Problem

Writing programs that target Clojure and ClojureScript involves a lot of copy and pasting. The usual approach is to copy the whole code of one implementation to a source file of the other implementation and to modify the platform dependent forms until they work on the other platform. Depending on the kind of program the platform specific code is often a fraction of the code that works on both platforms. A change to platform independent code requires a modification of two source files that have to be kept in sync. To solve this problem branching by target platform on a form level would help a lot.

See also ticket CLJS-27.

Current Solutions

cljx

cljx is an implementation of feature expressions that:

It has been used successfully by a number of projects (see the cljx README for a partial list).  cljx's limitations include:

lein-cljsbuild "crossovers"

lein-cljsbuild provides a (deprecated, to be removed) feature called "crossovers" that provides a very limited preprocessing of certain files during the cljsbuild build process; a special comment string is removed, allowing one to work around the -macros declarations required in ClojureScript ns forms.  Crossover files must otherwise be fully portable.  Language/runtime-specific code must be maintained in separate files.  However, (my) experience shows that this can quickly lead to the situation where one has to think a lot about in which file to put a specific function, in order to go though the whole preprocessing machinery. Functions are split into namespaces because of conditional compilation, and not because they belong to the same part or module of the program.

Potential Solution

A solution to this problem that is used by Common Lisp implementations are feature expressions. Each platform has a variable called *features* that contains keywords that indicate the supported features of the platform the code is running under. The branching on a platform or a platform specific feature is done via the reader macros #+ and #- followed by a feature condition. The feature condition is either a symbol or a form that combines symbols with the or, and or not operators. The feature condition is evaluated by looking up the symbols in the *features* variable. If the feature condition evaluates to true the next form will be passed through the reader and evaluated, otherwise it will be discarded.

The patches attached to the CLJS-27 ticket contain a proof of concept implementation of these feature expressions for Clojure and ClojureScript. With this extension one can branch on a form level and write code like illustrated by the following example:

(ns feature.expressions
  #+clojurescript (:require [goog.string :as gstring]))

(defn my-trim [s]
  #+clojure (.. s toString trim)
  #+clojurescript (gstring/trim s))

(my-trim " Hello CL? ")

The patches add a dynamic variable called *features* to the clojure.core and cljs.core namespaces, that should contain the supported features of the platform in question as keywords. Unlike in Common Lisp, the variable is a Clojure set and not a list. In Clojure the set contains at the moment the :clojure keyword, and in ClojureScript the :clojurescript keyword.

I would like to get feedback on the following issues: 

To run the ClojureScript tests, drop a JAR named "clojure.jar" that has the Clojure patch applied into ClojureScript's lib directory.

References

The Common Lisp Hyperspec about the Sharp Sign macros:

Examples of Common Lisp's Feature Expresions:

Maintaining Portable Lisp Programs:

Crossover files in lein-cljsbuild:

ClojureScript JIRA Ticket and patches with a proof of concept implementation of CL's feature expressions: