Implement stack resolver override for Haskell.

Closes PaulDance#13.

This patch also bundled some other changes:

- `-Wall` to `--ghc-options -Wall` as this is supposed to be
pased to GHC (stack can't recognize this flag)
- Removed `--trace` as it tends to be verbose and messes up hspec's output.

README.md

@@ -96,6 +96,12 @@ The usual workflow is as such:
  * When done for the day, you may run `xr clean` in order to remove all
    downloaded exercise submissions.
 
+## Track-specific settings
+
+### Haskell
+
+You can optionally override student's resolver by setting `XR_HASKELL_STACK_RESOLVER` to a valid resolver version.
+(e.g. `XR_HASKELL_STACK_RESOLVER=lts-16.21 xr t`)
 
 ## Contributing
 

tracks/haskell.sh

@@ -1,10 +1,33 @@
+function _override_stack_resolver() {
+    if [[ -z "$XR_HASKELL_STACK_RESOLVER" ]]; then
+        return
+    fi
+
+    # See https://docs.haskellstack.org/en/stable/pantry/#snapshot-location for resolver syntax.
+    # For the purpose of running exercises it's resonable just to allow lts and nightly.
+    if ! [[ $XR_HASKELL_STACK_RESOLVER =~ '^(lts-[0-9\.]+|nightly-[0-9-]+)$' ]]; then
+        echo Skipped resolver override as $XR_HASKELL_STACK_RESOLVER is not allowed.
+        return
+    fi
+
+    if ! stack --resolver="$XR_HASKELL_STACK_RESOLVER" eval '()' >/dev/null 2>/dev/null; then
+        echo Skipped resolver override as stack cannot recognize $XR_HASKELL_STACK_RESOLVER.
+        return
+    fi
+
+    echo Overriding resolver to $XR_HASKELL_STACK_RESOLVER.
+    sed -i 's/resolver: .*/resolver: '"$XR_HASKELL_STACK_RESOLVER"'/' stack.yaml
+}
+
 # Runs all the available tests.
 function _run_tests() {
-    stack test -Wall --trace \
+    _override_stack_resolver
+    stack test --ghc-options -Wall \
         && hlint .
 }
 
 # Just runs any available benchmark, importing still needs to be added.
 function _run_benches() {
-    stack bench -Wall --no-run-tests
+    _override_stack_resolver
+    stack bench --ghc-options -Wall --no-run-tests
 }