Understanding type hints for char primitives in clojure.

char-reflection.clj

user=> (set! *warn-on-reflection* true)
true
user=> (defn is-letter? [c] (Character/isLetter c))
Reflection warning, NO_SOURCE_PATH:1:22 - call to isLetter can't be resolved.
#'user/is-letter?

user=> (defn is-letter? [^char c] (Character/isLetter c))
user=> CompilerException java.lang.IllegalArgumentException: Only long and double primitives are supported, compiling:(NO_SOURCE_PATH:1:1) 

;; Ugh...no.
user=> (defn is-letter? [c] (let [^char c c] (Character/isLetter c)))
#'user/is-letter?

;; Note:
user=> (is-letter? "b")
ClassCastException java.lang.String cannot be cast to java.lang.Character  user/is-letter? (NO_SOURCE_FILE:1)

;; More concise (probably "idiomatic")
user=> (defn is-letter? [c] (Character/isLetter (char c)))
#'user/is-letter?

;; Same classcast exception (as expected)
user=> (is-letter? "b")
ClassCastException java.lang.String cannot be cast to java.lang.Number  clojure.lang.RT.charCast (RT.java:927)

;; You can avoid the use of (char) coercion, but this defeats the purpose
;; of the type hint, if you are processing a String seq.
user=> (defn is-letter? [^Character c] (Character/isLetter c))
#'user/is-letter?

;; Also, same (expected) classcast exception
user=> (is-letter? "b")
ClassCastException java.lang.String cannot be cast to java.lang.Character  user/is-letter? (NO_SOURCE_FILE:1)

Best solution is (Character/isLetter ^char c)

See:
https://twitter.com/puredanger/status/353924017662595075
https://gist.github.com/noahlz/5944853