evalenv.lsp

;;
;; The EVAL function in the original XLISP evaluated in the current lexical
;; context. This was changed to evaluate in the NIL (global) context to
;; match Common Lisp. But this created a problem: how do you EVAL an
;; expression in the current lexical context?
;;
;; The answer is you can use the evalhook facility. The evalhook function
;; will evaluate an expression using an environment given to it as an
;; argument. But then the problem is "how do you get the current
;; environment?" Well the getenv macro, below obtains the environment by
;; using an *evalhook* form.
;;
;; The following two macros do the job. Insteading of executing (eval <expr>)
;; just execute (eval-env <expr>). If you want, you can dispense with the
;; macros and execute:
;;
;;(evalhook <expr> nil nil (let ((*evalhook* (lambda (x env) env)))
;;                              (eval nil)))
;;
;; Tom Almy  10/91
;;

(defmacro getenv ()
  '(progv '(*evalhook*) 
          (list #'(lambda (exp env) env))
     (eval nil)))

; this didn't work, may be for a later (Almy) version of xlisp?
;(defmacro getenv ()
;          '(let ((*evalhook* (lambda (x env) env)))
;                (eval nil)))    ; hook function evaluates by returning
                                 ; environment

(defmacro eval-env (arg)        ; evaluate in current environment
          `(evalhook ,arg nil nil (getenv)))