Add execWriterT

Author: MichaelXavier

docs/Module.md

@@ -779,6 +779,8 @@
 
 ### Values
 
+    execWriterT :: forall w m a. (Monad m) => WriterT w m a -> m w
+
     liftCallCCWriter :: forall w m a b. (Monoid w) => (((Tuple a w -> m (Tuple b w)) -> m (Tuple a w)) -> m (Tuple a w)) -> ((a -> WriterT w m b) -> WriterT w m a) -> WriterT w m a
 
     liftCatchWriter :: forall w m e a. (m (Tuple a w) -> (e -> m (Tuple a w)) -> m (Tuple a w)) -> WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

src/Control/Monad/Writer/Trans.purs

@@ -13,6 +13,9 @@ newtype WriterT w m a = WriterT (m (Tuple a w))
 runWriterT :: forall w m a. WriterT w m a -> m (Tuple a w)
 runWriterT (WriterT x) = x
 
+execWriterT :: forall w m a. (Monad m) => WriterT w m a -> m w
+execWriterT m = runWriterT m >>= \(Tuple _ w) -> return w
+
 mapWriterT :: forall w1 w2 m1 m2 a b. (m1 (Tuple a w1) -> m2 (Tuple b w2)) -> WriterT w1 m1 a -> WriterT w2 m2 b
 mapWriterT f m = WriterT $ f (runWriterT m)