;;;; Macros: a defmacro in the file, called from the file.
;;;;
;;;; There is no interpreter, so every macro below was compiled into a shared
;;;; object and dlopened into the compiler before this file's first line was
;;;; parsed. What arrives here is the expansion; nothing at run time knows a
;;;; macro was involved.
;;;;
;;;; A macro takes one parameter, the slice of forms written at its call site,
;;;; and answers one form. That is where variadics come from in a language with
;;;; no &rest: (len args) is how many were written.

;; The simplest one there is: two forms, in order. It proves the call site's
;; arguments arrive as forms and come back as code.
(defmacro both [args]
  `(do ~(at args 0) ~(at args 1)))

;; Splicing, which is the only reason ~@ exists: the body is however many forms
;; were written, and they go where a list is expected.
(defmacro when2 [args]
  `(if ~(at args 0) (do ~@(form-rest args 1))))

;; Expansion is not hygienic -- Common Lisp's rule and Clojure's, settled in
;; plan.org's open decision 2 -- so a macro that needs a name of its own asks
;; for one. gensym is a prelude function the loaded module runs while it runs,
;; and the name it answers starts with ~, which is a delimiter, so no symbol
;; the reader can produce is able to collide with it.
;;
;; Without this, `twice` would bind `tmp` and the caller's own `tmp` would be
;; shadowed inside it. The two calls below are the difference.
(defmacro twice [args]
  (let [v (gensym)]
    `(let [~v ~(at args 0)]
       (+ ~v ~v))))

;; A macro that answers a call to another macro. This costs the pre-pass
;; nothing: `both` is inside the quasiquote, so it is part of what this macro
;; *returns* and is expanded again after it returns, and `announce` can be
;; compiled without `both` existing.
(defmacro announce [args]
  `(both (print "-> ") ~(at args 0)))

;; This is the one that makes the pre-pass a fixpoint rather than a sweep. The
;; call to `id` is not inside a quasiquote, so it runs while *this macro is
;; being compiled* -- which means `id` has to be compiled and dlopened first,
;; and until it is, `id` is a name nothing defines and this body will not
;; compile at all. So round 0 takes `id`, round 1 expands this against it, and
;; the module that finally answers a call holds both.
(defmacro id [args]
  (at args 0))

(defmacro quiet [args]
  (id `(println "a macro that called a macro")))

;; And a macro that expands into a call to itself, which is what every
;; conditional macro in every Lisp is. It gets smaller each time and stops at
;; the empty case, so the expander's fuel never comes into it.
(defmacro all-of [args]
  (if (= (len args) 0)
    `true
    `(if ~(at args 0) (all-of ~@(form-rest args 1)) false)))

(defn main [] i32
  (both (print "a") (println "b"))

  (when2 true (print "c") (println "d"))
  (when2 false (println "not printed"))

  ;; 21 + 21. The argument is evaluated once, into the gensym'd binding.
  (print (twice 21)) (println "")

  ;; The caller's own `tmp` is untouched by the one the macro bound, because
  ;; the macro did not bind `tmp`.
  (let [tmp 5]
    (print (twice tmp)) (print " ") (print tmp) (println ""))

  (announce (println "announced"))
  (quiet)

  (print (all-of)) (println "")
  (print (all-of true true true)) (println "")
  (print (all-of true false true)) (println "")
  0)