Problem

There are Clojure dialects (Clojure, ClojureScript, ClojureCLR) hosted on several different platforms. We wish to write libraries that can share as much portable code as possible while leaving flexibility to provide platform-specific bits as needed, then have this code run on all of them.

Use cases for platform-specific functionality:

Proposal: Feature expressions

Common Lisp approaches this problem using feature expressions.

Each platform has a variable called *features* that is a set of keywords to indicate supported features. 

The Reader understands a new kind of "feature expression". The reader macros #+ and #- are used to include or skip a form based on a feature expression:

Feature expressions evaluate as booleans and are defined as follows:

Skipping in the reader is performed by binding *read-suppressed* to true for the next form.

Example:

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

(defn my-trim [s]
  #+clj (.. s toString trim)
  #+cljs (gstring/trim s))

(my-trim " Hello CL? ")

Features

The platform feature will be one of: clj, cljs, or clr.  Users may supply their own features, which should, by convention, use namespaces. Platform-specified features will not be namespaced.

In Clojure, the initial feature set may be specified at start time with the system property clojure.features, which is a comma-delimited list of symbols to add as features. The platform feature will always be added, regardless of whether it is set in the system property. 

For example:

-Dclojure.features=arch/osx,my.app/prod,my.app/strictmath

will yield the runtime feature set: #{:clj :arch/osx :my.app/prod :my.app/strictmath}

In ClojureScript, there is a new build option with key :features that takes a set of keywords defining the features. The platform feature :cljs is always added to this set, as in Clojure.

In addition to setting the features initially, users may bind *features* around explicit calls to the reader.

Reading Unreadable Things

There may be tagged literals or classes that are not known or available on all platforms. The reader must be able to read but avoid constructing these entities.

Example:

(def init #+cljs #js {} 
          #-cljs nil)

The #js tagged literal is not known on the Clojure platform, but the reader should read and skip it without failure.

CLR extended symbol problem: Clojure CLR uses an expanded set of valid symbols in the reader. The #| reader extension from Common Lisp was implement in ClojureCLR to delimit symbols containing otherwise invalid characters. The corresponding symbols are not currently readable by Clojure or ClojureScript.  The #| extension support is described here: https://github.com/clojure/clojure-clr/wiki/Specifying-types. The #| reader extension would need to be supported in Clojure and ClojureScript readers to support the full set of valid ClojureCLR symbols in feature expressions.

Loading and File Extensions

When code needs to be loaded based on a namespace:

CLJS source extension problem: ClojureScript currently expects source files to end (only) in .cljs. There are a number of places in the code where this assumption is made:
 
CLJS accidental macro compilation problem: when we collect all the ClojureScript code to compile, we will (accidentally) include .clj macro files that should only be read for macro expansion.

We use a single classpath (often merged from several directories within a library) to load several kinds of ClojureScript resources:
Because mixed and macro files have the same extension, we need a way to indicate that macro files should not be compiled as ClojureScript. Possible solutions:
  1. Modify the way ClojureScript loads code to specify separate paths for ClojureScript code and macro code  NO
    1. Likely breaks many existing projects which mingle these into the same source path.
    2. Also breaks library publishing as a single jar.
  2. Add build option to specify names to skip during compilation (the macro namespaces).   NO
    1. require-macros would ignore this directive so require-macros would still load the ns'es
    2. Existing CLJS projects would still work if the code in the macro files happened to be compile-able by ClojureScript
    3. This capability would also make it easier to mingle Clojure and ClojureScript projects in the same project in a single source tree (with capability to ignore certain files for CLJS for reasons other than avoiding compilation of macro files)
    4. Main problem is that if published as a library, there is no way to get this ns exclusion list to users of the published library. Would have to also invent some way to specify that list of namespaces in jar metadata etc.
  3. Add marker in macro namespaces to identify them as being skipped for compilation  INVESTIGATING
    1. Add namespace meta or other indicator that this .clj file is only included for macros not for compilation - use existing parse-ns for this
    2. Could automatically be picked up by downstream users of code published as a library

Open Extension

One consequence of feature expressions is that a library must encode solutions for all of the supported platforms in the source code at packaging time (in other words, extension is closed to external users).
 
An alternate solution to specifying all variants in a single file with feature expressions is to swap whole namespaces in and out for different platforms. This allows others to provide implementations for different platforms (if, for example, the library author lacked the expertise to do so). This strategy is orthogonal to feature expressions but made possible by the extension preference checking as specified in the previous section.

Example:

Library files:
 src/my/core.clj - ns requires my.thing
 src/my/thing.clj - Clojure implementation of my.thing (loaded by Clojure)
 src/my/thing.cljs - ClojureScript implementation of my.thing (loaded by ClojureScript)

ClojureScript Tooling Support

ClojureScript tooling needs to be aware of the change in supported file extension names and possibly new build options. Need to ensure tools are able to support the new mixed-language projects well:

 

FAQ

  1. What about non-boolean expressions for things like Clojure version, JDK version, etc?  Out of scope. The "compile-if" trick covers many of those (relatively rare) cases already.

Patches

JIRA Tickets and patches:

Alternate Approaches

Copy / paste

One approach is to maintain two versions of the same file that are largely the same but modify the platform-specific parts in each copy. This obviously works but is gross.

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:

cljx expressions are typically applied:

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.

Tagged Literal

Define a custom tagged literal that implements conditional read-time expressions:

#feature/condf [ (and jdk-1.6+ clj-1.5.*) 
 (call-my-fast-reducer-code) 
 else (some-old-fashioned-code) ]

Proof of concept here:  https://github.com/miner/wilkins

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: