Python Itertools in Common Lisp


Anish Moorthy mailto:anlsh@protonmail.com

Python Itertools in Common Lisp (v1.0.0). Pronounced like "pickle"

An (almost) complete port of Python's itertools package, complete with laziness where applicable.

This project currently lives on Github. Pull requests welcome!

Objectives and Rationale

Other libraries, such as cl-itertools and snakes, provide similar functionality

Unfortunately cl-itertools remains very incomplete, and bothcl-itertoolsand snakes rely on cl-cont meaning that they wont always play nice with certain parts of CL (apparently)

PICL provides a near-complete port of itertools, complete with laziness, without any reliance on cl-cont or other such libraries


PICL isn't in Quicklisp yet, though that should change soon. For now, you'll probably want to clone/symlink it into your .quicklisp/local-projects directory so that you can do a (ql:quickload :picl)


All functions are annotated with markdown docstrings. Thanks to Staple you can read the documentation online or build it yourself like so

(staple:generate :picl :if-exists :supersede)

Just ignore any warnings you get about invalid definitions, all they mean is that you don't have PICL's dependencies loaded yet.


A fairly comprehensive test suite written with FiveAM is provided. You can run it yourself either manually or through asdf

;; The easy way
(asdf:test-system :picl)
;; The slightly less easy way
(ql:quickload :picl/tests)
(fiveam:run! 'picl/tests:suite)

Concepts and How-To

An "iterator" in PICL is simply a function taking no arguments and producing two values: the payload and the alive-indicator. The payload represent's the iterator's next item, and the alive-indicator should be truthy until after the iterator is consumed.

By example

(let ((it (make-iterator '(1 2))))
  (next it)  ;; (values 1 t)
  (next it)  ;; (values 2 t)
  (next it)) ;; (values nil nil)

After returning nil, all further next calls should also produce nil as quickly as possible. Furthermore when the alive indicator is nil, the payload should be ignored.

To create iterators over your own objects, specialize the make-iterator generic function appropriately to produce a function satisfying the conditions described above. For instance, the make-iterator definition for lists is

(defmethod make-iterator ((obj list))
  (lambda ()
    (if obj
        (values (prog1 (car obj) (setf obj (cdr obj))) t)
        (values nil nil))))

Specializations for lists and vectors are predefined. A universal in-it driver is also provided for Iterate through the picl/iterate system.

(ql:quickload '(:picl :picl/iterate))

    (iterate:for i in-it #(1 2 3))
    (iterate:collect i))
;; (1 2 3)

    (iterate:for i in-it (picl:permutations '(1 2 3)))
    (iterate:collect i))
;; (#(1 2 3) #(1 3 2) #(2 1 3) #(2 3 1) #(3 1 2) #(3 2 1))

though you should probably :use iterate so as not to have the iterate: prefix everywhere

Missing Functionality

The only functions which are still missing are groupby and accumulate.

A note on :use

This is a new library, and so I have the liberty to tell you this: do not, and I mean do not :use this package. This library might export new symbols in the future, and I'm not going to accomodate the use of :use.

Use package local nicknames instead, and maybe even consider adding similar disclaimers in any new libraries you release.

Future Work


Python, its itertools package, and all those who have contributed to it.

Shinmera, for creating Staple


This project is provided under the MIT License (see LICENSE.md)

System Information

Anish Moorthy

Definition Index