Clojure Tip of the Day – Episode 3: Threading Macros Tracing

The third episode of my Clojure Tip of the Day screencast is out.

You can find the video on YouTube: Clojure Tip of the Day – Episode 3: Threading macros tracing

The episode shows a quick “debugging” technique using the println function to print intermediate values flowing through the threading macros to the standard output.

TL;DR

  • For thread-first macro you can use (doto println) to quickly print intermediate value
  • Usually, it’s better and more convenient to introduce little helper function spy : (def spy #(do (println "DEBUG:" %) %)) which then works for all threading macros
  • If you want to use doto-like method then you need to wrap in an anonymous function: (#(doto % println)) (notice extract parentheses – see macro-expansion of threading macros)

 Credit

Thanks to Sean Corfield and Brandom Adams for providing the tips on the Clojurians slack channel.

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