Added Clojure macros

clojure-macros.html.markdown

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+---
+language: "clojure macros"
+filename: learnclojuremacros.clj
+contributors:
+    - ["Adam Bard", "http://adambard.com/"]
+---
+
+As with all Lisps, Clojure's inherent [homoiconicity](https://en.wikipedia.org/wiki/Homoiconic)
+gives you access to the full extent of the language to write code-generation routines
+called "macros". Macros provide a powerful way to tailor the language to your needs.
+
+Be careful though. It's considered bad form to write a macro when a function will do.
+Use a macro only when you need control over when or if the arguments to a form will
+be evaluated.
+
+You'll want to be familiar with Clojure. Make sure you understand everything in
+[Clojure in Y Minutes](/docs/clojure/).
+
+```clojure
+;; Define a macro using defmacro. Your macro should output a list that can
+;; be evaluated as clojure code.
+;;
+;; This macro is the same as if you wrote (reverse "Hello World")
+(defmacro my-first-macro []
+  (list reverse "Hello World"))
+
+;; Inspect the result of a macro using macroexpand or macroexpand-1.
+;;
+;; Note that the call must be quoted.
+(macroexpand '(my-first-macro))
+;; -> (#<core$reverse clojure.core$reverse@xxxxxxxx> "Hello World")
+
+;; You can eval the result of macroexpand directly:
+(eval (macroexpand '(my-first-macro)))
+; -> (\d \l \o \r \W \space \o \l \l \e \H)
+
+;; But you should use this more succinct, function-like syntax:
+(my-first-macro)  ; -> (\d \l \o \r \W \space \o \l \l \e \H)
+
+;; You can make things easier on yourself by using the more succinct quote syntax
+;; to create lists in your macros:
+(defmacro my-first-quoted-macro []
+  '(reverse "Hello World"))
+
+(macroexpand '(my-first-quoted-macro))
+;; -> (reverse "Hello World")
+;; Notice that reverse is no longer function object, but a symbol.
+
+;; Macros can take arguments.
+(defmacro inc2 [arg]
+  (list + 2 arg))
+
+(inc2 2) ; -> 4
+
+;; But, if you try to do this with a quoted list, you'll get an error, because
+;; the argument will be quoted too. To get around this, clojure provides a
+;; way of quoting macros: `. Inside `, you can use ~ to get at the outer scope
+(defmacro inc2-quoted [arg]
+  `(+ 2 ~arg))
+
+(inc2-quoted 2)
+
+;; You can use the usual destructuring args. Expand list variables using ~@
+(defmacro unless [arg & body]
+  `(if (not ~arg)
+     (do ~@body))) ; Remember the do!
+
+(macroexpand '(unless true (reverse "Hello World")))
+;; ->
+;; (if (clojure.core/not true) (do (reverse "Hello World")))
+
+;; (unless) evaluates and returns its body if the first argument is false.
+;; Otherwise, it returns nil
+
+(unless true "Hello") ; -> nil
+(unless false "Hello") ; -> "Hello"
+
+;; Used without care, macros can do great evil by clobbering your vars
+(defmacro define-x []
+  '(do
+     (def x 2)
+     (list x)))
+
+(def x 4)
+(define-x) ; -> (2)
+(list x) ; -> (2)
+
+;; To avoid this, use gensym to get a unique identifier
+(gensym 'x) ; -> x1281 (or some such thing)
+
+(defmacro define-x-safely []
+  (let [sym (gensym 'x)]
+    `(do
+       (def ~sym 2)
+       (list ~sym))))
+
+(def x 4)
+(define-x-safely) ; -> (2)
+(list x) ; -> (4)
+
+;; You can use # within ` to produce a gensym for each symbol automatically
+(defmacro define-x-hygenically []
+  `(do
+     (def x# 2)
+     (list x#)))
+
+(def x 4)
+(define-x-hygenically) ; -> (2)
+(list x) ; -> (4)
+
+;; It's typical to use helper functions with macros. Let's create a few to
+;; help us support a (dumb) inline arithmatic syntax
+(declare inline-2-helper)
+(defn clean-arg [arg]
+  (if (seq? arg)
+    (inline-2-helper arg)
+    arg))
+
+(defn apply-arg
+  "Given args [x (+ y)], return (+ x y)"
+  [val [op arg]]
+  (list op val (clean-arg arg)))
+
+(defn inline-2-helper
+  [[arg1 & ops-and-args]]
+  (let [ops (partition 2 ops-and-args)]
+    (reduce apply-arg (clean-arg arg1) ops)))
+
+;; We can test it immediately, without creating a macro
+(inline-2-helper '(a + (b - 2) - (c * 5))) ; -> (- (+ a (- b 2)) (* c 5))
+
+; However, we'll need to make it a macro if we want it to be run at compile time
+(defmacro inline-2 [form]
+  (inline-2-helper form)))
+
+(macroexpand '(inline-2 (1 + (3 / 2) - (1 / 2) + 1)))
+; -> (+ (- (+ 1 (/ 3 2)) (/ 1 2)) 1)
+
+(inline-2 (1 + (3 / 2) - (1 / 2) + 1))
+; -> 3 (actually, 3N, since the number got cast to a rational fraction with /)
+```
+
+### Further Reading
+
+Writing Macros from [Clojure for the Brave and True](http://www.braveclojure.com/)  
+[http://www.braveclojure.com/writing-macros/](http://www.braveclojure.com/writing-macros/)
+
+Official docs  
+[http://clojure.org/macros](http://clojure.org/macros)
+
+When to use macros?  
+[http://dunsmor.com/lisp/onlisp/onlisp_12.html](http://dunsmor.com/lisp/onlisp/onlisp_12.html)