Eliminate use of QuantifiedConstraints to support GHC < 8.6

Author: ali-abrar

hnix.cabal

@@ -404,6 +404,7 @@ library
     , binary >= 0.8.5 && < 0.9
     , bytestring >= 0.10.8 && < 0.11
     , comonad >= 5.0.4 && < 5.1
+    , constraints >= 0.11 && <0.13
     , containers >= 0.5.11.0 && < 0.7
     , data-fix >= 0.3.0 && < 0.4
     , deepseq >= 1.4.3 && <1.5

src/Nix/Standard.hs

@@ -5,7 +5,6 @@
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TypeApplications #-}
@@ -25,6 +24,8 @@ import           Control.Monad.Reader
 import           Control.Monad.Ref
 import           Control.Monad.State
 import           Data.Coerce
+import           Data.Constraint                ( (\\) )
+import           Data.Constraint.Forall         ( Forall, inst )
 import           Data.Functor.Identity
 import           Data.HashMap.Lazy              ( HashMap )
 import           Data.Text                      ( Text )
@@ -202,8 +203,12 @@ instance HasCitations1 m v Identity where
 
 type StandardT m = Fix1T StandardTF m
 
-instance (forall m. MonadTrans (t (Fix1T t m))) => MonadTrans (Fix1T t) where
-  lift = Fix1T . lift
+class MonadTrans (t (Fix1T t m)) => TransAtFix1T t m
+
+instance MonadTrans (t (Fix1T t m)) => TransAtFix1T t m
+
+instance Forall (TransAtFix1T t) => MonadTrans (Fix1T t) where
+  lift (x :: m a) = Fix1T $ (lift \\ inst @(TransAtFix1T t) @m) x
 
 mkStandardT
   :: StandardTFInner (Fix1T StandardTF m) m a

src/Nix/Utils/Fix1.hs

@@ -4,9 +4,10 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE UndecidableInstances #-}
-{-# LANGUAGE QuantifiedConstraints #-}
 
 module Nix.Utils.Fix1 where
 
@@ -22,6 +23,9 @@ import           Control.Monad.Catch            ( MonadCatch
                                                 , MonadThrow )
 import           Control.Monad.Reader           ( MonadReader )
 import           Control.Monad.State            ( MonadState )
+import           Data.Constraint                ( (\\) )
+import           Data.Constraint.Forall         ( Forall, inst )
+
 
 import Nix.Thunk --TODO: Move MonadTransWrap somewhere better, or find something that already exists
 
@@ -60,8 +64,12 @@ deriving instance MonadMask (t (Fix1T t m) m) => MonadMask (Fix1T t m)
 deriving instance MonadReader e (t (Fix1T t m) m) => MonadReader e (Fix1T t m)
 deriving instance MonadState s (t (Fix1T t m) m) => MonadState s (Fix1T t m)
 
-instance (forall m. MonadTransWrap (t (Fix1T t m))) => MonadTransWrap (Fix1T t) where
-  liftWrap f (Fix1T a) = Fix1T $ liftWrap f a
+class MonadTransWrap (t (Fix1T t m)) => TransWrapAtFix1T t m
+
+instance MonadTransWrap (t (Fix1T t m)) => TransWrapAtFix1T t m
+
+instance Forall (TransWrapAtFix1T t) => MonadTransWrap (Fix1T t) where
+  liftWrap (f :: forall x. m x -> m x) (Fix1T (a :: (t (Fix1T t m) m a))) = Fix1T $ liftWrap f a \\ inst @(TransWrapAtFix1T t) @m
 
 {-