A Better Version of IF-N-Z-P

Let’s improve this macro so that:

1. It evaluates the test expression only once;

2. It uses a temporary symbol for its local variable to avoid the possibility of “Variable Capture.”

(defmacro if-n-z-p (expr f1 f2 f3)

(let ((val (gensym)))

`(let ((,val ,expr))

(cond ((minusp ,val) ,f1)

((zerop ,val) ,f2)

((plusp ,val) ,f3) ) ) ) )

GENSYM creates a new symbol, distinct from all others currently in use. This is protection against the possibility of a “variable-capture” bug.

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