Common Lisp: Why progn is a special form?

Question

Since Common Lisp\'s function arguments evaluate in left-to-right order, why wouldn\'t use an ordinary function:

(defun progn2 (&rest body)
  (first (las         


        

Answer 1

There is also another feature of PROGN which you can't get with a function:

Imagine this code in a file of Common Lisp code:

(progn
  (defmacro foo () ))

vs.

(my-progn
  (defmacro foo () ))

With using PROGN the compiler will treat the DEFMACRO form as a top-level form. That means for example that the compiler notes that there is a macro definition and makes it available in the compile-time environment.

Using a function MY-PROGN, the compiler won't recognize the DEFMACRO form, because it is not at top-level.