loop is the built-in macro for iteration.
Its simplest form is (loop (print "hello")): this will print forever.
A simple iteration over a list is:
(loop for x in '(1 2 3)
do (print x))It prints what’s needed but returns nil.
If you want to return a list, use collect:
(loop for x in '(1 2 3)
collect (* x 10))
;; (10 20 30)The Loop macro is different than most Lisp expressions in having a complex internal domain-specific language that doesn’t use s-expressions. So you need to read Loop expressions with half of your brain in Lisp mode, and the other half in Loop mode. You love it or you hate it.
Think of Loop expressions as having four parts: expressions that set up variables that will be iterated, expressions that conditionally terminate the iteration, expressions that do something on each iteration, and expressions that do something right before the Loop exits. In addition, Loop expressions can return a value. It is very rare to use all of these parts in a given Loop expression, but you can combine them in many ways.
iterate is a
popular iteration macro that aims at being simpler, “lispier” and more
predictable than loop, besides being extensible. However it isn’t built-in,
so you have to import it:
(ql:quickload "iterate") (use-package :iterate)
Iterate looks like this:
(iter (for i from 1 to 5)
(collect (* i i)))(if you use loop and iterate in the same package, you might run into name conflicts)
Iterate also comes with display-iterate-clauses that can be quite handy:
(display-iterate-clauses '(for))
;; FOR PREVIOUS &OPTIONAL INITIALLY BACK Previous value of a variable
;; FOR FIRST THEN Set var on first, and then on subsequent iterations
;; ...Much of the examples on this page that are valid for loop are also valid for iterate, with minor modifications.
for is an extensible iteration macro that is often shorter than loop, that “unlike loop is extensible and sensible, and unlike iterate does not require code-walking and is easier to extend”.
It has the other advantage of having one construct that works for all
data structures (lists, vectors, hash-tables…): in doubt, just use
for… over:
(for:for ((x over <your data structure>))
(print …))You also have to quickload it:
(ql:quickload "for")
We’ll also give examples with =mapcar= and map, and eventually
with their friends mapcon, mapcan, maplist, mapc and mapl
which E. Weitz categorizes very well in his “Common Lisp Recipes”,
chap. 7. The one you are certainly accustomed to from other languages is
mapcar: it takes a function, one or more lists as arguments,
applies the function on each element of the lists one by one and
returns a list of result.
(mapcar (lambda (it) (+ it 10)) '(1 2 3))
(11 12 13)map is generic, it accepts list and vectors as arguments, and
expects the type for its result as first argument:
(map 'vector (lambda (it) (+ it 10)) '(1 2 3))
;; #(11 12 13)
(map 'list (lambda (it) (+ it 10)) #(1 2 3))
;; (11 12 13)
(map 'string (lambda (it) (code-char it)) '#(97 98 99))
;; "abc"The other constructs have their advantages in some situations ;) They either process the tails of lists, or concatenate the return values, or don’t return anything. We’ll see some of them.
If you like mapcar, use it a lot, and would like a quicker and
shorter way to write lambdas, then you might like one of those
lambda shorthand libraries.
Here is an example with cl-punch:
(mapcar ^(* _ 10) '(1 2 3))
;; (10 20 30)and voilà :) We won’t use this more in this recipe, but feel free to do.
Last but not least, you might like series, a library that describes itself as combining aspects of sequences, streams, and loops. Series expressions look like operations on sequences (= functional programming), but can achieve the same high level of efficiency as a loop. Series first appeared in “Common Lisp the Language”, in the appendix A (it nearly became part of the language). Series looks like this:
(collect
(mapping ((x (scan-range :from 1 :upto 5)))
(* x x)))
;; (1 4 9 16 25)series is good, but its function names are different from what we
find in functional languages today. You might like the “Generators
The Way I Want Them Generated”
library. It is a lazy sequences library, similar to series although
younger and not as complete, with a “modern” API with words like take, filter,
for or fold, and that is easy to use.
(range :from 20)
;; #<GTWIWTG::GENERATOR! {1001A90CA3}>
(take 4 (range :from 20))
;; (20 21 22 23)At the time of writing, GTWIWTG is licensed under the GPLv3.
(loop
(print "hello"))return can return a result:
(loop for i in '(1 2 3)
when (> i 1)
return i)
2(dotimes (n 10)
(print n))Here dotimes returns nil. The return value is evaluated at the end of the loop.
