Clojure Koans 24 Macros Notebook

{:hello (clojure-version)}

(defmacro hello [x]

    (str "Hello, " x))

(defmacro infix [form]

    (list (second form) (first form) (nth form 2)))

;; note: unused-public-var linter warning from clojure-lsp

(defmacro infix-concise [form]

    `(~(second form) ; Note the syntax-quote (`) and unquote (~) characters!

       ~(first form)

       ~(nth form 2)))

;; question 24008: What exactly does the "unquote" (~) character do?

;; question 24009: What exactly does the "syntax-qyote" (`) character do?

;; question 24010: How advanced exactly are macros, and what learning prerequisites are appropriate/suitable?

;; question 24011: What are great use cases for macros?

;; question 24012: What are common pitfalls for first time macro writers?

(defmacro recursive-infix [form]

    (cond (not (seq? form))

          form

          (= 1 (count form))

          `(recursive-infix ~(first form))

          :else

          (let [operator (second form)

                first-arg (first form)

                others (rest (rest form))]

              `(~operator

                   (recursive-infix ~first-arg)

                   (recursive-infix ~others)))))

;; "Macros are like functions created at compile time"

(= "Hello, Macros!" (hello "Macros!"))

;; ~~question 24001: What is the significance (and consequences) of macros being created at compile time?~~

;; answer 24001a: High level question, too broad, and not immediately helpful at this stage.

;; skill 24001: Invoke a macro

;; "I can haz infix?"

(= 10 (infix (9 + 1)))

;; description: The `infix` macro is used to take in the arguments `9`, `+`, and `1` to execute the addition operation `nine plus one` to get a result of ten. The text above of "I can haz infix?" is a reference to an Internet meme where the grammatically correct English would be be "Can I have infix?"

;; question 24002: Is infix notation useful or helpful to Clojure programmers?

;; answer 24002a: Perhaps when converting code from other languages or regular text... Though I'd still guess no.

;; skill 24002: Invoke a macro that enables non-(Clojure-)standard syntax

;; "Remember, these are nothing but code transformations"

(= '(+ 9 1) (macroexpand '(infix (9 + 1))))

;; question 24003: What exactly is happening here?

;; question 24004: Why is `infix` quoted?

;; answer 24004a: `infix` is quoted to prevent code execution from

;;    happening so that way the `macroexpand` can correctly expand `infix`.

;; question 24005: What is `macroexpand`?

;; answer 24005a: From the docs:

;;    "Repeatedly calls macroexpand-1 on form until it no longer represents

;;    a macro form, then returns it.  Note neither macroexpand-1 nor

;;    macroexpand expand macros in subforms."

;; question 24006: What is `macroexpand-1`?

;; answer 24006a: TBD...

;; skill 24003: Expand a macro by using `macroexpand`

;; "You can do better than that - hand crafting FTW!"

(= '(* 10 2) (macroexpand '(infix-concise (10 * 2))))

;; skill 24004: Write your own custom macro

;; "Things don't always work as you would like them to... "

(= '(+ 10 (2 * 3)) (macroexpand '(infix-concise (10 + (2 * 3)))))

;; question 24007: Is this Koan exercise in the wrong order?

;; "Really, you don't understand recursion until you understand recursion"

(= 36 (recursive-infix (10 + (2 * 3) + (4 * 5))))

;; skill 24005: Rewrite a macro as a function