Clojure Macros

clojure_macros.md

Clojure Macros

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This is nice and simple!

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String -> Reader -> Evaluator

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Example

(+ 1 2) 

gives me 3

• (+ 1 2) is a persistant list - an output from Reader.

• When evaluated, it takes the first symbol, identifies that it is a function.

• Evalutes the arguments and pass it to function +

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AST

The output from reader will be an AST, which is basically clojure's data structure typically List - one of the easiest to represent trees.

1 <--- (+) ---> 2 

Is that a clojure structure?

Yes! it is (list + 1 2)

list of symbols/forms/values - unevaluated!

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Tweak the Reader output


Hey Reader! You have to do a few more things before you pass the weird tree on to the evaluator!

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That means?

Typically something like this:

(def readoutput (read-string “(1 + 1)”)

(def input-for-eval 
(list (second readoutput) 
(first readoutput)
(last readoutput)) 

;; Passing on to the evaluator
eval(input-for-eval)

;; Result is 2

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Macros?

(defmacro thatisit! [argument] 
  (list (second argument) 
  (first argument) 
  (last argument)))

Macros give you a convenient way to manipulate lists before Clojure evaluates them.

;; gives you the expaned form that
;; is passed on to the evaluator
(macroexpand 'thatisit(1 + 2))

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And where to use?

That means you can use Clojure to extend itself so you can write programs however you please. In other words, macros enable syntactic abstraction.

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Syntactic abstraction?

Allowed	syntactic abstraction
(+ 1 2)	(1 + 2)
(filter ..((map ..[1 2]))	(-> [1 2] (map...)(filter...)

Those abstractions were made possible through macros