1 files changed, 86 insertions, 0 deletions
diff --git a/third_party/bazel/rules_haskell/examples/primitive/Data/Primitive/MutVar.hs b/third_party/bazel/rules_haskell/examples/primitive/Data/Primitive/MutVar.hs
new file mode 100644
index 0000000000..f707bfb630
--- /dev/null
+++ b/third_party/bazel/rules_haskell/examples/primitive/Data/Primitive/MutVar.hs
@@ -0,0 +1,86 @@
+{-# LANGUAGE MagicHash, UnboxedTuples, DeriveDataTypeable #-}
+
+-- |
+-- Module      : Data.Primitive.MutVar
+-- Copyright   : (c) Justin Bonnar 2011, Roman Leshchinskiy 2011-2012
+-- License     : BSD-style
+--
+-- Maintainer  : Roman Leshchinskiy <rl@cse.unsw.edu.au>
+-- Portability : non-portable
+--
+-- Primitive boxed mutable variables
+--
+
+module Data.Primitive.MutVar (
+  MutVar(..),
+
+  newMutVar,
+  readMutVar,
+  writeMutVar,
+
+  atomicModifyMutVar,
+  atomicModifyMutVar',
+  modifyMutVar,
+  modifyMutVar'
+) where
+
+import Control.Monad.Primitive ( PrimMonad(..), primitive_ )
+import GHC.Prim ( MutVar#, sameMutVar#, newMutVar#,
+                  readMutVar#, writeMutVar#, atomicModifyMutVar# )
+import Data.Primitive.Internal.Compat ( isTrue# )
+import Data.Typeable ( Typeable )
+
+-- | A 'MutVar' behaves like a single-element mutable array associated
+-- with a primitive state token.
+data MutVar s a = MutVar (MutVar# s a)
+  deriving ( Typeable )
+
+instance Eq (MutVar s a) where
+  MutVar mva# == MutVar mvb# = isTrue# (sameMutVar# mva# mvb#)
+
+-- | Create a new 'MutVar' with the specified initial value
+newMutVar :: PrimMonad m => a -> m (MutVar (PrimState m) a)
+{-# INLINE newMutVar #-}
+newMutVar initialValue = primitive $ \s# ->
+  case newMutVar# initialValue s# of
+    (# s'#, mv# #) -> (# s'#, MutVar mv# #)
+
+-- | Read the value of a 'MutVar'
+readMutVar :: PrimMonad m => MutVar (PrimState m) a -> m a
+{-# INLINE readMutVar #-}
+readMutVar (MutVar mv#) = primitive (readMutVar# mv#)
+
+-- | Write a new value into a 'MutVar'
+writeMutVar :: PrimMonad m => MutVar (PrimState m) a -> a -> m ()
+{-# INLINE writeMutVar #-}
+writeMutVar (MutVar mv#) newValue = primitive_ (writeMutVar# mv# newValue)
+
+-- | Atomically mutate the contents of a 'MutVar'
+atomicModifyMutVar :: PrimMonad m => MutVar (PrimState m) a -> (a -> (a,b)) -> m b
+{-# INLINE atomicModifyMutVar #-}
+atomicModifyMutVar (MutVar mv#) f = primitive $ atomicModifyMutVar# mv# f
+
+-- | Strict version of 'atomicModifyMutVar'. This forces both the value stored
+-- in the 'MutVar' as well as the value returned.
+atomicModifyMutVar' :: PrimMonad m => MutVar (PrimState m) a -> (a -> (a, b)) -> m b
+{-# INLINE atomicModifyMutVar' #-}
+atomicModifyMutVar' mv f = do
+  b <- atomicModifyMutVar mv force
+  b `seq` return b
+  where
+    force x = let (a, b) = f x in (a, a `seq` b)
+
+-- | Mutate the contents of a 'MutVar'
+modifyMutVar :: PrimMonad m => MutVar (PrimState m) a -> (a -> a) -> m ()
+{-# INLINE modifyMutVar #-}
+modifyMutVar (MutVar mv#) g = primitive_ $ \s# ->
+  case readMutVar# mv# s# of
+    (# s'#, a #) -> writeMutVar# mv# (g a) s'#
+
+-- | Strict version of 'modifyMutVar'
+modifyMutVar' :: PrimMonad m => MutVar (PrimState m) a -> (a -> a) -> m ()
+{-# INLINE modifyMutVar' #-}
+modifyMutVar' (MutVar mv#) g = primitive_ $ \s# ->
+  case readMutVar# mv# s# of
+    (# s'#, a #) -> let a' = g a in a' `seq` writeMutVar# mv# a' s'#
+