Unexpected behavior with loop macro and closures

Question

Why do these forms behave this way?

CL-USER>
(setf *closures*
      (loop for num in (list 1 2 3 4)
            collect (lambda ()
                      n         


        

Answer 1

The name num represents the same binding during the evaluation of LOOP. Maybe you want to write:

(mapcar 'constantly (list 1 2 3 4))

to get what you meant.

Answer 2

num is same variable shared by all lambdas.

Use

(setf *closures*
  (loop for num in (list 1 2 3 4)
        collect (let ((num1 num))
                  (lambda ()
                    num1))))

num1 is fresh variable for each iteration.

As of dolist, "It is implementation-dependent whether dolist establishes a new binding of var on each iteration or whether it establishes a binding for var once at the beginning and then assigns it on any subsequent iterations." (CLHS, Macro DOLIST). So it may work on one implementation and fail on other.