6.2.4 loop [Macro]

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-form—a compound form.

name—a symbol.

simple-var—a symbol (a variable name).

form, form1, form2, form3—a form.

step-fun—a form that evaluates to a function of one argument.

vector—a form that evaluates to a vector.

hash-table—a form that evaluates to a hash table.

package—a form that evaluates to a package designator.

type-specifier—a 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.

result—an 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.~%" n (sqrt n)))))
⇒  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)

See Also::

do , dolist , dotimes , return , go , throw , Destructuring

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}*)