*evalhook*

Type:	-	system variable
Source:	-	xleval.c

Syntax

 *evalhook*

Description

*evalhook* is a system variable whose value is user code that will intercept evaluations either through normal system evaluation or through calls to evalhook. The default value for *evalhook* is NIL , which specifies to use the built in system evaluator. If *evalhook* is non-NIL , the routine is called with expression and environment parameters. If the environment argument is NIL , then the the current global environment is used. The environment, if non-NIL , is a structure composed of dotted pairs constructed of the symbol and its value which have the form:

(((sym1 . val1) (sym2 . val2) ... )))

Examples

(defun myeval (exp env)           ; define MYEVAL routine
  (princ "exp: ") (print exp)
  (princ "env: ") (print env)
  (evalhook exp #'myeval NIL env))

(defun foo (a) (+ a a))           ; create simple function
(setq *evalhook* #'myeval)        ; and install MYEVAL as hook

(foo 1)                           ; prints exp: (FOO 1) env:NIL
                                  ;        exp: 1       env:NIL
                                  ;        exp: (+ A A) env:((((A . 1))))
                                  ;        exp: A       env:((((A . 1))))
                                  ;        exp: A       env:((((A . 1))))
                                  ; returns 2

(top-level)                       ; to clean up *evalhook*

Note: The evalhook function and *evalhook* system variable are very useful in the construction of debugging facilities within XLISP. The trace and untrace functions use evalhook and *evalhook* to implement their functionality. The other useful aspect of evalhook and *evalhook* is to help in understanding how XLISP works to see the expressions, their environment and how they are evaluated.

Caution: Be careful when using *evalhook* and evalhook. If you put in a bad definition into *evalhook*, you might not be able to do anything and will need to exit XLISP.

Unusual behaviour: The evalhook function and *evalhook* system variable, by their nature, cause some unusual things to happen. After you have set *evalhook* to some non-NIL value, your function will be called. However, when you are all done and set *evalhook* to NIL or some other new routine, it will never be set. This is because the 'xevalhook' function [in the 'xlbfun.c' source file] saves the old value of *evalhook* before calling your routine, and then restores it after the evaluation. The mechanism to reset *evalhook* is to execute the top-level function, which sets *evalhook* to NIL.

See the *evalhook* system variable in the XLISP 2.0 manual.

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