Refactoring with Free Monad

Paul Meng

Making a Slackbot

@iris: :help

iris: - help
      - load_metric
@iris: load_metrics

iris: loading now...
iris: 12345

Message Dispatch

dispatch :: ChannelId -> T.Text -> SlackAction ()
dispatch cid msg =
    case T.parse irisCmd "" msg of
      Left err -> sendMessage cid (T.pack "uncomprehensible command")
      Right cmd -> liftIO $ run cid cmd

Data type

data IrisCommand next = Help
                      | LoadMetric
                      deriving (Eq, Show)

Interpret in IO Monad

run :: ChannelId -> IrisCommand -> SlackAction ()
run cid (Help) = sendMessage cid (T.pack ":help")
run cid (LoadMetric) = do
  sendMessage cid (T.pack "loading now...")
  l <- liftIO getDailyMetric
  let resp = T.pack $ show l
  sendMessage cid resp

How to Test This?

  • It relies on sendMessage, which results to a call to Slack API.
  • In Ruby, you can use Rspec to write expect a method call.
  • It also connects to a database to fetch data, you have to setup a database and mock data.

How to Test This?

  • With Haskell it is hard to stub the message passing and its response without a runtime like Ruby.
  • Setting up a testing database works as most imperaitve languages do, the only drawback is slowdown.
  • It fallback to IO Monad. Is there an alternative before we use the last resort?

  • How about.. test it semantically? With the structure of AST, we know how the thing would happen.

Thinking in Javascript

  • We are loading an array of String, and decoding them into Function
  • It is able to be mapped over, so it's a Functor
var commands_in_str = ["ReplyLoading", "LoadMetrics"];

//after parsing

replyLoading = function() {
    console.log("in replayLoading");

loadMetrics = function() {
    console.log("in loadMetrics");

// functor
var commands = [replyLoading, loadMetrics]; {; });

//cheating by using window { window[fname].call(this); });

Thinking in Javascript

  • With a Functor, there exists an implicit Structure for Free
  • We have an implicit sequence compose the evaluation order.
// monad for free, visitor pattern
function node(type, next) {
    this.type = type; = next ? next : null;

node.prototype.accept = function(visitorObj) {

    if (;

Thinking in Javascript

  • Functor as Abstract Syntax Tree, and that stucture is actually a Monad. It's visitor pattern in OO term.
function visitor() {
    var that = this;
    this.visit = function(tgt) {
        if (tgt.type == "replyLoading") {

        if (tgt.type == "loadMetrics") {

    this.walk = function(tgt) {

var head = new node("replyLoading", (new node("loadMetrics")));
(new visitor()).walk(head);

Data type in Haskell

data IrisCommand next = Help'
                      | LoadMetric'
                      | ReplyLoading' next
                      deriving (Functor, Eq, Show)

type IrisCommandM = Free IrisCommand

makeFree ''IrisCommand

Data type in Haskell

  • It looks a lot like a list strcuture on data type level
data [a] = [] | a : [a]
data Free f a = Pure a | Roll (f (Free f a))

Interpret in IO Monad

runIris :: ChannelId -> IrisCommandM () -> SlackAction ()
runIris cid = iterM run
    run :: IrisCommand (SlackAction ()) -> SlackAction ()
    run (Help') = sendMessage cid (T.pack ":help")
    run (LoadMetric')= do
      l <- liftIO getDailyMetric
      let resp = T.pack $ show l
      sendMessage cid resp
    run (ReplyLoading' n) = do
      sendMessage cid (T.pack "loading now...")

Interpret in Test

type FakeResponse = [T.Text]

runIrisTest :: MonadState FakeResponse m => IrisCommandM () -> m ()
runIrisTest = iterM run
    run :: MonadState FakeResponse m => IrisCommand (m ()) -> m ()
    run (Help') = modify (λl -> T.pack ":help" : l)
    run (LoadMetric') = modify (λl -> T.pack "123456" : l)
    run (ReplyLoading' n) = do
      modify (λl -> T.pack "loading now..." : l)

Run with Hspec

spec :: Spec
spec = do
    describe "dispatch" $ do•
      context "receive help" $ do
        it "return help result" $ do
          (runState (I.runIrisTest') []) `shouldBe` ((), [":help"])

      context "receive load_metric" $ do
        it "return load_metric result" $ do
          (runState (I.runIrisTest (I.replyLoading' >> I.loadMetric')) []) `shouldBe` ((), ["123456", "loading now..."])

How to Test This?

  • Without resorting to external database Using local state to simulate the state and response.
  • Test it semantically, on the command level, but not the side-effect comes with the commands.
  • Suitable for small amount of API (get, set)

Thank you