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(cl)Loop Basics

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Loop Basics
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The `loop' macro essentially creates a mini-language within Lisp that
is specially tailored for describing loops.  While this language is a
little strange-looking by the standards of regular Lisp, it turns out
to be very easy to learn and well-suited to its purpose.

   Since `loop' is a macro, all parsing of the loop language takes
place at byte-compile time; compiled `loop's are just as efficient as
the equivalent `while' loops written longhand.

 - Special Form: loop clauses...
     A loop construct consists of a series of CLAUSEs, each introduced
     by a symbol like `for' or `do'.  Clauses are simply strung
     together in the argument list of `loop', with minimal extra
     parentheses.  The various types of clauses specify
     initializations, such as the binding of temporary variables,
     actions to be taken in the loop, stepping actions, and final
     cleanup.

     Common Lisp specifies a certain general order of clauses in a loop:

          (loop NAME-CLAUSE
                VAR-CLAUSES...
                ACTION-CLAUSES...)

     The NAME-CLAUSE optionally gives a name to the implicit block that
     surrounds the loop.  By default, the implicit block is named
     `nil'.  The VAR-CLAUSES specify what variables should be bound
     during the loop, and how they should be modified or iterated
     throughout the course of the loop.  The ACTION-CLAUSES are things
     to be done during the loop, such as computing, collecting, and
     returning values.

     The Emacs version of the `loop' macro is less restrictive about
     the order of clauses, but things will behave most predictably if
     you put the variable-binding clauses `with', `for', and `repeat'
     before the action clauses.  As in Common Lisp, `initially' and
     `finally' clauses can go anywhere.

     Loops generally return `nil' by default, but you can cause them to
     return a value by using an accumulation clause like `collect', an
     end-test clause like `always', or an explicit `return' clause to
     jump out of the implicit block.  (Because the loop body is
     enclosed in an implicit block, you can also use regular Lisp
     `return' or `return-from' to break out of the loop.)

   The following sections give some examples of the Loop Macro in
action, and describe the particular loop clauses in great detail.
Consult the second edition of Steele's "Common Lisp, the Language", for
additional discussion and examples of the `loop' macro.