Common Lisp the Language, 2nd Edition
The driving element of the Loop Facility is the loop macro. When Lisp encounters a loop macro call form, it invokes the Loop Facility and passes to it the loop clauses as a list of unevaluated forms, as with any macro. The loop clauses contain Common Lisp forms and loop keywords. The loop keywords are recognized by their symbol name, regardless of the packages that contain them. The loop macro translates the given form into Common Lisp code and returns the expanded form.
The expanded loop form is one or more lambda-expressions for the local binding of loop variables and a block and a tagbody that express a looping control structure. The variables established in the loop construct are bound as if by using let or lambda. Implementations can interleave the setting of initial values with the bindings. However, the assignment of the initial values is always calculated in the order specified by the user. A variable is thus sometimes bound to a harmless value of the correct data type, and then later in the prologue it is set to the true initial value by using setq.
The expanded form consists of three basic parts in the tagbody:
Expansion of the loop macro produces an implicit block (named nil). Thus, the Common Lisp macro return and the special form return-from can be used to return values from a loop or to exit a loop.
Within the executable parts of loop clauses and around the entire
loop form, you can still bind variables by using the Common Lisp
special form let.