1 (NonEmpty f) where
 
 instance unfoldable1NonEmpty :: Unfoldable f => Unfoldable1 (NonEmpty f) where
   unfoldr1 f b = uncurry (:|) $ unfoldr (map f) <$> f b
+
+-- | This is a lawful `Semigroup` instance that will behave sensibly for common nonempty
+-- | containers like lists and arrays. However, it's not guaranteed that `pure` will behave
+-- | sensibly alongside `<>` for all types, as we don't have any laws which govern their behavior.
+instance semigroupNonEmpty
+  :: (Applicative f, Semigroup (f a))
+  => Semigroup (NonEmpty f a) where
+  append (a1 :| f1) (a2 :| f2) = a1 :| (f1 <> pure a2 <> f2)

test/Main.purs

@@ -27,3 +27,4 @@ main = do
   assert $ oneOf (0 :| Nothing) == oneOf (0 :| Just 1)
   assert $ second (1 :| 2 :| [3, 4]) == 2
   assert $ U1.range 0 9 == (0 :| [1, 2, 3, 4, 5, 6, 7, 8, 9])
+  assert $ (0 :| [1,2]) <> (3 :| [4,5]) == (0 :| [1, 2, 3, 4, 5])