Quick demo project of how to use leiningen profiles to provide alternative implementations of a namespace... The project.clj defines two profiles :dev & :prod with alternative source paths for each. The main code should remain in the main /src folder and the namespace to be altered must be provided in both /src.dev and /src.prod

1-project.clj

(defproject profiletest "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url "http://example.com/FIXME"
  :license {:name "Eclipse Public License"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.5.1"]]
  :main profiletest.core
  :profiles {
        :dev {:source-paths ["src" "src.dev"]}
        :prod {:source-paths ["src" "src.prod"]}
  })

2-core.clj

;; the main/shared ns pulling in the correct version of profiletest.foo,
;; based on current lein profile settings
(ns profiletest.core
  (:require [profiletest.foo :as foo]))

(defn -main [] (println "active profile " (foo/blah)))

3-dev-foo.clj

;; src.dev/profiletest/foo.clj
;; implementation of profiletest.foo for :dev profile
(ns profiletest.foo)

(defn blah [] :dev)

4-prod-foo.clj

;; src.prod/profiletest/foo.clj
;; implementation of profiletest.foo for :prod profile
(ns profiletest.foo)

(defn blah [] :prod)

5-usage

$ lein with-profile dev run
Performing task 'run' with profile(s): 'dev'
active profile :dev

$ lein with-profile prod run
Performing task 'run' with profile(s): 'prod'
active profile :prod