Episode 004: Atomic Curls

Functional Design in Clojure

Christoph tries to make tic-tac-toe work over the Internet and discovers the power of the atom.

• Let's get a web framework, build a UI, hook that up to an HTTP API, throw in some websockets for notifications!
• "We'd end up with our first 12 hour podcast episode."
• A "tracer bullet": get something functioning to see the important parts of the solution.
• Can choose when to replace important parts by something more "production worthy".
• "Let's just ditch all the complexity of having a UI and just use curl."
• curl will print the text response. We have a terminal UI!
• "We're extending the command line out to the web!"
• "I just keep curling every day or every other day until it's my turn."
• "It's your morning curls!"
• How do we handle web requests? What is a route?
• Routes: /new, /show, /play?row=0&col=1
• "Super simple URL API. Who needs REST?"
• Let's run this on port 1337 to hide it from the Internet.
• One shared game state stored in an atom. It's the only game in town!
• Use an atom instead of a database to cut complexity while problem solving.
• The ! ("bang") in swap! and reset! indicates you're causing a side effect.
• Handler's sole job: take web requests and alter the game state using the game model.
• Function called by swap! should be pure. Don't throw exceptions!
• Dilemma: How much do you put inside the function called by "swap!"?
• For errors, the transaction function can:
  • Return unchanged reference. Use "triage" function to diagnose.
  • Have an "error" attribute in the state and set an "outcome" code like :success,:invalid-coordinate, etc.
• Tracer bullet shows lots of ways to complex-ify this into "Tic-Tac-Toe, Enterprise Edition"

Clojure in this episode:

• defonce
• atom
• deref
• swap! and reset!
• aleph and http-kit
• compojure