defined Alternative

josefs · Dec 22, 2023 · 30b13ae · 30b13ae
1 parent 6412fcb
commit 30b13ae
Showing 1 changed file with 4 additions and 0 deletions.
diff --git a/Control/Monad/ST/Trans/Internal.hs b/Control/Monad/ST/Trans/Internal.hs
@@ -105,6 +105,10 @@ instance (Monad m, Functor m) => Applicative (STT s m) where
                            (STTRet s3 x) <- n s2
                            return (STTRet s3 (f x))
 
+instance (Monad m, Alternative m) => Alternative (STT s m) where
+  empty = STT $ \_ -> empty
+  STT m <|> STT n = STT $ \s# -> m s# <|> n s#
+
 -- Instances of other monad classes
 
 instance (MonadError e m, Functor m) => MonadError e (STT s m) where