Clojure’s `case` selects a result by comparing one expression with fixed test constants: `(case expr test then ... default)`. Only the selected `then` form is evaluated. If nothing matches, it returns the optional default; without one, it throws `IllegalArgumentException`. Duplicate test constants also cause an exception. section 3.3.4
Tests are compile-time literals, not expressions evaluated to produce test values. For example, `(inc 0)` in a test position does not become `1`; its list elements act as alternative tests. This also makes quoted symbols surprising: use `alpha` rather than `'alpha` as a symbol test. Compared with `cond` and `condp`, `case` is intended for fast lookup among fixed alternatives. section 3.3.4
(defn error [& args]
(println "Unrecognized operator for" args))
(defn operator [op] ; #1
(case op
("+" "plus" "sum") +
("-" "minus" "subtract") -
("*" "x" "times") *
("/" "÷" "divide") /
error))
(defn execute [arg1 op arg2] ; #2
((operator op)
(Integer/valueOf arg1)
(Integer/valueOf arg2)))
(defn calculator [s] ; #3
(let [[arg1 op arg2] (clojure.string/split s #"\s+")]
(execute arg1 op arg2)))
(calculator "10 ÷ 5") ; #4
;; 2
The calculator maps several literal operator names to each function, then uses the selected function to evaluate an input string. section 3.3.4
