CLHS: Macro LOOP

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Macro LOOP

Syntax:

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 Section 6.1.1.7 (Destructuring).

result---an object.

Description:

For details, see Section 6.1 (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<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))))
    =>  SQUARE-ADVISOR
     (square-advisor)
    >>  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 6.1.1.7 (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*)

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

- LOOP-MISCELLANEOUS-REPAIRS:FIX
- LOOP-SYNTAX-OVERHAUL:REPAIR

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