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cl-itertools

This is a port of Python itertools, that is a part of standard library, to CL. Original Python docs: https://docs.python.org/2/library/itertools.html

Also it defines couple of primitives, that make writing iterators in CL a joy (at least, for me).

To avoid collisions with CL names the "i" prefix is added to all functions. Thus, they are:

Defined utils

The summary of things, that make working with iterators more convenient. For examples of use, see cl-itertools.lisp itself.

Technicalities

Iterators are implemented on top of CL-COROUTINE system.

To ease the work with iterators there is a universal "IN-IT" driver for ITERATE

(iter (for i in-it (irange 4 10))
      (collect i))
--> (4 5 6 7 8 9)

To ease definition of new iterators, there is "DEFITER" macro. The syntax is prettly self-explanatory, for example, ICOUNT is defined like

(defiter icount (start &optional (step 1))
  (let ((cur start))
    (iter (while t)
	  (yield cur)
	  (incf cur step))))

On each successful invocation of iterator (underlying iterator coroutine), YIELD form has a value, which is supplied to the coroutine on this invocation.

For example, let's define iterator that just prints whatever is supplied to it (and returns successive integers:

(defiter just-a-printer ()
  (iter (for i from 1)
  	(format t "~a~%" (yield i))))

(defparameter *a* (just-a-printer))
(inext-noexit *a* 'a)
  1
  T
(inext-noexit *a* 'b)
  B
  2
  T
(inext-noexit *a* 'c)
  C
  3
  T

This example also shows use of INEXT-NOEXIT macro, which fetches next value from iterator. Note how symbol A is not printed on first call to INEXT-NOEXIT (because control flow is returned from YIELD, before FORMAT is entered). On second call to INEXT-NOEXIT symbol 'B is correctly printed and the next integer -- 2 -- is returned. Second value T is an artefact of realization.

We can modify this example, so that it would also print symbol A

(defiter just-a-printer ()
  (format t "~a~%" (yield-last-value))
  (iter (for i from 1)
  	(format t "~a~%" (yield i))))

(defparameter *a* (just-a-printer))
(inext-noexit *a* 'a)
  A
  1
  T
(inext-noexit *a* 'b)
  B
  2
  T

For this we need to use (YIELD-LAST-VALUE) macrolet, that always gives the last value, supplied to iterator (to the coroutine) by its caller. Thus, before we encountered any YIELDs in the control flow of the iterator, this value is equal to the value first supplied to the iterator (i.e. 'A).

TODO