Question

I have the following code

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

A call

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

returns 55.0 (the correct value) in SBCL but crashes with the following stack in emacs lisp

Debugger entered--Lisp error: (void-variable f)   (funcall f x)   (+ (funcall f x) x)   (/ (+ (funcall f x) x) 2.0)   (lambda (x) (/ (+ ... x) 2.0))(10)   funcall((lambda (x) (/ (+ ... x) 2.0)) 10)   eval((funcall (avg-damp (function ...)) 10))   eval-last-sexp-1(nil)   eval-last-sexp(nil)   call-interactively(eval-last-sexp) 

How can I make it work in Emacs lisp?

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.