Info Catalog cl: Loop Facility cl: Loop Examples

cl: Loop Basics

 
 4.7.1 Loop Basics
 -----------------
 
 The ‘cl-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 ‘cl-loop’ is a macro, all parsing of the loop language takes
 place at byte-compile time; compiled ‘cl-loop’s are just as efficient as
 the equivalent ‘while’ loops written longhand.
 
  -- Macro: cl-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 ‘cl-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 ‘cl-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 ‘cl-return’ or
      ‘cl-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 for additional discussion and
 examples.
 

Info Catalog cl: Loop Facility cl: Loop Examples