A loop macro form expands into a form containing one or more binding forms (that establish bindings of loop variables) and a block and a tagbody (that express a looping control structure). The variables established in loop are bound as if by 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 meaningless value of the correct type, and then later in the prologue it is set to the true initial value by using setq.
One implication of this interleaving is that it is implementation-dependent whether the lexical environment in which the initial value forms (variously called the form1, form2, form3, step-fun, vector, hash-table, and package) in any for-as-subclause, except for-as-equals-then, are evaluated includes only the loop variables preceding that form or includes more or all of the loop variables; the form1 and form2 in a for-as-equals-then form includes the lexical environment of all the loop variables.
After the form is expanded, it consists of three basic parts in the tagbody: the loop prologue, the loop body, and the loop epilogue.
The loop prologue contains forms
that are executed before iteration begins, such as
any automatic variable initializations prescribed
by the variable clauses, along with any initially clauses
in the order they appear in the source.
The loop body contains those forms that are executed during iteration, including application-specific calculations, termination tests, and variable stepping_1.
The loop epilogue contains forms that are executed after iteration
terminates, such as finally clauses, if any, along
with any implicit return value from an accumulation clause or
an termination-test clause.
Some clauses from the source form contribute code only to the loop prologue; these clauses must come before other clauses that are in the main body of the loop form. Others contribute code only to the loop epilogue. All other clauses contribute to the final translated form in the same order given in the original source form of the loop.
Expansion of the loop macro produces an implicit block named nil
unless named is supplied.
Thus, return-from (and sometimes return) can be used to return values from loop or to exit loop.