Implement case*

[jeaye]

Clojure's special case* form is used for constant type matching on literal values. It's wrapped by a case macro which breaks down the inputs into the necessary bits needed for case*. For example:

user=> (require '[clojure.pprint :as p])

user=> (p/with-pprint-dispatch
  p/code-dispatch
  (p/pprint (macroexpand '(case foo 1 "one" 2 "two"))))

;;;
(let*
  [G__170 foo]
  (case*
    G__170
    0
    0
    (throw
      (java.lang.IllegalArgumentException.
        (clojure.core/str "No matching clause: " G__170)))
    {1 [1 "one"], 2 [2 "two"]}
    :compact
    :int))

It has certain optimizations for when all of the values are integers. For example, if we add a string branch to the above:

user=> (p/with-pprint-dispatch
  p/code-dispatch
  (p/pprint (macroexpand '(case foo 1 "one" 2 "two" "three" :hmmm))))

;;;
(let*
  [G__176 foo]
  (case*
    G__176
    1
    3
    (throw
      (java.lang.IllegalArgumentException.
        (clojure.core/str "No matching clause: " G__176)))
    {0 [1 "one"], 1 [2 "two"], 3 ["three" :hmmm]}
    :compact
    :hash-equiv
    nil))

Resources

• Slack discussion on this: https://clojurians.slack.com/archives/C03S1KBA2/p1554335491138400
• case macro: https://github.com/clojure/clojure/blob/58d0225dc33e7e61837c63fd581f63b2f42a3886/src/clj/clojure/core.clj#L6789
• CaseExpr AST node: https://github.com/clojure/clojure/blob/58d0225dc33e7e61837c63fd581f63b2f42a3886/src/jvm/clojure/lang/Compiler.java#L9285
• In general, look into minimal perfect hashing functions

[jianlingzhong]

i can look into this one.