Add new MonadInspectMVar class with an instance for IO.

io-classes/CHANGELOG.md

@@ -1,5 +1,14 @@
 # Revsion history of io-classes
 
+## next version
+
+### Non-breaking changes
+
+* Add new `MonadInspectMVar` class with an `inspectMVar` function for accessing
+  an `MVar` in an underlying monad (if applicable). This is mainly useful for
+  `io-sim`, since the underlying monad is `ST`. `IO` has no underlying monad, so
+  the provided instance for `IO` defaults `inspectMVar` to `tryReadMVar`.
+
 ## 1.1.0.0
 
 ### Breaking changes

io-classes/src/Control/Concurrent/Class/MonadMVar.hs

@@ -1,9 +1,12 @@
 {-# LANGUAGE DefaultSignatures      #-}
+{-# LANGUAGE FlexibleContexts       #-}
 {-# LANGUAGE QuantifiedConstraints  #-}
 {-# LANGUAGE TypeFamilyDependencies #-}
-{-# LANGUAGE TypeOperators          #-}
 
-module Control.Concurrent.Class.MonadMVar (MonadMVar (..)) where
+module Control.Concurrent.Class.MonadMVar
+  ( MonadMVar (..)
+  , MonadInspectMVar (..)
+  ) where
 
 import qualified Control.Concurrent.MVar as IO
 import           Control.Monad.Class.MonadThrow
@@ -127,6 +130,9 @@ class Monad m => MonadMVar m where
       return b
   {-# INLINE modifyMVarMasked #-}
 
+--
+-- IO instance
+--
 
 instance MonadMVar IO where
     type MVar IO      = IO.MVar
@@ -181,8 +187,22 @@ instance ( MonadMask m
     modifyMVarMasked (WrappedMVar v) f = ReaderT $ \r ->
       modifyMVarMasked v (\a -> runReaderT (f a) r)
 
+--
+-- MonadInspectMVar
+--
 
-
+-- | This type class is intended for
+-- ['io-sim'](https://hackage.haskell.org/package/io-sim), where one might want
+-- to access an 'MVar' in the underlying 'ST' monad.
+class (MonadMVar m, Monad (InspectMVarMonad m)) => MonadInspectMVar m where
+  type InspectMVarMonad m :: Type -> Type
+  -- | Return the value of an 'MVar' as an 'InspectMVarMonad' computation. Can
+  -- be 'Nothing' if the 'MVar' is empty.
+  inspectMVar :: proxy m -> MVar m a -> InspectMVarMonad m (Maybe a)
+
+instance MonadInspectMVar IO where
+  type InspectMVarMonad IO = IO
+  inspectMVar _ = tryReadMVar
 
 --
 -- Utilities