One man's constant is another man's variable.I remember trying to understand this very idea in high school calculus, but my teacher had no idea what I was getting at. Who knew all I needed was the lambda calculus?
Making something variable is easy. Controlling duration of constancy is the trick.
As Will Rogers would have said, "There is no such thing as a free variable."
There is no difference between a variable and a constant except the lexical context in which you are looking at it. In this function:
the variable k is a constant, right?(lambda (n)
(+ n k))
(define (make-adder k)
(+ n k)))