You can use return inside of it:
(dotimes (i 10)
(if (> i 3)
(return)
(print i)))(loop repeat 10
do (format t "Hello!~%"))This prints 10 times “hello” and returns nil.
(loop repeat 10 collect (random 10))
;; (5 1 3 5 4 0 7 4 9 1)with collect, this returns a list.
(iterate ((n (scan-range :below 10)))
(print n))We can build an infinite list by setting its last element to the list itself:
(loop with list-a = '(1 2 3)
with infinite-list = (setf (cdr (last list-a)) list-a)
for item in infinite-list
repeat 8
collect item)
;; (1 2 3 1 2 3 1 2)Illustration: (last '(1 2 3)) is (3), a list, or rather a cons cell, whose car is 3 and cdr is NIL. See the data-structures chapter for a reminder. This is the representation of (list 3):
[o|/] | 3
The representation of (list 1 2 3):
[o|o]---[o|o]---[o|/] | | | 1 2 3
By setting the cdr of the last element to the list itself, we make it recur on itself.
A notation shortcut is possible with the #= syntax:
(defparameter *list-a* '#1=(1 2 3 . #1#))
(setf *print-circle* t) ;; don't print circular lists forever
*list-a*If you need to alternate only between two values, use for … then:
(loop repeat 4
for up = t then (not up)
do (print up))
T
NIL
T
NILFor lists and vectors:
(iter (for item in '(1 2 3))
(print item))
(iter (for i in-vector #(1 2 3))
(print i))Looping over a hash-table is also straightforward:
(let ((h (let ((h (make-hash-table)))
(setf (gethash 'a h) 1)
(setf (gethash 'b h) 2)
h)))
(iter (for (k v) in-hashtable h)
(print k)))
;; b
;; aIn fact, take a look here,
or (display-iterate-clauses '(for)) to know about iterating over
- symbols in-package
- forms - or lines, or whatever-you-wish - in-file, or in-stream
- elements in-sequence - sequences can be vectors or lists
(dolist (item '(1 2 3))
(print item))dolist returns nil.
with in, no surprises:
(loop for x in '(a b c)
do (print x))
;; A
;; B
;; C
;; NIL(loop for x in '(a b c)
collect x)
;; (A B C)With on, we loop over the cdr of the list:
(loop for i on '(1 2 3) do (print i))
;; (1 2 3)
;; (2 3)
;; (3)(mapcar (lambda (x)
(print (* x 10)))
'(1 2 3))
10
20
30
(10 20 30)mapcar returns the results of the lambda function as a list.
(iterate ((item (scan '(1 2 3))))
(print item))scan-sublists is the equivalent of loop for ... on:
(iterate ((i (scan-sublists '(1 2 3))))
(print i))(loop for i across #(1 2 3) do (print i))(iterate ((i (scan #(1 2 3))))
(print i))We create a hash-table:
(defparameter h (make-hash-table))
(setf (gethash 'a h) 1)
(setf (gethash 'b h) 2)Looping over keys:
(loop for k being the hash-key of h do (print k))
;; b
;; asame with hash-value.
Looping over key-values pairs:
(loop for k
being the hash-key
using (hash-value v) of h
do (format t "~a ~a~%" k v))
b 2
a 1Use in-hashtable:
(iter (for (key value) in-hashtable h)
(collect (list key value)))the same with for:
(for:for ((it over h))
(print it))
(A 1)
(B 2)
NILThe lambda function of maphash takes two arguments: the key and the
value:
(maphash (lambda (key val)
(format t "key: ~a val:~a~&" key val))
h)
;; key: A val:1
;; key: B val:2
;; NILSee also with-hash-table-iterator.
Only because we like this topic, we introduce another library, trivial-do. It has the dohash macro, that ressembles dolist:
(dohash (key value h)
(format t "key: ~A, value: ~A~%" key value))(iterate (((k v) (scan-hash h)))
(format t "~&~a ~a~%" k v))(loop for x in '(a b c)
for y in '(1 2 3)
collect (list x y))
;; ((A 1) (B 2) (C 3))To return a flat list, use nconcing instead of collect:
(loop for x in '(a b c)
for y in '(1 2 3)
nconcing (list x y))
(A 1 B 2 C 3)If a list is smaller than the other one, loop stops at the end of the small one:
(loop for x in '(a b c)
for y in '(1 2 3 4 5)
collect (list x y))
;; ((A 1) (B 2) (C 3))We could loop over the biggest list and manually access the elements
of the smaller one by index, but it would quickly be
inefficient. Instead, we can tell loop to extend the short list.
