CLCS
Macro

loop

The "simple" loop form:

loop {compound-form}*{result}*

The "extended" loop form:

loop [!name-clause] {!variable-clause}* {!main-clause}* ⇒ {result}*

name-clause ::=named name
variable-clause ::=!with-clause | !initial-final | !for-as-clause
with-clause ::=with var1 [type-spec] [form1] {and var2 [type-spec] [= form2]}*
main-clause ::=!unconditional | !accumulation | !conditional | !termination-test | !initial-final
initial-final ::=initially {compound-form}^+ | finally {compound-form}^+
unconditional ::={do | doing} {compound-form}^+ | return {form | it}
accumulation ::=!list-accumulation | !numeric-accumulation
list-accumulation ::={collect | collecting | append | appending | nconc | nconcing} {form | it}                       [into simple-var]
numeric-accumulation ::={count | counting | sum | summing | }                          maximize | maximizing | minimize | minimizing {form | it}                          [into simple-var] [type-spec]
conditional ::={if | when | unless} form !selectable-clause {and !selectable-clause}*                 [else !selectable-clause {and !selectable-clause}*]                 [end]
selectable-clause ::=!unconditional | !accumulation | !conditional
termination-test ::=while form | until form | repeat form | always form | never form | thereis form
for-as-clause ::={for | as} !for-as-subclause {and !for-as-subclause}*
for-as-subclause ::=!for-as-arithmetic | !for-as-in-list | !for-as-on-list | !for-as-equals-then |                      !for-as-across | !for-as-hash | !for-as-package
for-as-arithmetic ::=var [type-spec] !for-as-arithmetic-subclause
for-as-arithmetic-subclause ::=!arithmetic-up | !arithmetic-downto | !arithmetic-downfrom
arithmetic-up ::=[[{from | upfrom} form1 | {to | upto | below} form2 | by form3]]^+
arithmetic-downto ::=[[{from form1}^1 | {{downto | above} form2}^1 | by form3]]
arithmetic-downfrom ::=[[{downfrom form1}^1 | {to | downto | above} form2 | by form3]]
for-as-in-list ::=var [type-spec] in form1 [by step-fun]
for-as-on-list ::=var [type-spec] on form1 [by step-fun]
for-as-equals-then ::=var [type-spec] = form1 [then form2]
for-as-across ::=var [type-spec] across vector
for-as-hash ::=var [type-spec] being {each | the}                 {{hash-key | hash-keys} {in | of} hash-table                 [using (hash-value other-var)] |                  {hash-value | hash-values} {in | of} hash-table                 [using (hash-key other-var)]}
for-as-package ::=var [type-spec] being {each | the}                    {symbol | symbols |                    present-symbol | present-symbols |                    external-symbol | external-symbols}                     [{in | of} package]
type-spec ::=!simple-type-spec | !destructured-type-spec
simple-type-spec ::=fixnum | float | t | nil
destructured-type-spec ::=of-type d-type-spec
d-type-spec ::=type-specifier | (d-type-spec . d-type-spec)
var ::=!d-var-spec
var1 ::=!d-var-spec
var2 ::=!d-var-spec
other-var ::=!d-var-spec
d-var-spec ::=simple-var | nil | (!d-var-spec . !d-var-spec)

Arguments and Values

compound-forma compound form.
namea symbol.
simple-vara symbol (a variable name).
form, form1, form2, form3a form.
step-funa form that evaluates to a function of one argument.
vectora form that evaluates to a vector.
hash-tablea form that evaluates to a hash table.
packagea form that evaluates to a package designator.
type-specifiera type specifier. This might be either an atomic type specifier or a compound type specifier, which introduces some additional complications to proper parsing in the face of destructuring; for further information, see Destructuring.
resultan object.

Description

For details, see The LOOP Facility.

Examples

;; An example of the simple form of LOOP.
 (defun sqrt-advisor ()
   (loop (format t "~&Number: ")
         (let ((n (parse-integer (read-line) :junk-allowed t)))
           (when (not n) (return))
           (format t "~&The square root of ~D is ~D.~
⇒  SQRT-ADVISOR
 (sqrt-advisor)
 |>  Number: |>>5[<–~]<<|
 |>  The square root of 5 is 2.236068.
 |>  Number: |>>4[<–~]<<|
 |>  The square root of 4 is 2.
 |>  Number: |>>done[<–~]<<|
⇒  NIL

;; An example of the extended form of LOOP.
 (defun square-advisor ()
   (loop as n = (progn (format t "~&Number: ")
                       (parse-integer (read-line) :junk-allowed t))
         while n
         do (format t "~&The square of ~D is ~D.~
⇒  SQUARE-ADVISOR
 (square-advisor)
 |>  Number: |>>4[<–~]<<|
 |>  The square of 4 is 16.
 |>  Number: |>>23[<–~]<<|
 |>  The square of 23 is 529.
 |>  Number: |>>done[<–~]<<|
⇒  NIL

;; Another example of the extended form of LOOP.
 (loop for n from 1 to 10
       when (oddp n)
         collect n)
⇒  (1 3 5 7 9)

Notes

Except that loop-finish cannot be used within a simple loop form, a simple loop form is related to an extended loop form in the following way:

 (loop {compound-form}*) ≡ (loop do {compound-form}*)