;;;; nil <-> None at (Option T) boundaries -- M2 queue item 4.
;;;;
;;;; nil is dyn's own absence and None is (Option T)'s; this is the boundary
;;;; where the checker decides they are the same absence. [absent] and
;;;; [opt-of] take it in at the two annotated sites that are not a function
;;;; argument -- a global's declared type and a return type; [via-param]
;;;; takes it in at the third, a parameter. [as-dyn] is the other direction:
;;;; a written (Option i64) crossing into dyn becomes nil or the boxed
;;;; payload. [box-it]/[unbox-opt] round-trip a value through both crossings.
;;;;
;;;; The last line is the trap: a dyn that is nil only once the program runs,
;;;; reaching a bare i64. The literal [nil] two lines above it would have been
;;;; refused at compile time instead -- see test_flan.ml and
;;;; test_acceptance.ml's "nil at a bare T, compile time" row for that half.

(defonce absent (Option i64) nil)

(defn opt-of [flag bool] (Option i64)
  (if flag (Some 7) nil))

(defn via-param [o (Option i64)] i64
  (match o (Some v) v None -1))

(defn as-dyn [o (Option i64)] dyn o)

(defn box-it [x i64] dyn x)
(defn unbox-opt [d dyn] (Option i64) d)

(defn maybe-nil [flag bool] dyn (if flag 5 nil))
(defn take-i64 [n i64] i64 n)

(defn show [o (Option i64)] ()
  (print (match o (Some v) v None -1)) (println ""))

(defn main [] ()
  ;; nil -> None, at a global's declared type, a return type and a parameter.
  (show absent)                                       ; -1
  (show (opt-of true))                                ; 7
  (show (opt-of false))                                ; -1
  (print (via-param nil)) (println "")                 ; -1
  (print (via-param (Some 3))) (println "")             ; 3

  ;; None -> nil, crossing into dyn; Some x -> the boxed x.
  (print (= (as-dyn None) nil)) (println "")            ; true
  (print (as-dyn (Some 9))) (println "")                ; 9

  ;; A value round-tripped through both crossings: typed -> dyn -> (Option T).
  (show (unbox-opt (box-it 42)))                        ; 42

  ;; The trap: a dyn that turns out to be nil only when the program runs,
  ;; reaching a bare i64. flan_dyn_need_i64 owns the wording.
  (print (take-i64 (maybe-nil false)))
  (println ""))