clojure.core/str version that shows how it works

mystr.clj

(defn mystr
  "With no args, returns the empty string. With one arg x, returns
  x.toString().  (str nil) returns the empty string. With more than
  one arg, returns the concatenation of the str values of the args."
  {:tag String}
  ([] (println "0:")
     "")
  ([#^Object x] (println "1: x:" x)
     (if (nil? x) "" (. x (toString))))
  ([x & ys] (println "2: x: " x ", ys: " ys)
     ((fn [#^StringBuilder sb more]
	(if more
	  (do (println "2i: sb: " sb ", more: " more)
	      (recur (. sb (append (mystr (first more)))) (next more)))
	  (do (println "2i: sb: " sb)
	      (mystr sb))))
      (new StringBuilder #^String (mystr x)) ys)))

out0.clj

user> (mystr)
0:
""

out1.clj

user> (mystr \a)
1: x: a
"a"

out2.clj

user> (mystr \a \b)
2: x:  a , ys:  (b)
1: x: a
2i: sb:  #<StringBuilder a> , more:  (b)
1: x: b
2i: sb:  #<StringBuilder ab>
1: x: #<StringBuilder ab>
"ab"

out3.clj

user> (mystr \a \b \c)
2: x:  a , ys:  (b c)
1: x: a
2i: sb:  #<StringBuilder a> , more:  (b c)
1: x: b
2i: sb:  #<StringBuilder ab> , more:  (c)
1: x: c
2i: sb:  #<StringBuilder abc>
1: x: #<StringBuilder abc>
"abc"