The problem here is that case
's test constants, as described in the docs, " must be compile-time literals". So, rather than resolving java.util.concurrent.TimeUnit/MILLISECONDS
, the literal symbol 'java.util.concurrent.TimeUnit/MILLISECONDS
is being tested against.
(foo java.util.concurrent.TimeUnit/MILLISECONDS) ; IllegalArgumentException
(foo 'java.util.concurrent.TimeUnit/MILLISECONDS) ; yes!
Instead, the solution is to dispatch on the .ordinal
of the Enum
instance, which is what Java itself does when compiling switch
statements over enums:
(defn foo [x]
(case (.ordinal x)
2 "yes!"))
You can wrap this pattern in a macro which correctly evaluates the case ordinals for you:
(defmacro case-enum
"Like `case`, but explicitly dispatch on Java enum ordinals."
[e & clauses]
(letfn [(enum-ordinal [e] `(let [^Enum e# ~e] (.ordinal e#)))]
`(case ~(enum-ordinal e)
~@(concat
(mapcat (fn [[test result]]
[(eval (enum-ordinal test)) result])
(partition 2 clauses))
(when (odd? (count clauses))
(list (last clauses)))))))