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37 lines
1.3 KiB
Common Lisp
37 lines
1.3 KiB
Common Lisp
;;
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;; The EVAL function in the original XLISP evaluated in the current lexical
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;; context. This was changed to evaluate in the NIL (global) context to
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;; match Common Lisp. But this created a problem: how do you EVAL an
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;; expression in the current lexical context?
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;;
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;; The answer is you can use the evalhook facility. The evalhook function
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;; will evaluate an expression using an environment given to it as an
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;; argument. But then the problem is "how do you get the current
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;; environment?" Well the getenv macro, below obtains the environent by
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;; using an *evalhook* form.
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;;
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;; The following two macros do the job. Insteading of executing (eval <expr>)
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;; just execute (eval-env <expr>). If you want, you can dispense with the
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;; macros and execute:
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;;
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;;(evalhook <expr> nil nil (let ((*evalhook* (lambda (x env) env)))
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;; (eval nil)))
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;;
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;; Tom Almy 10/91
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;;
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(defmacro getenv ()
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'(progv '(*evalhook*)
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(list #'(lambda (exp env) env))
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(eval nil)))
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; this didn't work, may be for a later (Almy) version of xlisp?
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;(defmacro getenv ()
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; '(let ((*evalhook* (lambda (x env) env)))
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; (eval nil))) ; hook function evaluates by returning
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; environment
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(defmacro eval-env (arg) ; evaluate in current environment
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`(evalhook ,arg nil nil (getenv)))
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