Closures and Lexical Binding

ToC

Allegro CL

ANSI Common Lisp

3 Evaluation and Compilation

3.1 Evaluation

3.1.4 Closures and Lexical Binding

A lexical closure is a function that can refer to and alter the values of lexical bindings established by binding forms that textually include the function definition.

Consider this code, where x is not declared special:

 (defun two-funs (x)
   (list (function (lambda () x))
         (function (lambda (y) (setq x y)))))
 (setq funs (two-funs 6))
 (funcall (car funs))  6
 (funcall (cadr funs) 43)  43
 (funcall (car funs))  43

The function special form coerces a lambda expression into a closure in which the lexical environment in effect when the special form is evaluated is captured along with the lambda expression.

The function two-funs returns a list of two functions, each of which refers to the binding of the variable x created on entry to the function two-funs when it was called. This variable has the value 6 initially, but setq can alter this binding. The lexical closure created for the first lambda expression does not "snapshot" the value 6 for x when the closure is created; rather it captures the binding of x. The second function can be used to alter the value in the same (captured) binding (to 43, in the example), and this altered variable binding then affects the value returned by the first function.

In situations where a closure of a lambda expression over the same set of bindings may be produced more than once, the various resulting closures may or may not be identical, at the discretion of the implementation. That is, two functions that are behaviorally indistinguishable might or might not be identical. Two functions that are behaviorally distinguishable are distinct. For example:

 (let ((x 5) (funs '()))
   (dotimes (j 10)                          
     (push #'(lambda (z)                        
               (if (null z) (setq x 0) (+ x z)))
           funs))
   funs)

The result of the above form is a list of ten closures. Each requires only the binding of x. It is the same binding in each case, but the ten closure objects might or might not be identical. On the other hand, the result of the form

 (let ((funs '()))     
   (dotimes (j 10)
     (let ((x 5))
       (push (function (lambda (z)
                        (if (null z) (setq x 0) (+ x z))))
             funs)))
  funs)

is also a list of ten closures. However, in this case no two of the closure objects can be identical because each closure is closed over a distinct binding of x, and these bindings can be behaviorally distinguished because of the use of setq.

The result of the form

 (let ((funs '()))
   (dotimes (j 10)
     (let ((x 5))
       (push (function (lambda (z) (+ x z)))
            funs)))
   funs)

is a list of ten closure objects that might or might not be identical. A different binding of x is involved for each closure, but the bindings cannot be distinguished because their values are the same and immutable (there being no occurrence of setq on x). A compiler could internally transform the form to

 (let ((funs '()))
   (dotimes (j 10)
     (push (function (lambda (z) (+ 5 z)))
           funs))
  funs)

where the closures may be identical.

It is possible that a closure does not close over any variable bindings. In the code fragment

 (mapcar (function (lambda (x) (+ x 2))) y)

the function (lambda (x) (+ x 2)) contains no references to any outside object. In this case, the same closure might be returned for all evaluations of the function form.