Core: derive 'MonadFail X' instance for GHC 8.6.x and beyond

A side effect of that change is that our code no longer compiles with GHC
versions prior to 8.0.x. We could work around that, no doubt, but the resulting
code would require CPP and Cabal flags and whatnot. It feels more reasonable to
just require a moderately recent compiler instead of going through all that
trouble.

.travis.yml

@@ -2,7 +2,7 @@
 #
 #   runghc make_travis_yml_2.hs '-o' '.travis.yml' 'xmonad.cabal' 'libxrandr-dev'
 #
-# For more information, see https://github.com/hvr/multi-ghc-travis
+# For more information, see https://github.com/haskell-CI/haskell-ci
 #
 language: c
 sudo: false
@@ -28,6 +28,9 @@ before_cache:
 
 matrix:
   include:
+    - compiler: "ghc-8.6.1"
+      env: GHCHEAD=true
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-8.6.1,libxrandr-dev], sources: [hvr-ghc]}}
     - compiler: "ghc-8.4.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.4.3,libxrandr-dev], sources: [hvr-ghc]}}
@@ -37,15 +40,9 @@ matrix:
     - compiler: "ghc-8.0.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.0.2,libxrandr-dev], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.10.3"
-    # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.10.3,libxrandr-dev], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.8.4"
-    # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.8.4,libxrandr-dev], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.6.3"
-    # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.6.3,libxrandr-dev], sources: [hvr-ghc]}}
+
+  allow_failures:
+    - compiler: "ghc-8.6.1"
 
 before_install:
   - HC=${CC}
@@ -69,6 +66,24 @@ install:
   - travis_retry cabal update -v
   - "sed -i.bak 's/^jobs:/-- jobs:/' ${HOME}/.cabal/config"
   - rm -fv cabal.project cabal.project.local
+  # Overlay Hackage Package Index for GHC HEAD: https://github.com/hvr/head.hackage
+  - |
+    if $GHCHEAD; then
+      sed -i 's/-- allow-newer: .*/allow-newer: *:base/' ${HOME}/.cabal/config
+      for pkg in $($HCPKG list --simple-output); do pkg=$(echo $pkg | sed 's/-[^-]*$//'); sed -i "s/allow-newer: /allow-newer: *:$pkg, /" ${HOME}/.cabal/config; done
+
+      echo 'repository head.hackage'                                                        >> ${HOME}/.cabal/config
+      echo '   url: http://head.hackage.haskell.org/'                                       >> ${HOME}/.cabal/config
+      echo '   secure: True'                                                                >> ${HOME}/.cabal/config
+      echo '   root-keys: 07c59cb65787dedfaef5bd5f987ceb5f7e5ebf88b904bbd4c5cbdeb2ff71b740' >> ${HOME}/.cabal/config
+      echo '              2e8555dde16ebd8df076f1a8ef13b8f14c66bad8eafefd7d9e37d0ed711821fb' >> ${HOME}/.cabal/config
+      echo '              8f79fd2389ab2967354407ec852cbe73f2e8635793ac446d09461ffb99527f6e' >> ${HOME}/.cabal/config
+      echo '   key-threshold: 3'                                                            >> ${HOME}/.cabal.config
+
+      grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
+
+      cabal new-update head.hackage -v
+    fi
   - grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
   - "printf 'packages: \".\"\\n' > cabal.project"
   - touch cabal.project.local

CHANGES.md

@@ -11,6 +11,15 @@
     and to install them into proper locations where their users expect them.
     [https://github.com/xmonad/xmonad/pull/127]
 
+### Bug Fixes
+
+  * Add support for GHC 8.6.x by providing an instance for 'MonadFail X'. A
+    side effect of that change is that our code no longer compiles with GHC
+    versions prior to 8.0.x. We could work around that, no doubt, but the
+    resulting code would require CPP and Cabal flags and whatnot. It feels more
+    reasonable to just require a moderately recent compiler instead of going
+    through all that trouble.
+
 ## 0.14 (July 30, 2018)
 
 ### Bug Fixes

src/XMonad/Core.hs

@@ -37,6 +37,7 @@ import Prelude
 import Control.Exception.Extensible (fromException, try, bracket, throw, finally, SomeException(..))
 import qualified Control.Exception.Extensible as E
 import Control.Applicative(Applicative, pure, (<$>), (<*>))
+import Control.Monad.Fail
 import Control.Monad.State
 import Control.Monad.Reader
 import Data.Semigroup
@@ -147,7 +148,7 @@ data ScreenDetail   = SD { screenRect :: !Rectangle } deriving (Eq,Show, Read)
 -- instantiated on 'XConf' and 'XState' automatically.
 --
 newtype X a = X (ReaderT XConf (StateT XState IO) a)
-    deriving (Functor, Monad, MonadIO, MonadState XState, MonadReader XConf, Typeable)
+    deriving (Functor, Monad, MonadFail, MonadIO, MonadState XState, MonadReader XConf, Typeable)
 
 instance Applicative X where
   pure = return

xmonad.cabal

@@ -28,13 +28,7 @@ cabal-version:      >= 1.8
 bug-reports:        https://github.com/xmonad/xmonad/issues
 build-type:         Simple
 
-tested-with:
-                    GHC==7.6.3,
-                    GHC==7.8.4,
-                    GHC==7.10.3,
-                    GHC==8.0.2,
-                    GHC==8.2.2,
-                    GHC==8.4.3
+tested-with:        GHC==8.0.2, GHC==8.2.2, GHC==8.4.3, GHC==8.6.1
 
 source-repository head
   type:     git
@@ -62,7 +56,7 @@ library
                         XMonad.StackSet
     other-modules:      Paths_xmonad
 
-    build-depends: base >= 4.6 && < 4.12,
+    build-depends: base >= 4.6 && < 5,
                    containers,
                    data-default,
                    directory,
@@ -86,6 +80,8 @@ library
         -- needed for XMonad.Config's instance Default (XConfig a)
     if flag(testing)
         buildable: False
+    if !impl(ghc > 8)
+        build-depends: fail
 
 executable xmonad
     main-is:        Main.hs