Lisp Macros in 20 Minutes (Featuring Clojure)

Lisp Macros in 20 minutes (featuring ‘clojure) phillip calçado http://fragmental.tw http://thoughtworks.com --:-- *LISP Macros in 20 minutes* http://fragmental.tw (Presentation)--------------------------------------------------------

Clojure

Clojure •homoiconic •fairly functional •runtime polymorphism •jvm language •software transactional memory •agent-based asynchronous concurrency

Clojure •homoiconic •fairly functional •runtime polymorphism •jvm language Code is Data •software transactional memory •agent-based asynchronous concurrency Data is Code

Example: LINQ Envy

C# string[] names = { quot;Burkequot;, quot;Connorquot;, quot;Frankquot;, quot;Everettquot;, quot;Albertquot;, quot;Georgequot;, quot;Harrisquot;, quot;Davidquot; }; IEnumerable<string> query = from n in names where n.Length == 5 orderby n select n.ToUpper(); foreach (string item in query) Console.WriteLine(item); }

Java - Quaere String[] names={quot;Burkequot;, quot;Connorquot;, quot;Frankquot;, quot;Everettquot;, quot;Albertquot;, quot;Georgequot;, quot;Harrisquot;, quot;Davidquot;}; Iterable<String> query= from(quot;nquot;).in(names). where(eq(quot;n.length()quot;,5). select(quot;n.toUpperCase()quot;); for (String n: query) { System.out.println(n); }

Ruby - Quick Hack names = [quot;Burkequot;, quot;Connorquot;, quot;Frankquot;, quot;Everettquot;, quot;Albertquot;, quot;Georgequot;, quot;Harrisquot;, quot;Davidquot;] query = from :n => names do where n.length => 5 orderby n select n.upcase end query.each{|e| puts e }

Clojure (def names '(quot;Burkequot;, quot;Connorquot;, quot;Frankquot;, quot;Everettquot;, quot;Albertquot;, quot;Georgequot;, quot;Harrisquot;, quot;Davidquot;)) (def query (from n in names where (= (. n length) 5) orderby n select (. n toUpperCase))) (doseq [n query] (println n))

Ruby Hack - Implementation class Parameter def from(binding, &spec) def method_missing(method, *args) var = binding.keys.first method list = binding.values.last end query = Query.new var end query.instance_eval &spec list.select do |a| class Query a.send(query.condition[:method]) == attr_reader :condition, :criteria, :action query.condition[:value] end.sort do |a,b| def initialize(var) if(query.criteria) singleton_class.send(:define_method, var) a.send(query.criteria) <=> b.send(query.criteria) { Parameter.new } else end a <=> b end def singleton_class; class << self; self; end; end end.map do |a| a.send(query.action) def where(cond) end @condition = {:method => cond.keys.first, :value => end cond.values.last} end def orderby(criteria) @criteria = criteria unless criteria.kind_of? Parameter end def select(action) @action = action end end a <=> b end end.map do |a| a.send(query.action) end end

Clojure - Implementation (defmacro from [var _ coll _ condition _ ordering _ desired-map] `(map (fn [~var] ~desired-map) (sort-by (fn[~var] ~ordering) (filter (fn[~var] ~condition) ~coll))))

Code is Data Data is Code “ InfoQ: [...] many modern programming languages like Ruby are claiming big influences from Lisp Have you seen those languages or do you have any ideas about the current state of programming languages? McCarthy: [...] I don't know enough for example about Ruby to know in what way it's related to Lisp. Does it use, for example, list structures as data? InfoQ: No. McCarthy: So if you want to compute with sums and products, you have to parse every time? InfoQ: Yes. McCarthy: So, in that respect Ruby still isn't up to where Lisp was in 1960. Adapted From: http://www.infoq.com/interviews/mccarthy-elephant-2000*

Everything is a (List)

(1 2 3 4 5) (+ 1 2) (+ (- 3 2) 10)

List { (1 2 3 4 5) Number

List { (+ 1 2) Function Number

List { (+ (- 3 2) 10) { List Function Number

{ (defn- run-variant[variant] (let [result (wrap-and-run List (:impl variant) (:args variant))] (struct-map variant-result :args (:args variant) :result (first result) :exception (second result))))

Code is Data Data is Code

Example: Implementing If

(defn they-are-the-same [] (println quot;They are the same!quot;)) (defn they-are-different [] (println quot;They are different!quot;)) (my-if (= 2 2) (they-are-the-same) (they-are-different))

First Try: Function (defn my-if [condition succ fail] (cond condition succ :else fail)) user> ;;;; (my-if (= 2 2) (they-are-the- same) (they-are ... They are the same! They are different!

Second Try: Macro (defmacro my-if [condition succ fail] (cond condition succ :else fail)) user> ;;;; (my-if (= 2 2) (they-are-the- same) (they-are ... They are the same!

Why? Dump Function Arguments (defn my-if [condition succ fail] (println quot;Parameters are: quot; condition succ fail)) user> user> ;;;; (my-if (= 2 2) (they-are-the- same) (they-are ... They are the same! They are different! Parameters are: true nil nil

Why? Dump Macro Arguments (defmacro my-if [condition succ fail] (println quot;Parameters are: quot; condition succ fail)) user> user> ;;;; (My-if (= 2 2) (they-are-the- same) (they-are ... Parameters are: (= 2 2) (they-are-the- same) (they-are-different)

Macro Expansion (println (macroexpand-1 '(my-if (= 2 2) (they-are-the-same) (they-are-different))) user> user> (they-are-the-same)

(my-if (= 2 2) (they-are-the-same) (they-are-different)) (defmacro my-if [condition succ fail] (cond condition succ :else fail)) (they-are-the-same)

Revisiting LINQ

Clojure - Implementation (defmacro from [var _ coll _ condition _ ordering _ desired-map] `(map (fn [~var] ~desired-map) (sort-by (fn[~var] ~ordering) (filter (fn[~var] ~condition) ~coll))))

(def query (from n in names where (= (. n length) 5) orderby n select (. n toUpperCase))) (defmacro from [var _ coll _ condition _ ordering _ desired-map] `(map (fn [~var] ~desired-map) (sort-by (fn[~var] ~ordering) (filter (fn[~var] ~condition) ~coll)))) (map (fn [n] (. n toUpperCase)) (sort-by (fn [n] n) (filter (fn [n] (= (. n length) 5)) names)))

More? http://www.pragprog.com/titles/shcloj/ programming-clojure http://www.lisperati.com/casting.html http://groups.google.com/group/clojure http://www.gigamonkeys.com/book/ http://mitpress.mit.edu/sicp/ http://github.com/pcalcado/fato/tree/master

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Lisp Macros in 20 Minutes (Featuring Clojure)

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