Clojure Koans Walkthrough

1. Introduction
2. Lists
3. Vectors
4. Sets
5. Maps
6. Functions
7. Conditionals
8. Higher Order Functions
9. Runtime Polymorphism
10. Lazy Sequences
11. Sequence Comprehensions
12. Creating Functions
13. Recursion
14. Destructuring
15. Refs
16. Atoms
17. Macros
18. Datatypes
19. Java Interop
20. Partition

Recommended Books

• ← Atoms
• Datatypes →

17. Macros - Koans Walkthrough

This video introduces macros, which are a key feature of Clojure, and any Lisp. In Clojure, you'll notice that all the source code is actually valid Clojure data (homo-iconicity!), written in terms of basic data structures. Macros give you the unparalleled power to manipulate those "data structures" at compile-time and generate new code. In fact, many of Clojure's "operators" are not built into the language, but are actually implemented as macros. And that same power is available to you! http://clojure.org/macros

Here is the 17_macros.clj source code:

(defmacro hello [x]
  (str "Hello, " x))

(defmacro infix [form]
  (list (second form) (first form) (nth form 2)))

(defmacro infix-better [form]
  `(~(second form) ; Note the syntax-quote (`) and unquote (~) characters!
    __
    __ ))

(defmacro r-infix [form]
  (cond (not (seq? form))
        __
        (= 1 (count form))
        `(r-infix ~(first form))
        :else
        (let [operator (second form)
              first-arg (first form)
              others __]
          `(~operator
            (r-infix ~first-arg)
            (r-infix ~others)))))

(meditations
  "Macros are like functions created at compile time"
  (= __ (hello "Macros!"))

  "I can haz infix?"
  (= __ (infix (9 + 1)))

  "Remember, these are nothing but code transformations"
  (= __ (macroexpand '(infix (9 + 1))))

  "You can do better than that - hand crafting FTW!"
  (= __ (macroexpand '(infix-better (10 * 2))))

  "Things don't always work as you would like them to... "
  (= __ (macroexpand '(infix-better ( 10 + (2 * 3)))))

  "Really, you don't understand recursion until you understand recursion"
  (= 36 (r-infix (10 + (2 * 3) + (4 * 5)))))