loop (ANSI and GNU Common Lisp Document)

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#### 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}*)

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