(loop for y in '(1 2 3 4 5)
for x-list = '(a b c) then (cdr x-list)
for x = (or (car x-list) 'z)
collect (list x y))
;; ((A 1) (B 2) (C 3) (Z 4) (Z 5))The trick is that the notation for … = … then (cdr …) (note the ===
and the role of then) shortens our intermediate list at each
iteration (thanks to cdr). It will first be '(a b c), the initial
value, then we will get the cdr: (2 3), then (3), then
NIL. And both (car NIL) and (cdr NIL) return NIL, so we are
good.
(mapcar (lambda (x y)
(list x y))
'(a b c)
'(1 2 3))
;; ((A 1) (B 2) (C 3))or simply:
(mapcar #'list
'(a b c)
'(1 2 3))
;; ((A 1) (B 2) (C 3))Return a flat list:
(mapcan (lambda (x y)
(list x y))
'(a b c)
'(1 2 3))
;; (A 1 B 2 C 3)(collect
(#Mlist (scan '(a b c))
(scan '(1 2 3))))A more efficient way, when the lists are known to be of equal length:
(collect
(mapping (((x y) (scan-multiple 'list
'(a b c)
'(1 2 3))))
(list x y)))Return a flat list:
(collect-append ; or collect-nconc
(mapping (((x y) (scan-multiple 'list
'(a b c)
'(1 2 3))))
(list x y)))(loop for x from 1 to 3
collect (loop for y from 1 to x
collect y))
;; ((1) (1 2) (1 2 3))To return a flat list, use nconcing instead of the first collect.
(iter outer
(for i below 2)
(iter (for j below 3)
(in outer (collect (list i j)))))
;; ((0 0) (0 1) (0 2) (1 0) (1 1) (1 2))(collect
(mapping ((x (scan-range :from 1 :upto 3)))
(collect (scan-range :from 1 :upto x))))Use ===.
With for:
(loop for x from 1 to 3
for y = (* x 10)
collect y)
;; (10 20 30)With with, the difference being that the value is computed only
once:
(loop for x from 1 to 3
for y = (* x 10)
with z = x
collect (list x y z))
;; ((1 10 1) (2 20 1) (3 30 1))The HyperSpec defines the with clause like this:
with-clause::= with var1 [type-spec] [= form1] {and var2 [type-spec] [= form2]}*
so it turns out we can specify the type before the === and chain the with with and:
(loop for x from 1 to 3
for y integer = (* x 10)
with z integer = x
collect (list x y z))(loop for x upto 3
with foo = :foo
and bar = :bar
collect (list x foo bar))We can also give for a then clause that will be called at each iteration:
(loop repeat 3
for intermediate = 10 then (incf intermediate)
do (print intermediate))
10
11
12Here’s a trick to alternate a boolean:
(loop repeat 4
for up = t then (not up)
do (print up))
T
NIL
T
NILIterate through a list, and have a counter iterate in parallel. The length of the list determines when the iteration ends. Two sets of actions are defined, one of which is executed conditionally.
* (loop for x in '(a b c d e)
for y from 1
when (> y 1)
do (format t ", ")
do (format t "~A" x)
)
A, B, C, D, E
NILWe could also write the preceding loop using the IF construct.
* (loop for x in '(a b c d e)
for y from 1
if (> y 1)
do (format t ", ~A" x)
else do (format t "~A" x)
)
A, B, C, D, E
NILBy iterating on multiple series in parallel, and using an infinite range, we can make a counter.
(iterate ((x (scan '(a b c d e)))
(y (scan-range :from 1)))
(when (> y 1) (format t ", "))
(format t "~A" x))from… to…:
(loop for i from 0 to 10
do (print i))
;; 0 1 2 3 4 5 6 7 8 9 10from… below…: this stops at 9:
(loop for i from 0 below 10
do (print i))Similarly, use from 10 downto 0 (10…0) and from 10 above 0 (10…1).
