MacWorld 1999 January - Disc 2/Macworld (1999-01) (Disk 2).dmg/Serious Demos/Symbolic Composer 4.2/Environment/Projects/Questions & Answers/Q&A Basic/Delayed evaluation

Text File | 1998-10-26 | 1KB | 74 lines

>i was wondering how to use scom functions with delayed evaluation arguments.
>that is, instead of the argument being evaluated, and then passed as a
>parameter to the function, i want the function to take the argument as a
>function, and evaluate it during the function's evaluation.  does this make
>sense?

You can make it a macro and evaluate the parameter with eval. Here you
supply the function call (rnd2 0 1) to the delayed-evaluation which
then evaluates and prints the result 10 times. Notice that since this
is a macro it will return a value which will be evaluated, since
macros are usually used for making new Lisp forms which expand and
evaluate. To overcome this make your macro returning nil.

(defmacro delayed-evaluation (function)
  (dotimes (i 10)
    (print (eval function)))
  nil)

(delayed-evaluation (rnd2 0 1))

0.7697816208819859
0.915418354110443
0.8008900594140869
0.9490920328971697
0.5024546815548092
0.7643523601145716
0.8911941803817172
0.7534653782931855
0.49140359536977485
0.33869340586534236
nil
?

If you want to evaluate any number of arguments in a similar way
then make it this way.

(defmacro delayed-evaluation2 (&rest body)
  (dotimes (i 10)
    (eval `(progn ,@body)))
  nil)

(delayed-evaluation2 (print 1) (print 2) (print 3))

1
2
3
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3
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2
3
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3
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3