joy-in-the-morning/language/joy/eval.scm

;;; eval.scm -- Joy runtime.
;;; Copyright © 2016 Eric Bavier <bavier@member.fsf.org>
;;;
;;; Joy is free software; you can redistribute it and/or modify it under
;;; the terms of the GNU General Public License as published by the Free
;;; Software Foundation; either version 3 of the License, or (at your
;;; option) any later version.
;;;
;;; Joy is distributed in the hope that it will be useful, but WITHOUT
;;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
;;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
;;; License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with Joy.  If not, see <http://www.gnu.org/licenses/>.

(define-module (language joy eval)
  #:use-module (srfi srfi-1)
  #:replace (eval))

(define *primitives* '(language joy primitives))

(define (eval x S)
  "Evaluate joy term X with the stack S."
  (cond
   ((symbol? x)
    (let ((v (or (and=> (module-variable
			 (resolve-interface *primitives*) x)
			variable-ref)
		 (module-ref (resolve-module '(joy)) x))))
      (if (procedure? v)
          ;; Joy primitives are procedures that must be applied
	  (apply v S)
          ;; Variables from the 'DEFINE ...' syntax or 'def' primitive
          ;; are lists of factors to be evaluated.
	  (fold eval S v))))
   (else
    (cons x S))))