xlisponly/lsp/evalenv.lsp

   1 ;;
   2 ;; The EVAL function in the original XLISP evaluated in the current lexical
   3 ;; context. This was changed to evaluate in the NIL (global) context to
   4 ;; match Common Lisp. But this created a problem: how do you EVAL an
   5 ;; expression in the current lexical context?
   6 ;;
   7 ;; The answer is you can use the evalhook facility. The evalhook function
   8 ;; will evaluate an expression using an environment given to it as an
   9 ;; argument. But then the problem is "how do you get the current
  10 ;; environment?" Well the getenv macro, below obtains the environent by
  11 ;; using an *evalhook* form.
  12 ;;
  13 ;; The following two macros do the job. Insteading of executing (eval <expr>)
  14 ;; just execute (eval-env <expr>). If you want, you can dispense with the
  15 ;; macros and execute:
  16 ;;
  17 ;;(evalhook <expr> nil nil (let ((*evalhook* (lambda (x env) env)))
  18 ;;                              (eval nil)))
  19 ;;
  20 ;; Tom Almy  10/91
  21 ;;
  22
  23 (defmacro getenv ()
  24           '(let ((*evalhook* (lambda (x env) env)))
  25                 (eval nil)))    ; hook function evaluates by returning
  26                                 ; environment
  27
  28 (defmacro eval-env (arg)        ; evaluate in current environment
  29           `(evalhook ,arg nil nil (getenv)))
  30
  31