;; lib/guest/reflective/quoting.sx — quasiquote walker.
;;
;; Extracted from Kernel's `knl-quasi-walk` and Scheme's `scm-quasi-walk`,
;; which differ only in:
;;   - the unquote keyword name (Kernel: "$unquote" / "$unquote-splicing";
;;     Scheme: "unquote" / "unquote-splicing")
;;   - the host evaluator function (`kernel-eval` vs `scheme-eval`)
;;
;; Algorithm is identical. Adapter cfg parameterises the two
;; language-specific knobs.
;;
;; Adapter cfg keys
;; ----------------
;;   :unquote-name           — string, name of the unquote keyword
;;   :unquote-splicing-name  — string, name of the splice keyword
;;   :eval                   — fn (form env) → value
;;
;; Public API
;;   (refl-quasi-walk-with CFG FORM ENV)
;;     Top-level walker. Returns FORM with unquotes evaluated in ENV.
;;
;;   (refl-quasi-walk-list-with CFG FORMS ENV)
;;     Walks a list of forms, splicing unquote-splicing results inline.
;;
;;   (refl-quasi-list-concat XS YS)
;;     Pure-SX list append (no host append/append! needed).

(define
  refl-quasi-list-concat
  (fn
    (xs ys)
    (cond
      ((or (nil? xs) (= (length xs) 0)) ys)
      (:else (cons (first xs) (refl-quasi-list-concat (rest xs) ys))))))

(define
  refl-quasi-walk-with
  (fn
    (cfg form env)
    (cond
      ((not (list? form)) form)
      ((= (length form) 0) form)
      ((and (string? (first form)) (= (first form) (get cfg :unquote-name)))
        (cond
          ((not (= (length form) 2))
            (error
              (str (get cfg :unquote-name) ": expects exactly 1 argument")))
          (:else ((get cfg :eval) (nth form 1) env))))
      (:else (refl-quasi-walk-list-with cfg form env)))))

(define
  refl-quasi-walk-list-with
  (fn
    (cfg forms env)
    (cond
      ((or (nil? forms) (= (length forms) 0)) (list))
      (:else
        (let
          ((head (first forms)))
          (cond
            ((and (list? head) (= (length head) 2) (string? (first head)) (= (first head) (get cfg :unquote-splicing-name)))
              (let
                ((spliced ((get cfg :eval) (nth head 1) env)))
                (cond
                  ((not (list? spliced))
                    (error
                      (str
                        (get cfg :unquote-splicing-name)
                        ": value must be a list")))
                  (:else
                    (refl-quasi-list-concat
                      spliced
                      (refl-quasi-walk-list-with cfg (rest forms) env))))))
            (:else
              (cons
                (refl-quasi-walk-with cfg head env)
                (refl-quasi-walk-list-with cfg (rest forms) env)))))))))