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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}*]  
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 Section (Destructuring).

result---an object.


For details, see Section 6.1 (The LOOP Facility).


;; 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)))))
>>  Number: 5<NEWLINE>
>>  The square root of 5 is 2.236068.
>>  Number: 4<NEWLINE>
>>  The square root of 4 is 2.
>>  Number: done<NEWLINE>
=>  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.~%" n (* n n))))
>>  Number: 4<NEWLINE>
>>  The square of 4 is 16.
>>  Number: 23<NEWLINE>
>>  The square of 23 is 529.
>>  Number: done<NEWLINE>
=>  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)

Affected By: None.

Exceptional Situations: None.

See Also:

do, dolist, dotimes, return, go, throw, Section (Destructuring)


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*)

The following X3J13 cleanup issues, not part of the specification, apply to this section:

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