From dcf722d0515e7aa3c21ef8caaa6b493b0364a50d Mon Sep 17 00:00:00 2001
From: Anton-Latukha <anton.latukha@gmail.com>
Date: Tue, 9 Feb 2021 04:00:31 +0200
Subject: [PATCH] Eliminate use of QuantifiedConstraints to support GHC < 8.6

---
 hnix.cabal             |  1 +
 src/Nix/Scope/Basic.hs |  1 +
 src/Nix/Standard.hs    | 11 ++++++++---
 src/Nix/Thunk.hs       | 15 +++++++++++++--
 src/Nix/Utils/Fix1.hs  |  7 +++++--
 5 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/hnix.cabal b/hnix.cabal
index 873e6c396..66a2c50ba 100644
--- a/hnix.cabal
+++ b/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
diff --git a/src/Nix/Scope/Basic.hs b/src/Nix/Scope/Basic.hs
index bf19ac6f9..983bbcd36 100644
--- a/src/Nix/Scope/Basic.hs
+++ b/src/Nix/Scope/Basic.hs
@@ -15,6 +15,7 @@
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE StandaloneDeriving #-}
+
 module Nix.Scope.Basic where
 
 import Control.Applicative
diff --git a/src/Nix/Standard.hs b/src/Nix/Standard.hs
index eaf7b1125..578ec92c7 100644
--- a/src/Nix/Standard.hs
+++ b/src/Nix/Standard.hs
@@ -8,7 +8,6 @@
 {-# LANGUAGE DeriveTraversable #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
@@ -30,6 +29,8 @@ 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 )
@@ -173,8 +174,12 @@ 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
diff --git a/src/Nix/Thunk.hs b/src/Nix/Thunk.hs
index cecb12cd4..79d9f8dc0 100644
--- a/src/Nix/Thunk.hs
+++ b/src/Nix/Thunk.hs
@@ -1,3 +1,7 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE LambdaCase #-}
@@ -18,6 +22,8 @@ import           Control.Monad.Trans.State
 import           Control.Monad.Trans.Writer
 import           Data.Typeable                  ( Typeable )
 import           Nix.Utils.Fix1
+import           Data.Constraint                ( (\\) )
+import           Data.Constraint.Forall         ( Forall, inst )
 
 class MonadTransWrap t where
   --TODO: Can we enforce that the resulting function is as linear as the provided one?
@@ -47,8 +53,13 @@ instance MonadTransWrap (StateT s) where
     put new
     pure result
 
-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
 
 
 class ( Monad m
diff --git a/src/Nix/Utils/Fix1.hs b/src/Nix/Utils/Fix1.hs
index 4f853e66b..d8c72bfe1 100644
--- a/src/Nix/Utils/Fix1.hs
+++ b/src/Nix/Utils/Fix1.hs
@@ -7,7 +7,9 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE UndecidableInstances #-}
 
 module Nix.Utils.Fix1 where
@@ -30,6 +32,7 @@ import           Control.Monad.Reader           ( MonadReader )
 import           Control.Monad.State            ( MonadState )
 
 
+
 -- | The fixpoint combinator, courtesy of Gregory Malecha.
 --   https://gist.github.com/gmalecha/ceb3778b9fdaa4374976e325ac8feced
 newtype Fix1 (t :: (k -> *) -> k -> *) (a :: k) = Fix1 { unFix1 :: t (Fix1 t) a }
@@ -50,6 +53,8 @@ deriving instance MonadState s (t (Fix1 t)) => MonadState s (Fix1 t)
 newtype Fix1T (t :: (k -> *) -> (* -> *) -> k -> *) (m :: * -> *) (a :: k)
   = Fix1T { unFix1T :: t (Fix1T t m) m a }
 
+type MonadFix1T t m = (MonadTrans (Fix1T t), Monad (t (Fix1T t m) m))
+
 deriving instance Functor (t (Fix1T t m) m) => Functor (Fix1T t m)
 deriving instance Applicative (t (Fix1T t m) m) => Applicative (Fix1T t m)
 deriving instance Alternative (t (Fix1T t m) m) => Alternative (Fix1T t m)
@@ -65,8 +70,6 @@ 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)
 
-type MonadFix1T t m = (MonadTrans (Fix1T t), Monad (t (Fix1T t m) m))
-
 instance (MonadFix1T t m, MonadRef m) => MonadRef (Fix1T t m) where
   type Ref (Fix1T t m) = Ref m
   newRef  = lift . newRef