2.26.2 Closures

Closures for local functions can be allocated on the stack if the local function is declared dynamic-extent, and the closure appears as an argument in the call of a named function. In the example:

(defun foo (x)
  (flet ((bar () x))
    (declare (dynamic-extent #'bar))
    (baz #'bar)))

the closure passed to function baz is allocated on the stack. Likewise in the example:

(defun foo (x)
  (flet ((bar () x))
    (baz #'bar)
    (locally (declare (dynamic-extent #'bar))
      (baz #'bar))))

Stack-allocation of closures can also automatically take place when calling certain known CL functions taking function arguments, for example some or find-if.