Add Control.Monad.STM.Class

Author: mitchellwrosen

Control/Monad/STM/Class.hs

@@ -0,0 +1,48 @@
+{-# LANGUAGE CPP #-}
+
+#if __GLASGOW_HASKELL__ >= 701
+{-# LANGUAGE Trustworthy #-}
+#endif
+
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Monad.STM
+-- Copyright   :  (c) The University of Glasgow 2004
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
+--
+-- Maintainer  :  libraries@haskell.org
+-- Stability   :  experimental
+-- Portability :  non-portable (requires STM)
+--
+-- Class of monads based on @STM@.
+-----------------------------------------------------------------------------
+
+module Control.Monad.STM.Class (
+        MonadSTM(..)
+  ) where
+
+import GHC.Conc (STM, atomically)
+
+
+-- | Monads in which 'STM' computations may be embedded.
+-- Any monad built by applying a sequence of monad transformers to the
+-- 'STM' monad will be an instance of this class.
+--
+-- Instances should satisfy the following laws, which state that 'liftSTM'
+-- is a transformer of monads:
+--
+-- * @'liftSTM' . 'return' = 'return'@
+--
+-- * @'liftSTM' (m >>= f) = 'liftSTM' m >>= ('liftSTM' . f)@
+
+class Monad m => MonadSTM m where
+    -- | Lift a computation from the 'STM' monad.
+    liftSTM :: STM a -> m a
+
+-- | @since FIXME
+instance MonadSTM STM where
+    liftSTM = id
+
+-- | @since FIXME
+instance MonadSTM IO where
+    liftSTM = atomically

changelog.md

@@ -1,5 +1,11 @@
 # Changelog for [`stm` package](http://hackage.haskell.org/package/stm)
 
+## Unreleased
+
+  * Added `Control.Monad.STM.Class` module
+
+    * Before release, update `@since FIXME` annotations
+
 ## 2.5.0.0 *Sep 2018*
 
   * Removed `alwaysSucceeds` and `always`, GHC's invariant checking primitives. (GHC #14324)

stm.cabal

@@ -60,6 +60,7 @@ library
         Control.Concurrent.STM.TBQueue
         Control.Concurrent.STM.TSem
         Control.Monad.STM
+        Control.Monad.STM.Class
     other-modules:
         Control.Sequential.STM