diff --git a/monad-supply.cabal b/monad-supply.cabal
--- a/monad-supply.cabal
+++ b/monad-supply.cabal
@@ -1,6 +1,6 @@
 Name:          monad-supply
 Build-type:    Simple
-Version:       0.2
+Version:       0.3
 Cabal-Version: >= 1.6
 Synopsis:      Stateful supply monad.
 Description:   Support for computations which consume values from a (possibly infinite) supply.
diff --git a/src/Control/Monad/Supply.hs b/src/Control/Monad/Supply.hs
--- a/src/Control/Monad/Supply.hs
+++ b/src/Control/Monad/Supply.hs
@@ -5,7 +5,7 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
 
 -- | Support for computations which consume values from a (possibly infinite)
--- supply. See http://www.haskell.org/haskellwiki/New_monads/MonadSupply for
+-- supply. See <http://www.haskell.org/haskellwiki/New_monads/MonadSupply> for
 -- details.
 module Control.Monad.Supply
 ( MonadSupply (..)
@@ -23,10 +23,11 @@
 import Control.Monad.Error
 import Control.Monad.Reader
 import Control.Monad.Writer
-import Data.Monoid
 
 class Monad m => MonadSupply s m | m -> s where
   supply :: m s
+  peek :: m s
+  exhausted :: m Bool
 
 -- | Supply monad transformer.
 newtype SupplyT s m a = SupplyT (StateT [s] m a)
@@ -40,23 +41,34 @@
   supply = SupplyT $ do (x:xs) <- get
                         put xs
                         return x
+  peek = SupplyT $ gets head
+  exhausted = SupplyT $ gets null
 
 -- Monad transformer instances
 instance (Error e,MonadSupply s m) => MonadSupply s (ErrorT e m) where
   supply = lift supply
+  peek = lift peek
+  exhausted = lift exhausted
 
 instance MonadSupply s m => MonadSupply s (StateT st m) where
   supply = lift supply
+  peek = lift peek
+  exhausted = lift exhausted
 
 instance MonadSupply s m => MonadSupply s (ReaderT r m) where
   supply = lift supply
+  peek = lift peek
+  exhausted = lift exhausted
 
 instance (Monoid w, MonadSupply s m) => MonadSupply s (WriterT w m) where
   supply = lift supply
+  peek = lift peek
+  exhausted = lift exhausted
 
--- | Monoid instance for the supply monad. Actually any monad/monoid pair gives
--- rise to this monoid instance, but we can't write it like that because it
--- would conflict with existing instances provided by Data.Monoid.
+-- | Monoid instance for the supply monad. Actually any monad/monoid pair
+-- gives rise to this monoid instance, but we can't write its type like that
+-- because it would conflict with existing instances provided by Data.Monoid.
+--instance (Monoid a, Monad m) => Monoid (m a) where
 instance (Monoid a) => Monoid (Supply s a) where
   mempty = return mempty
   m1 `mappend` m2 = do