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} |   {{downto | above} form2} |   by form3]]

arithmetic-downfrom::= [[{downfrom form1} |   {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)

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 6.1.1.7 Destructuring.

result - an object.

;; 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[Return]
 The square root of 5 is 2.236068.
 Number: 4[Return]
 The square root of 4 is 2.
 Number: done[Return]
  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))))
  SQUARE-ADVISOR
 (square-advisor)
 Number: 4[Return]
 The square of 4 is 16.
 Number: 23[Return]
 The square of 23 is 529.
 Number: done[Return]
  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)

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

See cl:loop and the for-as-in-sequnce subclause for looping over sequences for a description of the additional for-as-in-sequence clause in Allegro CL. Note the links are to the documentation for the current Allegro CL version. Replace /current/ in the URL with the Allegro CL version number to see similar documentation is earlier releases.