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type: system variable 
location: built-in
source file: xleval.c
Common LISP compatible: related
supported on: all machines




*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> ) ... ))).


(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.

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