Dispatch in Clojure

Dispatch in Clojure

• 1.
  Dispatch in Clojure Carlo Sciolla, Product Lead @ Backbase DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  (ns aot (:gen-class)) (defn -main [& args] (dorun (map println (seq args)))) javap -c aot.class A quick journey in function calling We all learn to divide our code in functions, and invoke them when it’s their time on the stage of data processing. We define units of computations, ready to be executed. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  public static voidmain(java.lang.String[]); [..] 4:! invokevirtual!#66; [..] 26:!invokestatic! #104; 29:!invokeinterface! #108, 2; [..] 44:!invokespecial!#115; (defn -main [& args] [..] (dorun (map println (seq args)))) The JVM executes our code We’ll leave it to the JVM to figure out which code to actually run upon function call. It’s not always a straightforward job, and there are several ways to get to the code. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  public static voidmain(java.lang.String[]); [..] 4:! invokevirtual!#66; [..] 26:!invokestatic! #104; 29:!invokeinterface! #108, 2; [..] 44:!invokespecial!#115; [..] [clojure.lang.RT] static public ISeq seq(Object coll) Static dispatch When there’s nothing to choose from, the compiler emits a static dispatch bytecode. All calls will always result in the same code being executed. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  public static voidmain(java.lang.String[]); [..] 4:! invokevirtual!#66; [..] 26:!invokestatic! #104; 29:!invokeinterface! #108, 2; [..] 44:!invokespecial!#115; [..] Dynamic dispatch Most often the compiler can’t figure out the proper method implementation to call, and the runtime will get its chance to dynamically dispatch the call. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  * dispatch by arity * object oriented inheritance * multimethods * custom is-a hierarchies * protocols The options at hand Clojure provides a rich interface to dynamic dispatch, allowing programmers to have control over the dispatch logic at different degrees to find the optimal balance on the performance trade off scale. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  * dispatch by arity * object oriented inheritance (defn sum-them * multimethods ([x y] (+ x y)) * custom is-a hierarchies ([x y z] (+ x y z))) * protocols The good old arity Being a dynamically typed language, Clojure only checks on the number of arguments provided in the function call to find the right implementation to call. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  * dispatch by arity (defn stringify [x] * object oriented inheritance (.toString x)) * multimethods * custom is-a hierarchies (stringify (HashMap.)) ; “{}” * protocols (stringify (HashSet.)) ; “[]” More than Java™ Thanks to Clojure intimacy with Java, object inheritance is easily achieved. Thanks to Clojure dynamic typing, it also allows functions to traverse multiple inheritance trees. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  (defn stringify* [^HashMap x] (.toString x)) * dispatch by arity * object oriented inheritance (stringify* (HashMap.)) * multimethods => “{}” * custom is-a hierarchies (stringify* (TreeMap.)) * protocols => “{}” (stringify* (HashSet.)) => ClassCastException Forcing virtual dispatch to improve performance Being a dynamic language has a number of benefits, but performance isn’t one of them. To avoid reflection calls needed by default by the dynamic dispatch you can use type hints. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  (defn dispatch-fn [{:keys [version]}] * dispatch by arity version) * object oriented inheritance * multimethods (defmulti multi-call * custom is-a hierarchies dispatch-fn) * protocols http://bit.ly/multi-tests Full power Multimethods allow you to define your own dispatch strategy as a plain Clojure function. Their limit is the sky: they perform quite bad, and your dispatch fn can’t be changed or extended in user code. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  (defmethod multi-call ::custom-type * dispatch by arity [x] [...]) * object oriented inheritance * multimethods (derive java.util.HashSet * custom is-a hierarchies ::custom-type) * protocols http://bit.ly/multi-tests Inheritance à la carte Java types hierarchies defined outside your code can be “altered” by custom is-a? relationships created as needed. You can either use the default hierarchy or use make-hierarchy to restrict its scope. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  (defprotocol Registered (register [this])) * dispatch by arity * object oriented inheritance (extend-type String * multimethods Registered * custom is-a hierarchies (register [this] [...])) * protocols http://bit.ly/proto-tests Inheritance à la carte Java types hierarchies defined outside your code can be “altered” by custom is-a? relationships created as needed. You can either use the default hierarchy or use make-hierarchy to restrict its scope. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  * never used when compiling pure Java * moves type checking at run time * the compiler doesn’t resolve the method * user code to perform the dispatch * eligible for JIT optimizations http://bit.ly/headius-invokedynamic Bonus track: invokedynamic Java7 introduced a new bytecode instruction to help JVM languages designers: invokedynamic. There’s a long standing discussion as which benefits it can provide to Clojure. DISPATCH IN CLOJURE | August 8, 2012 | @skuro
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  Q/A DISPATCH IN CLOJURE| August 8, 2012 | @skuro
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  Thanks! Amsterdam Clojurians Carlo Sciolla Product Lead http://skuro.tk @skuro http://bit.ly/amsclojure DISPATCH IN CLOJURE | August 8, 2012 | @skuro