Getting Clojure

An Eval of Your Own

The wonderful thing about eval is that it’s simultaneously a gateway to the entire Clojure programming language and a very ordinary function. That eval is just an ordinary function raises an interesting question: can we—as an intellectual exercise—implement our own version of eval?

Remarkably, we can. We’ve already seen that if you hand eval a string or a keyword or a number, you get the same string, keyword, or number back, unchanged. So here’s a start on our own toy eval function:

(defn reval [expr]
  (cond
    (string? expr) expr
    (keyword? expr) expr
    (number? expr) expr
    :else :completely-confused))

Note that the real eval throws an exception when you hand it something it doesn’t understand, but to keep the example ...