Value: fx MonadFail for GHC 8.6

Author: Anton-Latukha

src/Nix/Reduce.hs

@@ -32,26 +32,39 @@ module Nix.Reduce
 
 import           Control.Applicative
 import           Control.Arrow                  ( second )
-import           Control.Monad
-import           Control.Monad.Catch
+import           Control.Monad                  ( MonadPlus
+                                                , join
+                                                )
+import           Control.Monad.Catch            ( MonadCatch(catch) )
 #if !MIN_VERSION_base(4,13,0)
+import           Prelude                 hiding ( fail )
 import           Control.Monad.Fail
 #endif
-import           Control.Monad.Fix
-import           Control.Monad.IO.Class
-import           Control.Monad.Reader
+import           Control.Monad.Fix              ( MonadFix )
+import           Control.Monad.IO.Class         ( MonadIO(liftIO) )
+import           Control.Monad.Reader           ( MonadReader(local)
+                                                , ReaderT(runReaderT)
+                                                , asks
+                                                )
 import           Control.Monad.State.Strict     ( MonadState
                                                 , StateT
                                                 , gets
                                                 , modify
                                                 , evalStateT
                                                 )
 import           Data.Bifunctor                 ( first )
-import           Data.Fix                       ( Fix(..), foldFix, foldFixM )
+import           Data.Fix                       ( Fix(..)
+                                                , foldFix
+                                                , foldFixM
+                                                )
 import           Data.HashMap.Lazy              ( HashMap )
 import qualified Data.HashMap.Lazy             as M
 import qualified Data.HashMap.Strict           as MS
-import           Data.IORef
+import           Data.IORef                     ( IORef
+                                                , newIORef
+                                                , readIORef
+                                                , writeIORef
+                                                )
 import           Data.List.NonEmpty             ( NonEmpty(..) )
 import qualified Data.List.NonEmpty            as NE
 import           Data.Maybe                     ( fromMaybe
@@ -74,12 +87,25 @@ import           System.Directory
 import           System.FilePath
 
 newtype Reducer m a = Reducer
-    { runReducer :: ReaderT (Maybe FilePath, Scopes (Reducer m) NExprLoc)
-                           (StateT (HashMap FilePath NExprLoc, MS.HashMap Text Text) m) a }
-    deriving (Functor, Applicative, Alternative, Monad, MonadPlus,
-              MonadFix, MonadIO, MonadFail,
-              MonadReader (Maybe FilePath, Scopes (Reducer m) NExprLoc),
-              MonadState (HashMap FilePath NExprLoc, MS.HashMap Text Text))
+    { runReducer ::
+        ReaderT
+          ( Maybe FilePath
+          , Scopes (Reducer m) NExprLoc
+          )
+          ( StateT
+              ( HashMap FilePath NExprLoc
+              , MS.HashMap Text Text
+              )
+            m
+          )
+          a
+    }
+  deriving
+    ( Functor, Applicative, Alternative
+    , Monad, MonadPlus, MonadFix, MonadIO, MonadFail
+    , MonadReader (Maybe FilePath, Scopes (Reducer m) NExprLoc)
+    , MonadState (HashMap FilePath NExprLoc, MS.HashMap Text Text)
+    )
 
 staticImport
   :: forall m

src/Nix/Utils.hs

@@ -40,6 +40,9 @@ import           Prelude                       as X
                                          hiding ( putStr
                                                 , putStrLn
                                                 , print
+#if !MIN_VERSION_base(4,13,0)
+                                                , fail
+#endif
                                                 )
 trace :: String -> a -> a
 trace = const id

src/Nix/Value.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE ConstraintKinds #-}
@@ -34,6 +35,9 @@ import           Control.Monad.Free             ( Free(..)
 import           Control.Monad.Trans.Class      ( MonadTrans
                                                 , lift
                                                 )
+#if !MIN_VERSION_base(4,13,0)
+import           Control.Monad.Fail
+#endif
 import qualified Data.Aeson                    as Aeson
 import           Data.Functor.Classes           ( Show1
                                                 , liftShowsPrec