A type hint supplies Java type information where Clojure’s compiler cannot infer it. Its practical use is to resolve Java method calls without reflection. Turn on `*warn-on-reflection*` to find calls that need attention; a value’s runtime type alone does not necessarily tell the compiler its type. section 2.1.1 section 21.7.6
Place a hint where the compiler needs the type:
- Hint a function argument used as a Java method target, as in `(defn get-bytes [^String s] (.getBytes s (StandardCharsets/UTF_8)))`. A `^Mac` hint on the `mac` argument similarly resolves `.doFinal`. section 2.1.1
- Hint the method name in `memfn`, as in `(memfn ^String toLowerCase)`. `memfn` propagates that metadata to the target object of the generated function. section 2.2.7
- Hint a primitive array at the call site, as in `(aset ^ints a 0 9)`, or use the typed setter `(aset-int a 0 9)`. The setter is a convenient alternative when an array lookup has obscured its type. section 21.7.6
Find and remove a reflection warning
With reflection warnings enabled, calls to `.getBytes` on an unhinted `s` and `.doFinal` on an unhinted `mac` report unknown target classes. Adding `^String` and `^Mac` to those parameters removes the warnings. In the illustrated signing benchmark, the elapsed time falls from about 1054 ms to 449 ms for 100,000 requests; those figures are observations from that run, not a timing guarantee. section 2.1.1
(defn get-bytes [^String s] ; #1 (.getBytes s (StandardCharsets/UTF_8))) (defn compute-hmac [^Mac mac canonical] ; #2 (.doFinal mac (get-bytes canonical))) (time (dotimes [i 100000] (sign-request (str "http://example.com/tx/" i)))) ;; "Elapsed time: 449.417098 msecs" ; #3
Shows both parameter hints and the timed signing call after the warnings are addressed. section 2.1.1
Hints are not a substitute for making an operation visible at the right point. An ordinary wrapper around an overloaded Java method can select the generic `Object` overload even when its caller converts arguments to `int`; an inlined wrapper lets that call-site type information reach Java method selection. Use hints or typed operations to resolve a specific interop problem, then check the resulting warnings and behavior. section 4.5 section 21.7.6
