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