platypope.org

I lost the original version of this post. C’est la vie. The gist is that Clojure’s standard case macro somewhat surprisingly (to me) does not support arbitrary expressions as test conditions – only read-time constants. It does this because it’s primary use from the Clojure-implementation perspective is to support fast dispatch on symbol/keyword values.

Other people have blogged about this and provided their own less-surprising implementations, include cemerick.

I didn’t like any of the ones I found elsewhere, so here’s mine:

(defmacro case-expr
  "Like case, but only supports individual test expressions, which are
evaluated at macro-expansion time."
  [e & clauses]
  `(case ~e
     ~@(concat
        (mapcat (fn [[test result]]
                  [(eval `(let [test# ~test] test#)) result])
                (partition 2 clauses))
        (when (odd? (count clauses))
          (list (last clauses))))))

Enjoy!