Question

I’m looking for the ability to have the REPL print the current definition of a function. Is there any way to do this?

For example, given:

(defn foo [] (if true "true"))

I’d like to say something like

(print-definition foo)

and get something along the lines of

(foo [] (if true "true"))

printed.

Answer 1

An alternative to source (which should be available via clojure.repl/source when starting a REPL, as of 1.2.0. If you’re working with 1.1.0 or lower, source is in clojure.contrib.repl-utils.), for REPL use, instead of looking at functions defined in a .clj file:

(defmacro defsource
  "Similar to clojure.core/defn, but saves the function's definition in the var's
   :source meta-data."
  {:arglists (:arglists (meta (var defn)))}
  [fn-name & defn-stuff]
  `(do (defn ~fn-name ~@defn-stuff)
       (alter-meta! (var ~fn-name) assoc :source (quote ~&form))
       (var ~fn-name)))

(defsource foo [a b] (+ a b))

(:source (meta #'foo))
;; => (defsource foo [a b] (+ a b))

A simple print-definition:

(defn print-definition [v]
  (:source (meta v)))

(print-definition #'foo)

#' is just a reader macro, expanding from #'foo to (var foo):

(macroexpand '#'reduce)
;; => (var reduce)