How to set local function definition using function (or closure) objects?

Question

The problem with flet is that the functions bound therein must be defined inline. In other words, there's no way to do this:

(new-flet ((a (lambda (f x)
                (funcall f (* x 2))))
           (b (function-generator)))
    (a #'b 10))

I considered defining such a macro myself, but the problem is that flet seems to be the only way to set local function values. symbol-function always gets the global definition only, and function can't be used with setf. Anyone have an idea how this can be done fairly cleanly, if at all?

Answer 1

You can easily build a trampoline

(defun function-generator (x)
  (lambda (y) (* x y)))

(let ((fg (function-generator 42)))
  (flet ((a (f x) (funcall f (* x 2)))
         (b (x) (funcall fg x)))
    (a #'b 10)))

A macro implementation of new-flet with this approach is

(defmacro new-flet (bindings &body body)
  (let ((let-bindings (list))
        (flet-bindings (list))
        (args (gensym)))
    (dolist (binding bindings)
      (let ((name (gensym)))
        (push `(,name ,(second binding))
              let-bindings)
        (push `(,(first binding) (&rest ,args)
                 (apply ,name ,args))
              flet-bindings)))
    `(let ,(nreverse let-bindings)
       (flet ,(nreverse flet-bindings)
         ,@body))))

that expands in your example case as

(macroexpand-1 '(new-flet ((a (lambda (f x) (funcall f (* x 2))))
                           (b (function-generator)))
                  (a #'b 10)))

==> (LET ((#:G605 (LAMBDA (F X)
                    (FUNCALL F (* X 2))))
          (#:G606 (FUNCTION-GENERATOR)))
      (FLET ((A (&REST #:G604)
               (APPLY #:G605 #:G604))
             (B (&REST #:G604)
               (APPLY #:G606 #:G604)))
        (A #'B 10)))