:from ... :upto, including the upper limit:
(iterate ((i (scan-range :from 0 :upto 10)))
(print i)):from ... :below, excluding the upper limit:
(iterate ((i (scan-range :from 0 :below 10)))
(print i))with by:
(loop for i from 1 to 10 by 2
do (print i))if you use by (1+ (random 3)), the random is evaluated only once, as
if it was in a closure:
(let ((step (random 3)))
(loop for i from 1 to 10 by (+ 1 step)
do (print i)))The step must always be a positive number. If you want to count down, see above.
with :by
~+lisp
(iterate ((i (scan-range :from 1 :upto 10 :by 2)))
(print i))+~
with if, else and finally:
;; https://riptutorial.com/common-lisp/example/11095/conditionally-executing-loop-clauses
(loop repeat 10
for x = (random 100)
if (evenp x)
collect x into evens
else
collect x into odds
finally (return (values evens odds)))(42 82 24 92 92) (55 89 59 13 49)
Combining multiple clauses in an if body requires special syntax (and do, and count):
(loop repeat 10
for x = (random 100)
if (evenp x)
collect x into evens
and do (format t "~a is even!~%" x)
else
collect x into odds
and count t into n-odds
finally (return (values evens odds n-odds)))46 is even! 8 is even! 76 is even! 58 is even! 0 is even! (46 8 76 58 0) (7 45 43 15 69) 5
Translating (or even writing!) the above example using iterate is straight-forward:
(iter (repeat 10)
(for x = (random 100))
(if (evenp x)
(progn
(collect x into evens)
(format t "~a is even!~%" x))
(progn
(collect x into odds)
(count t into n-odds)))
(finally (return (values evens odds n-odds))))The preceding loop would be done a bit differently in Series. split
sorts one series into multiple according to provided boolean series.
(let* ((number (#M(lambda (n) (random 100))
(scan-range :below 10)))
(parity (#Mevenp number)))
(iterate ((n number) (p parity))
(when p (format t "~a is even!~%" n)))
(multiple-value-bind (evens odds) (split number parity)
(values (collect evens)
(collect odds)
(collect-length odds))))Note that although iterate and the three collect expressions are
written sequentially, only one iteration is performed, the same as the
example with loop.
(loop initially
(format t "~a " 'loop-begin)
for x below 3
do (format t "~a " x))
;; LOOP-BEGIN 0 1 2initially also exists with iterate.
(loop for x in '(1 2 3 4 5)
until (> x 3)
collect x)
;; (1 2 3)the same, with while:
(loop for x in '(1 2 3 4 5)
while (< x 4)
collect x)We truncate the series with until-if, then collect from its result.
(collect
(until-if (lambda (i) (> i 3))
(scan '(1 2 3 4 5))))do and collect can be combined in one expression
(loop for x in '(1 2 3 4 5)
while (< x 4)
do (format t "x is ~a~&" x)
collect x)
x is 1
x is 2
x is 3
(1 2 3)By mapping, we can perform a side effect and also collect items
(collect
(mapping ((x (until-if (complement (lambda (x) (< x 4)))
(scan '(1 2 3 4 5)))))
(format t "x is ~a~&" x)
x))The special loop named foo syntax allows you to create a loop that
you can exit early from. The exit is performed using return-from,
and can be used from within nested loops.
;; useless example
(loop named loop-1
for x from 0 to 10 by 2
do (loop for y from 0 to 100 by (1+ (random 3))
when (< x y)
do (return-from loop-1 (values x y))))
0
2Sometimes, you want to return early but execute the finally clause
anyways. Use =loop-finish=.
(loop for x from 0 to 100
do (print x)
when (>= x 3)
return x
finally (print :done)) ;; <-- not printed
;; 0
;; 1
;; 2
;; 3
;; 3
(loop for x from 0 to 100
do (print x)
when (>= x 3)
do (loop-finish)
finally (print :done)
(return x))
;; 0
;; 1
;; 2
;; 3
;; :DONE
;; 3It is most needed when some computation must take place in the finally clause.
Several actions provide shorthands for combinations of when/return:
* (loop for x in '(foo 2)
thereis (numberp x))
T* (loop for x in '(foo 2)
never (numberp x))
NIL* (loop for x in '(foo 2)
always (numberp x))
NILThey correspond to the functions some, notany and every:
(some #'numberp '(foo 2))
(notany #'numberp '(foo 2))
(every #'numberp '(foo 2))A block is manually created and returned from.
