Episode 008: Twitter, Plated

Functional Design in Clojure

Christoph tries to figure out why Twitter stopped talking about Clojure.

• "Are you twitterpated?"
• Building on where we left off last episode.
• Runs and just stops working.
• "I was pretty sure it stopped working because people on Twitter just stopped talking about Clojure. After about a day of that, I realized people were talking about Clojure, and I just wasn't seeing it."
• The auth token expired! What should we do?
• Why should the main loop have to deal with getting a new auth token?
• "The Twitter wrapper should be concerned with all of the warts and complexities of dealing with Twitter."
• "What problems should bubble up, and which ones shouldn't?"
• The wrapper should handle the retry.
• It's like a kitchen in a restaurant. What are the steps of fulfilling an order? The customer doesn't care.
• "There's a side-effect: the freezer mutates."
• The wrapper gets to worry about all the steps:
  • turning the order into the specific request for the kitchen
  • do the I/O to fetch and fulfill the request
  • the "input transform" takes the mass of data and picks out the relevant parts
  • the "internal" version is returned
• "Like all good metaphors, they stretch to the point where they break, like a rubber band."
• Maybe avoid expired tokens by authenticating every time? Too much overhead.
• If the handle is mutable, then retry can just update the handle with the new token.
• A mutable handle does allow the wrapper to control the concern.
• The "handle" is the state of the wrapper. The term "handle" comes from I/O libraries.
• Instead of mutation, have the search function return [updated-handle, result].
• search can catch an auth exception, retry, and return a new auth handle.
• Instead of search retrying, the fetcher can do it! Then it works for all kinds of requests.
• Better yet, leave fetch simple, and have a fetch-with-retry function that uses fetch.
• Can have even more policy functions like, fetch-with-retry-forever.
• "Keep calm, and assoc on."
• "I'm never going to miss another Clojure tweet. I'm going to read them all!"

Clojure in this episode:

• loop, recur
• try, catch
• atom
• assoc