diff -ruN orig/Control/Monad/Primitive.hs new/Control/Monad/Primitive.hs
--- orig/Control/Monad/Primitive.hs	2014-08-01 14:49:04.175318972 +0300
+++ new/Control/Monad/Primitive.hs	2014-08-01 14:49:03.000000000 +0300
@@ -44,7 +44,9 @@
 primitive_ :: PrimMonad m
               => (State# (PrimState m) -> State# (PrimState m)) -> m ()
 {-# INLINE primitive_ #-}
-primitive_ f = primitive (\s# -> (# f s#, () #))
+primitive_ f = primitive (\s# ->
+    case f s# of
+        s'# -> (# s'#, () #))
 
 instance PrimMonad IO where
   type PrimState IO = RealWorld
diff -ruN orig/primitive.cabal new/primitive.cabal
--- orig/primitive.cabal	2014-08-01 14:49:04.175318972 +0300
+++ new/primitive.cabal	2014-08-01 14:49:03.000000000 +0300
@@ -50,6 +50,16 @@
   if arch(i386) || arch(x86_64)
       cc-options: -msse2
 
+test-suite test
+  Default-Language: Haskell2010
+  hs-source-dirs: test
+  main-is: main.hs
+  type: exitcode-stdio-1.0
+  build-depends: base
+               , ghc-prim
+               , primitive
+  ghc-options: -O2
+
 source-repository head
   type:     git
   location: https://github.com/haskell/primitive
diff -ruN orig/test/main.hs new/test/main.hs
--- orig/test/main.hs	1970-01-01 02:00:00.000000000 +0200
+++ new/test/main.hs	2014-08-01 14:49:03.000000000 +0300
@@ -0,0 +1,24 @@
+{-# LANGUAGE MagicHash, UnboxedTuples #-}
+import Control.Monad.Primitive
+import Data.Primitive.Array
+import GHC.IO
+import GHC.Prim
+
+-- Since we only have a single test case right now, I'm going to avoid the
+-- issue of choosing a test framework for the moment. This also keeps the
+-- package as a whole light on dependencies.
+
+main :: IO ()
+main = do
+    arr <- newArray 1 'A'
+    let unit =
+            case writeArray arr 0 'B' of
+                IO f ->
+                    case f realWorld# of
+                        _ -> ()
+    c1 <- readArray arr 0
+    return $! unit
+    c2 <- readArray arr 0
+    if c1 == 'A' && c2 == 'B'
+        then return ()
+        else error $ "Expected AB, got: " ++ show (c1, c2)