(block loop-1
(iterate ((x (scan-range :from 0 :upto 10 :by 2)))
(iterate ((y (scan-range :from 0 :upto 100 :by (1+ (random 3)))))
(when (< x y)
(return-from loop-1 (values x y))))))(loop for i from 1 to 3 count (oddp i))
;; 2(collect-length (choose-if #'oddp (scan-range :from 1 :upto 3)))(loop for i from 1 to 3 sum (* i i))
;; 14Summing into a variable:
(loop for i from 1 to 3
sum (* i i) into total
do (print i)
finally (print total))
1
2
3
14(collect-sum (#M(lambda (i) (* i i))
(scan-range :from 1 :upto 3)))(loop for i from 1 to 3 maximize (mod i 3))
;; 2and minimize.
(collect-max (#M(lambda (i) (mod i 3))
(scan-range :from 1 :upto 3)))and collect-min.
(loop for (a b) in '((x 1) (y 2) (z 3))
collect (list b a) )
;; ((1 X) (2 Y) (3 Z))(loop for (x . y) in '((1 . a) (2 . b) (3 . c)) collect y)
;; (A B C)Use nil to ignore a term:
(loop for (a nil) in '((x 1) (y 2) (z 3))
collect a )
;; (X Y Z)To iterate over a list, 2 items at a time we use a combination of on, by and destructuring.
We use on to loop over the rest (the cdr) of the list.
(loop for rest on '(a 2 b 2 c 3)
collect rest)
;; ((A 2 B 2 C 3) (2 B 2 C 3) (B 2 C 3) (2 C 3) (C 3) (3))We use by to skip one element at every iteration ((cddr list) is equivalent to (rest (rest list)))
(loop for rest on '(a 2 b 2 c 3) by #'cddr
collect rest)
;; ((A 2 B 2 C 3) (B 2 C 3) (C 3))Then we add destructuring to bind only the first two items at each iteration:
(loop for (key value) on '(a 2 b 2 c 3) by #'cddr
collect (list key (* 2 value)))
;; ((A 2) (B 4) (C 6))In general, with destructuring-bind:
(collect
(mapping ((l (scan '((x 1) (y 2) (z 3)))))
(destructuring-bind (a b) l
(list b a))))But for alists, scan-alist is provided:
(collect
(mapping (((a b) (scan-alist '((1 . a) (2 . b) (3 . c)))))
b))iterate has some other things unique to it.
If you are a newcomer in Lisp, it’s perfectly OK to keep you this section for later. You could very well spend your career in Lisp without resorting to those features… although they might turn out useful one day.
loop requires that all for clauses appear before the loop body,
for example before a while. It’s ok for iter to not follow this
order:
(iter (for x in '(1 2 99)
(while (< x 10))
(for y = (print x))
(collect (list x y)))collect, appending and other accumulating clauses can appear anywhere:
(iter (for x in '(1 2 3))
(case x
(1 (collect :a))
;; ^^ iter keyword, nested in a s-expression.
(2 (collect :b))))iterate has finders.
A finder is a clause whose value is an expression that meets some condition.
We can use finding followed by maximizing, minimizing or such-that.
Here’s how to find the longest list in a list of lists:
(iter (for elt in '((a) (b c d) (e f)))
(finding elt maximizing (length elt)))
=> (B C D)The rough equivalent in LOOP would be:
(loop with max-elt = nil
with max-key = 0
for elt in '((a) (b c d) (e f))
for key = (length elt)
do
(when (> key max-key)
(setf max-elt elt
max-key key))
finally (return max-elt))
=> (B C D)There could be more than one such-that clause:
(iter (for i in '(7 -4 2 -3))
(if (plusp i)
(finding i such-that (evenp i))
(finding (- i) such-that (oddp i))))
;; => 2We can also write such-that #'evenp and such-that #'oddp.
It is like “continue” and loop doesn’t have it.
Skips the remainder of the loop body and begins the next iteration of the loop.
iterate also has first-iteration-p and (if-first-time then else).
See control flow.
Use generate and next. A generator is lazy, it goes to the next value when said explicitly.
(iter (for i in '(1 2 3 4 5))
(generate c in-string "black")
(if (oddp i) (next c))
(format t "~a " c))
;; b b l l a
;; NILiterate allows us to get the previous value of a variable:
(iter (for el in '(a b c d e))
(for prev-el previous el)
(collect (list el prev-el)))
;; => ((A NIL) (B A) (C B) (D C) (E D))In this case however we can do it with loop’s parallel binding and, which is unsupported in iterate:
(loop for el in '(a b c d e)
and prev-el = nil then el
collect (list el prev-el))in-stringcan be used explicitly to iterate character by character over a string. With loop, useacross.
