Fix Travis jobs for GHC 8: Add fake Functor constraint

app/Main.hs

@@ -4,7 +4,7 @@
 module Main (main) where
 
 import           Control.Concurrent.Async
-import           Control.Lens
+import           Control.Lens.Compat
 import           Control.Monad.Reader
 import           Data.Maybe
 import           Data.Monoid

src/Control/Lens/Compat.hs

@@ -0,0 +1,30 @@
+-- | In lens-4.14, the constraint @'Functor' f@ is missing from the definition
+-- of 'to'. When compiling with GHC 8.0, this leads to warnings for definitions
+-- like
+--
+-- @
+-- foo :: Getter Bar Foo
+-- foo = to fooFromBar
+-- @
+--
+-- because of the redundant @'Functor' f@ constraint. This module exports an
+-- alternative definition with the added @'Functor' f@ Constraint.
+module Control.Lens.Compat
+  ( to
+  , module Control.Lens
+  ) where
+
+import           Control.Lens hiding (to)
+import qualified Control.Lens as Lens
+
+
+-- | Build an (index-preserving) 'Getter' from an arbitrary Haskell function.
+--
+-- Identical to "Control.Lens".'Lens.to' except for the additional constraint
+-- @'Functor' f@.
+to :: (Profunctor p, Functor f, Contravariant f) => (s -> a) -> Optic' p f s a
+to k = fakeFunctorConstraintToDisableGhc8WarningWithLens4_14 . Lens.to k
+  where
+    fakeFunctorConstraintToDisableGhc8WarningWithLens4_14 getter =
+        let _ = rmap (fmap undefined) getter
+        in  getter

src/Vgrep/Environment.hs

@@ -11,7 +11,7 @@ module Vgrep.Environment
     , module Graphics.Vty.Prelude
     ) where
 
-import Control.Lens
+import Control.Lens.Compat
 import Graphics.Vty.Prelude
 
 import Vgrep.Environment.Config

src/Vgrep/Environment/Config.hs

@@ -2,7 +2,7 @@
 {-# LANGUAGE TemplateHaskell #-}
 module Vgrep.Environment.Config where
 
-import Control.Lens
+import Control.Lens.Compat
 import Control.Monad.IO.Class
 import Data.Maybe
 import Data.Monoid

src/Vgrep/Text.hs

@@ -8,7 +8,7 @@ module Vgrep.Text (
     , expandLineForDisplay
     ) where
 
-import           Control.Lens
+import           Control.Lens.Compat
 import           Control.Monad.Reader.Class
 import           Data.Char
 import           Data.Text                  (Text)

src/Vgrep/Widget/HorizontalSplit.hs

@@ -22,9 +22,9 @@ module Vgrep.Widget.HorizontalSplit (
     , rightWidgetFocused
     ) where
 
-import Control.Lens
+import Control.Lens.Compat
 import Data.Monoid
-import Graphics.Vty.Image hiding (resize)
+import Graphics.Vty.Image  hiding (resize)
 import Graphics.Vty.Input
 
 import Vgrep.Environment

src/Vgrep/Widget/HorizontalSplit/Internal.hs

@@ -19,8 +19,8 @@ module Vgrep.Widget.HorizontalSplit.Internal (
     , (%)
     ) where
 
-import Control.Lens
-import Data.Ratio   ((%))
+import Control.Lens.Compat
+import Data.Ratio          ((%))
 
 
 -- $setup

src/Vgrep/Widget/Pager.hs

@@ -15,7 +15,7 @@ module Vgrep.Widget.Pager (
     , replaceBufferContents
     ) where
 
-import           Control.Lens         hiding ((:<), (:>))
+import           Control.Lens.Compat  hiding ((:<), (:>))
 import           Data.Foldable
 import           Data.Sequence        (Seq, (><))
 import qualified Data.Sequence        as Seq

src/Vgrep/Widget/Pager/Internal.hs

@@ -12,10 +12,10 @@ module Vgrep.Widget.Pager.Internal (
     , highlighted
     ) where
 
-import Control.Lens
-import Data.Sequence (Seq)
-import Data.Set      (Set)
-import Data.Text     (Text)
+import Control.Lens.Compat
+import Data.Sequence       (Seq)
+import Data.Set            (Set)
+import Data.Text           (Text)
 
 
 -- | Keeps track of the lines of text to display, the current scroll

src/Vgrep/Widget/Results.hs

@@ -27,7 +27,7 @@ module Vgrep.Widget.Results (
     ) where
 
 import           Control.Applicative
-import           Control.Lens
+import           Control.Lens.Compat
 import           Control.Monad.State.Extended
 import           Data.Foldable
 import           Data.Maybe

src/Vgrep/Widget/Results/Internal.hs

@@ -24,7 +24,7 @@ module Vgrep.Widget.Results.Internal (
     ) where
 
 import           Control.Applicative
-import           Control.Lens        (Getter, pre, to, _Just)
+import           Control.Lens.Compat (Getter, pre, to, _Just)
 import           Data.Foldable
 import           Data.Function
 import           Data.List           (groupBy)

test/Test/Case.hs

@@ -19,7 +19,7 @@ module Test.Case (
     , TestTree ()
     ) where
 
-import Control.Lens
+import Control.Lens.Compat
 import Test.QuickCheck.Monadic
 import Test.Tasty
 import Test.Tasty.QuickCheck

test/Test/Vgrep/Widget/Pager.hs

@@ -2,7 +2,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 module Test.Vgrep.Widget.Pager (test) where
 
-import           Control.Lens
+import           Control.Lens.Compat
 import qualified Data.Sequence           as S
 import           Data.Text.Testable      ()
 import qualified Data.Text.Testable      as T

test/Test/Vgrep/Widget/Results.hs

@@ -2,7 +2,7 @@
 {-# LANGUAGE LambdaCase       #-}
 module Test.Vgrep.Widget.Results (test) where
 
-import           Control.Lens            (Getter, over, to, view, views, _1)
+import           Control.Lens.Compat     (Getter, over, to, view, views, _1)
 import           Data.Map.Strict         ((!))
 import qualified Data.Map.Strict         as Map
 import           Data.Monoid             ((<>))

vgrep.cabal

@@ -35,6 +35,7 @@ library
   default-extensions:  LambdaCase
                      , MultiWayIf
   exposed-Modules:     Control.Concurrent.STM.TPQueue
+                     , Control.Lens.Compat
                      , Control.Monad.State.Extended
                      , Pipes.Concurrent.PQueue
                      , Vgrep.App