elisp functions as parameters and as return value

Question

I have the following code

(defun avg-damp(f) 
    #\'(lambda(x) (/ (+ (funcall f x) x) 2.0)))

A call

(funcall (avg-damp #         


        

Answer 1

A tricky question, but finally got this figured out. The problem is that #' in the definition of avg-damp makes the compiler compile the lambda function at the time when avg-damp itself is compiled, before the actual value of f is known. You need to delay the compilation of this function to a later point in time, when avg-damp is called, like this:

(defun avg-damp (f)
   `(lambda(x) (/ (+ (funcall ,f x) x) 2.0)))

(funcall (avg-damp #'(lambda(v) (* v v))) 10)

Backquoting does the trick.

Edit: Of course, the whole problem goes away if you define avg-damp in an uncurried form, such as this:

(defun avg-damp (f x)
   (/ (+ (funcall f x) x) 2.0))

(funcall 'avg-damp #'(lambda(v) (* v v)) 10)

But I guess you have your reasons not to do so.