37 lines
1.3 KiB
Common Lisp
37 lines
1.3 KiB
Common Lisp
;;
|
|
;; 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)))
|
|
|