Merge pull request #14 from purescript-contrib/lazy

Add Lazy instance for StateT

Author: paf31

docs/Module.md

@@ -532,6 +532,8 @@
 
     instance functorStateT :: (Monad m) => Functor (StateT s m)
 
+    instance lazy1StateT :: Lazy1 (StateT s m)
+
     instance monadPlusStateT :: (MonadPlus m) => MonadPlus (StateT s m)
 
     instance monadStateT :: (Monad m) => Monad (StateT s m)

src/Control/Monad/State/Trans.purs

@@ -5,6 +5,7 @@ import Control.Alternative
 import Control.Plus
 import Control.Monad.Trans
 import Control.MonadPlus
+import Control.Lazy
 import Data.Tuple
 
 newtype StateT s m a = StateT (s -> m (Tuple a s))
@@ -55,6 +56,9 @@ instance monadTransStateT :: MonadTrans (StateT s) where
     x <- m
     return $ Tuple x s
 
+instance lazy1StateT :: Lazy1 (StateT s m) where
+  defer1 f = StateT $ \s -> runStateT (f unit) s
+
 liftCatchState :: forall s m e a. (m (Tuple a s) -> (e -> m (Tuple a s)) -> m (Tuple a s)) -> StateT s m a -> (e -> StateT s m a) -> StateT s m a
 liftCatchState catch m h = StateT $ \s -> catch (runStateT m s) (\e -> runStateT (h e) s)