Clementson's Blog

Loop extensibility

Sunday, October 3, 2004

A few days ago, Zach pointed to the announcement on c.l.l. of a new implementation of Loop. The new Loop implementation code was interesting in itself, as was the Sacla project which it is part of (Sacla is an attempt to create a complete implemenation of CL in CL). However, I've been reading the discussion on c.l.l. which followed the initial announcement, and the thread has some interesting comments on how to extend Loop.

Carl Shapiro noted that Lucid had documented writing extensions to the Loop macro in their Loop Facility manual. Unfortunately, the extensibility mechanism does not appear to be present in all CLs today; however, it does appear to exist (and I did manage to test it) in LispWorks. This User extension mechanism allows a programmer to extend the predefined loop itereation capabilities by using loop methods. For example, the following types of extensions are possible:

• Use any loop iteration control keyword or loop method name to initiate loop processing:

  CL-USER> (define-loop-macro for)
  FOR
  CL-USER> (for x in '(1 2 3 4) by #'cddr collect x)
  (1 3)

• Define a custom mechanism for iterating over a sequence. For example, to iterate over a simple string:

  CL-USER> (defloop (schar schars) schar length simple-string string-char)
  (SCHAR SCHARS)
  CL-USER> (loop for c being each schar of "Temp String" count c)
  11
  instead of writing:
  CL-USER> (loop for c being each element of 
  	      (the simple-string "Temp String") 
  	      count c)
  11