travis: Move to stack-based Travis-CI configuration

Move to independent, isolated tests based on stack.
Uses the script `stackalize` in _test to generate
individual projects for each exercise.

Author: rbasso

.travis.yml

@@ -1,50 +1,53 @@
-# The Travis docs (https://docs.travis-ci.com/user/languages/haskell)
-# suggest that we use `language: haskell` and `ghc:` to specify GHC versions.
-# However, as of this writing, Travis only supported versions up to 7.8.
-# If Travis supports all versions we would like to test, then we could switch.
-# Until then, we use the https://github.com/hvr/multi-ghc-travis setup.
-language: c
-sudo: false
+sudo: false                      # Use the container-based infrastructure.
+
 addons:
-  apt: &apt
-    sources:
-      - hvr-ghc
-cabal: &cabal cabal-install-1.22
-env:
-  global:
-    - CABALVER=1.22
-matrix:
-  include:
-    - env: GHCVER=7.6.3
-      addons:
-        apt:
-          <<: *apt
-          packages: [ghc-7.6.3, *cabal]
-    - env: GHCVER=7.8.4
-      addons:
-        apt:
-          <<: *apt
-          packages: [ghc-7.8.4, *cabal]
-    - env: GHCVER=7.10.3
-      addons:
-        apt:
-          <<: *apt
-          packages: [ghc-7.10.3, *cabal]
-    - env: GHCVER=8.0.1
-      addons:
-        apt:
-          <<: *apt
-          packages: [ghc-8.0.1, *cabal]
+  apt:
+    packages:
+      - libgmp-dev               # Stack's GHC depends on this.
+
 cache:
   directories:
-    - '$HOME/.ghc'
-    - '$HOME/.cabal'
+    - $HOME/.stack               # Global stack's cache.
+    - $HOME/.foldercache         # Per exercise `.stack-work` cache.
+
+env:
+ - RESOLVER="lts-2.22"           # Last GHC 7.8.x snapshot.
+ - RESOLVER="lts-6.4"            # Last checked LTS snapshot (GHC 7.10.3).
+ - RESOLVER="nightly-2016-06-21" # Last checked nightly snapshot (GHC 8.0.1).
+ - RESOLVER="lts"                # Latest LTS snapshot.
+ - RESOLVER="nightly"            # Latest nightly snapshot.
+
+matrix:
+  allow_failures:                # The snapshots `lts` and `nightly` are
+    - env: RESOLVER="lts"        # aliases to the most recent versions.
+    - env: RESOLVER="nightly"    # We don't want to fail when a new version
+  fast_finish: true              # is released, so we list them here.
+
 before_install:
-  - './_test/bootstrap.sh'
+ - mkdir -p ${HOME}/bin                          # Create folder for stack.
+ - export PATH="${HOME}/bin:$PATH"               # For stack
+ - export PATH="${TRAVIS_BUILD_DIR}/bin:$PATH"   # For {,fetch-}configlet.
+ - export PATH="${TRAVIS_BUILD_DIR}/_test:$PATH" # For stackalize.
+
 install:
-  - true
+ - travis_retry fetch-configlet
+ - travis_retry curl -L https://www.stackage.org/stack/linux-x86_64 -o pack.tgz
+ - tar xzf pack.tgz --wildcards --strip-components=1 -C ${HOME}/bin '*/stack'
+
 script:
-  - 'export PATH=/opt/cabal/$CABALVER/bin:/opt/ghc/$GHCVER/bin:$PATH'
-  - 'runhaskell -Wall ./_test/check-exercises.hs'
-  - './bin/fetch-configlet'
-  - './bin/configlet .'
+ - |
+    set -e
+    configlet .
+    for exercise in ${TRAVIS_BUILD_DIR}/exercises/* ; do
+        pushd ${exercise}
+        # `stack --word-dir` fails if not targeting a subdirectory, so we just
+        # symbolic-link `.stack-work` to a subfolder in the cache directory.
+        mkdir -p "${HOME}/.foldercache/${exercise}/.stack-work"
+        ln -f -s "${HOME}/.foldercache/${exercise}/.stack-work"
+        stackalize --resolver lts-0.0       `# This resolver is not used.   `
+        stack test --resolver "${RESOLVER}" `# Select the correct resolver. `\
+                   --install-ghc            `# Download GHC if not in cache.`\
+                   --no-terminal            `# Terminal detection is broken.`\
+                   --pedantic               `# Enable -Wall and -Werror.    `
+        popd
+    done