(iter (for c in-string "hello")
(collect c))
;; => (#\h #\e #\l #\l #\o)loopofferscollecting,nconcing, andappending.iteratehas these and alsoadjoining,unioning,nunioning, andaccumulating.
(iter (for el in '(a b c a d b))
(adjoining el))
;; => (A B C D)(adjoin is a set operation)
loophassumming,counting,maximizing, andminimizing.iteratealso includesmultiplyingandreducing. reducing is the generalized reduction builder:
(iter (with dividend = 100)
(for divisor in '(10 5 2))
(reducing divisor by #'/ initial-value dividend))
;; => 1(defmacro dividing-by (num &keys (initial-value 0))
`(reducing ,num by #'/ initial-value ,initial-value))
(iter (for i in '(10 5 2))
(dividing-by i :initial-value 100))
=> 1but there is more to it, see the documentation.
We saw libraries extending loop, for example CLSQL, but they are
full of feature flag checks (#+(or allegro clisp-aloop cmu openmcl sbcl scl)) and they call internal modules
(ansi-loop::add-loop-path, sb-loop::add-loop-path etc).
If we often scan the same type of object, we can write our own scanner for it: the iteration itself can be factored out. Taking the example above, of scanning a list of two-element lists, we’ll write a scanner that returns a series of the first elements and a series of the second.
(defun scan-listlist (listlist)
(declare (optimizable-series-function 2))
(map-fn '(values t t)
(lambda (l)
(destructuring-bind (a b) l
(values a b)))
(scan listlist)))
(collect
(mapping (((a b) (scan-listlist '((x 1) (y 2) (z 3)))))
(list b a)))Consider this series expression:
(collect-sum (mapping ((i (scan-range :length 5)))
(* i 2)))It’s a bit longer than it needs to be, the mapping form’s only
purpose is to bind the variable i, and i is used in only one
place. Series has a “hidden feature” that allows us to simplify this
expression to the following:
(collect-sum (* 2 (scan-range :length 5)))This is called implicit mapping and can be enabled in the call to
series::install:
(series::install :implicit-map t)When using implicit mapping, the #M reader macro demonstrated above
becomes redundant.
- the keyword
it, often used in functional constructs, can be recognized as a loop keyword. Don’t use it inside a loop.
It breaks on the function count:
(iter (for i from 1 to 10)
(sum (count i '(1 3 5))))It doesn’t recognize the built-in count function and instead signals a condition.
It works in loop:
(loop for i from 1 to 10
sum (count i '(1 3 5 99)))
;; 3Name Clause
named
Variable Clauses
initially finally for as with
Main Clauses
do collect collecting append appending nconc nconcing into count counting sum summing maximize return loop-finish maximizing minimize minimizing doing thereis always never if when unless repeat while until
These don’t introduce clauses:
= and it else end from upfrom above below to upto downto downfrom in on then across being each the hash-key hash-keys of using hash-value hash-values symbol symbols present-symbol present-symbols external-symbol external-symbols fixnum float t nil of-type
But note that it’s the parsing that determines what is a keyword. For example in:
(loop for key in hash-values)Only for and in are keywords.
©Dan Robertson on Stack Overflow.
- Tutorial for the Common Lisp Loop Macro by Peter D. Karp
- Common Lisp’s Loop Macro Examples for Beginners by Yusuke Shinyama
- Section 6.1 The LOOP Facility, of the draft Common Lisp Standard (X3J13/94-101R) - the (draft) standard provides background information on Loop development, specification and examples. Single PDF file available
- 26. Loop by Jon L White, edited and expanded by Guy L. Steele Jr. - from the book “Common Lisp the Language, 2nd Edition”. Strong connection to the draft above, with supplementing comments and examples.
- The Iterate Manual -by Jonathan Amsterdam and Luís Oliveira
- iterate - Pseudocodic Iteration - by Shubhamkar Ayare
- Loop v Iterate - SabraOnTheHill
- Comparing loop and iterate - by Stephen Bach (web archive)
- Common Lisp the Language (2nd Edition) - Appendix A. Series
- SERIES for Common Lisp - Richard C. Waters
- See also: more functional constructs (do-repeat, take,…)