Add 'cabal: true' option to enable caching

  * Support both cabal and stack

  * Copy `hashFiles` until exposed exposed in the actions/cache API

  * Saving cache as a post-script

  * Additional action inputs: `cache-paths` and `cache-keys`

  * Add `output.stack-root` in `action.yml`

setup/action.yml

@@ -23,6 +23,17 @@ inputs:
   stack-setup-ghc:
     required: false
     description: 'If specified, enable-stack must be set. Will run stack setup to install the specified GHC'
+  cache:
+    required: false
+    description: 'If specified, automatically caches cabal/stack-related directories.'
+  cache-keys:
+    required: false
+    description: 'List of caching keys, where the first is the "primary" key and the rest are "restore" keys to fall back on.'
+    default: 'The default primary key is `$os-$ghc$hash(cabal.project.freeze|stack.yaml.lock)-$github-sha`, and the restores keys are its prefixes.'
+  cache-paths:
+    required: false
+    description: 'List of directories to cache.'
+    default: 'For cabal: `outputs.cabal-store` and `dist-newstyle`. For stack: `outputs.stack-root`.'
 outputs:
   ghc-path:
     description: 'The path of the ghc executable _directory_'
@@ -32,6 +43,8 @@ outputs:
     description: 'The path of the stack executable _directory_'
   cabal-store:
     description: 'The path to the cabal store'
+  stack-root:
+    description: 'The path to the stack root'
   ghc-exe:
     description: 'The path of the ghc _executable_'
   cabal-exe:
@@ -41,3 +54,5 @@ outputs:
 runs:
   using: 'node12'
   main: 'dist/index.js'
+  post: 'dist/save-cache.js'
+  post-if: success()