Parsing Loop Clauses (The Universe Known as LISP)

15.25.3 Parsing Loop Clauses

Clauses

The syntactic parts of a loop construct are called clauses; the scope of each clause is determined by the top-level parsing of that clause’s keyword.

The following example shows a loop construct with six clauses:

(loop for i from 1 to (compute-top-value)         ;First clause
      while (not (unacceptable i))                ;Second clause
      collect (square i)                          ;Third clause
      do (format t "Working on ~D now" i)         ;Fourth clause
      when (evenp i)                              ;Fifth clause
        do (format t "~D is a non-odd number" i)
      finally (format t "About to exit!"))        ;Sixth clause

Each loop keyword introduces either

a compound loop clause or
a simple loop clause that can consist of
- a loop keyword followed by
- a single Lisp form.

The number of forms in a clause is determined by the loop keyword that begins the clause and by the auxiliary keywords (“prepositions”) in the clause.

The keywords

do,
initially, and
finally

are the only loop keywords that can take any number of Lisp forms and group them as if in a single progn form.

Prepositions or Auxiliary Keywords

Loop clauses can contain auxiliary keywords, which are sometimes called prepositions.

For example, the first clause in the preceding code includes the prepositions from and to, which mark the value from which stepping begins and the value at which stepping ends.