Episode 012: Embrace the REPL

Functional Design in Clojure

Christoph complicated development by misunderstanding the REPL.

• We go back to the roots of Christoph's experience with Clojure...
• How do I run Clojure code?
• The REPL is fun for evaluation, but how do you run a "real" program?
• Experience in other languages: edit the file, compile, rerun the program
• Mentality: get through the edit, compile, run loop as fast as possible!
• Autobuilder is the logical end.
• "Where's my autobuilder in Clojure?!"
• The REPL model is fundamentally different.
• "[The REPL] is like cutting the Gordian Knot. You change the problem and that's how you solve it."
• "The reason why the problem I wanted solved, wasn't 'solved', is because nobody has that problem because they do things differently here."
• Tactic 1: edit, save, restart the REPL
• "Restarting the REPL isn't super slow, but let's just say it's not instantaneous."
• Tactic 2: edit, save, run (use 'the.namespace :reload) in the REPL
• "Now I have a REPL history of use statements!"
• Problems:
  • forgetting to reload dependent namespaces
  • loading dependencies in the wrong order
  • old definitions built up
• Big Problem: function renames left the old version around, so accidental calls using the old name produced no errors and ran old behavior!
• Back to quitting the REPL to clean out the cruft. Ugh!
• Discovered clojure.tools.namespace! Reloads everything and cleans out the cruft!
• Tactic 3: edit, save, (use '[clojure.tools.namespace.repl :only (refresh)]), (refresh)
• Problem: refresh would purge itself!
• "I don't know why it took me so long to discover the magical user namespace!"
• The REPL will automatically use the user namespace.
• "I can put code into a user namespace and it will be there for me."
• Christoph would switch namespaces in the REPL without even stopping to wonder what namespace he started in.
• "It's like walking out of a door and not even thinking about the fact you're in a building. Oh wait! What room did I just leave?"
• Tactic 4: make sure refresh is in the user namespace, edit, save, (refresh)
• However, Christoph still didn't understand the REPL.
• Just thought the REPL was for:
  • reloading the code and restarting
  • evaluating snippets of code
  • inspecting stuff
• Nate's big ah-ha moment: "Not only is my application inspectable, it's fungible. I can change it in place as it's flying along! I don't have to restart it!"
• Christoph's hint that there was still more came from seeing comment blocks in code and reading about vim-fireplace.
• "There's a new room you can explore. That room is editor integration with the REPL."

Clojure in this episode:

• use
• clojure.tools.namespace.repl/refresh
• clojure.pprint/pprint

Related projects:

• clojure.tools.namespace
• vim-fireplace
• Cider for Emacs