Chas Emerick @cemerick ClojureWest March 25, 2014 Targeting Clojure & ClojureScript Targeting Clojure & ClojureScript from a Single Codebase from a Single Codebase 

Writing portable Clojure[Script]: Writing portable Clojure[Script]: Why? Why? ● Validation ● JavaScript environments are useful for general-purpose computation – node.js – Browsers: OpenGL, IndexedDB, WebRTC, typed arrays, web workers, etc… ● JavaScript environments can go where the JVM can't 

A Brief History of ClojureScript A Brief History of ClojureScript ● First commit 3+ years after Clojure became public ● Implemented assuming protocols – Simpler, easier set of base abstractions ● Portability not an objective, so lots of oh-so-close naming differences – cljs.core vs. clojure.lang – INamed vs. Named – c.l.IPersistentCollection → #{ICollection ICounted IEmptyableCollection} 

ClojureScript's material differences ClojureScript's material differences ● Hybrid compilation model – Macros written and evaluated in Clojure – :require-macros, :use-macros ● No runtime namespaces ● JavaScript host – “missing” primitives – Different, less capable execution model – All the wat you could want 

Writing portable Clojure[Script]: Writing portable Clojure[Script]: Options & Problems Options & Problems ● Don't – Maintain separate codebases ● Carefully keep portable and non-portable code separate – Single non-portable call/type/name wags the namespace dog – Lowest common denominator – lein-cljsbuild “crossovers” ● Write code once, translate it for each compilation target 

Preprocessors in 60 seconds Preprocessors in 60 seconds Preprocessor Source files Compilation environment Env-suitable sources Compiler 

#ifdef SHARC millis = sharc_dtime(); #else if defined(VxWorks) && defined(X86) millis = read_386_counter(); #else if defined(VxWorks) millis = read_moto_counter(); #else if defined(WIN_32) millis = read_windows_timer(); #endif 30 seconds to go… 30 seconds to go… 

Feature expressions in LISP Feature expressions in LISP (cons #+Lispm "x" #+Spice "y" #-(or Lispm Spice) "z" x) When `Lispm` is defined: (CONS "x" X) When `Spice` is defined: (CONS "y" X) When neither are defined: (CONS "z" X) 

cljx: feature expressions for cljx: feature expressions for Clojure[Script] Clojure[Script] ● Operates entirely outside of (before) the reading, macroexpansion, and compilation of Clojure[Script] ● Transformation rules applied to a lossless representation of Clojure[Script] source code (Christophe Grand's sjacket) ● Downstream dependencies only ever see the generated Clojure or ClojureScript code; cljx doesn't leak out of your project ● Available via Leiningen and in the (n)REPL 

cljx .cljx sources project.clj config, nREPL session type ClojureScript sources ClojureScript Compiler Clojure sources Clojure Compiler 

cljx example cljx example (ns cemerick.pprng #+cljs (:require math.seedrandom [cljs.core :as lang]) #+clj (:require [clojure.core :as lang]) #+clj (:import java.util.Random) (:refer-clojure :exclude (double float int long boolean))) 

Transformation targeting Clojure Transformation targeting Clojure (ns cemerick.pprng (:require math.seedrandom [cljs.core :as lang]) (:refer-clojure :exclude (double float int long boolean))) 

Transformation targeting Transformation targeting ClojureScript ClojureScript (ns cemerick.pprng (:require [clojure.core :as lang]) (:import java.util.Random) (:refer-clojure :exclude (double float int long boolean))) 

(n)REPL support (n)REPL support ● Problem: want to be able to load/eval cljx code in a REPL session ● Problem': want to be able to load cljx files — if they exist — that correspond to namespaces interactively loaded via ns, require, load-namespace, etc. ● Solution: cljx nREPL middleware hooks clojure.core/load + ClojureScript's – This is safer than it sounds 

cljx in the wild: notable projects cljx in the wild: notable projects schema formative pprng sablano double-check garden hickory validateur frak cellular enoki inflections-clj pathetic 

cljx challenges and opportunities cljx challenges and opportunities ● Macros ● Line noise ● Feature expression flexibility: custom transformations ● #lang-style syntax control (viz. Racket) 

Resources Resources ● cljx: https://github.com/lynaghk/cljx ● “Official” wiki re: feature expressions in Clojure[Script]: http://dev.clojure.org/display/design/Feature+Expressions ● Common Lisp hyperspec, feature expressions: http://www.lispworks.com/documentation/lw50/CLHS/Body/24_aba.htm Thank you! Thank you! Chas Emerick http://cemerick.com http://twitter.com/cemerick http://quilt.org