Add Semigroup and Monoid instances (#51)

* Add Semigroup and Monoid instances

* Update CHANGELOG.md

Author: purefunctor

CHANGELOG.md

@@ -7,6 +7,7 @@ Notable changes to this project are documented in this file. The format is based
 Breaking changes:
 
 New features:
+- Add Semigroup and Monoid instances (#51 by @PureFunctor)
 
 Bugfixes:
 

src/Control/Monad/ST/Internal.purs

@@ -15,6 +15,7 @@ module Control.Monad.ST.Internal
 
 import Prelude
 
+import Control.Apply (lift2)
 import Control.Monad.Rec.Class (class MonadRec, Step(..))
 import Partial.Unsafe (unsafePartial)
 
@@ -72,6 +73,12 @@ instance monadRecST :: MonadRec (ST r) where
         Loop _ -> true
         _ -> false
 
+instance semigroupST :: Semigroup a => Semigroup (ST r a) where
+  append = lift2 append
+
+instance monoidST :: Monoid a => Monoid (ST r a) where
+  mempty = pure mempty
+
 -- | Run an `ST` computation.
 -- |
 -- | Note: the type of `run` uses a rank-2 type to constrain the phantom