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?
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.