From 3f72482cf401bb70b3c20ab1d62d4904385670f2 Mon Sep 17 00:00:00 2001
From: Gregor Kleen <gkleen@yggdrasil.li>
Date: Mon, 13 Nov 2017 17:48:00 +0100
Subject: Comment on language extensions & fix compilation

---
 edit-lens/src/Control/Lens/Edit.lhs | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/edit-lens/src/Control/Lens/Edit.lhs b/edit-lens/src/Control/Lens/Edit.lhs
index 69a6f7f..973f409 100644
--- a/edit-lens/src/Control/Lens/Edit.lhs
+++ b/edit-lens/src/Control/Lens/Edit.lhs
@@ -3,14 +3,16 @@ Dabei werden wir sowohl die Definitionen aus \cite{hofmann2012edit} sowohl in na
 
 \begin{code}
 {-# LANGUAGE TypeFamilies
-           , KindSignatures
            , FlexibleContexts
            , FlexibleInstances
            , MultiParamTypeClasses
            , FunctionalDependencies
-           , AllowAmbiguousTypes
-           , GADTs
   #-}
+-- Allow more complicated type families
+{-# LANGUAGE AllowAmbiguousTypes #-}
+-- AmbiguousTypes are useful if we expect functions to be called via TypeApplication
+{-# LANGUAGE GADTs #-}
+-- For allowing constraints on constructors
 
 module Control.Lens.Edit
   ( Module(..)
@@ -47,7 +49,7 @@ class Monoid m => Module m where
   -- prop> m `apply` (e `mappend` e') = (m `apply` e) `apply` e'
   init :: Domain m
   -- ^ 'init @m' (TypeApplication) is the initial element of 'm'
-  divInit :: Domain m -> Del m
+  divInit :: Domain m -> m
   -- ^ Calculate a representation of an element of 'Domain m' in 'Del m'
   --
   -- prop> init `apply` divInit m = m
@@ -131,7 +133,7 @@ class (Module m, Module n) => HasEditLens l m n | l -> m, l -> n where
   -- ^ Map edits of 'n' to changes of 'm', maintaining some state from 'Complement l'
 
 -- | Inspect the components of an edit lens (e.g. 'EditLens')
-instance HasEditLens (EditLens c m n) where
+instance (Module m, Module n) => HasEditLens (EditLens c m n) m n where
   type Complement (EditLens c m n) = c
   ground (EditLens g _ _) = g
   propR  (EditLens _ r _) = r
-- 
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