[LISPWORKS][Common Lisp HyperSpec (TM)] [Previous][Up][Next]



Forum: Cleanup

References: DEFMACRO (CLtL pp145-151),

The LOOP Facility (X3J13/89-004)

Category: ADDITION

Edit history: 24-Jan-89, Version 1 by Pitman

25-Jan-89, Version 2 by Pitman

29-Mar-89, Version 3, by Moon, amended based on poll

Problem Description:

Common Lisp programmers have frequently complained that the

destructuring facility used by DEFMACRO is not made available

for use in ordinary programming situations involving list data.

The presence of a destructuring facility in the recently adopted

LOOP facility will be likely to make the absence of a separable

destructuring facility all the more apparent.

Prior to the introduction of LET into Maclisp, many people wrote

their own LET macros. A popular expansion was in terms of a DO

which did not iterate. eg,

(LET ((A 3)) (+ A A)) ==> (DO ((A 3)) () (RETURN (+ A A)))

While this practice `worked,' it was not perspicuous and contributed

substantially to non-readability: not only were the macros hard to

understand, but the surface interface itself was not standardized

and varied in subtle ways. For example, some LET macros allowed GO

statements while others did not.

There is now considerable danger that a lot of people will write

DESTRUCTURING-BIND variants in terms of a LOOP expression that

immediately returns.



Since the destructuring offered by LOOP is different in subtle ways

from the destructuring offered by DESTRUCTURING-BIND in implementations

offering that primitive natively, gratuitous headaches could result.


Provide a macro called DESTRUCTURING-BIND which behaves like the

destructuring bind in DEFMACRO. Specifically...

DESTRUCTURING-BIND lambda-list expression {decl}* {form}* [Macro]

Binds the variables specified in LAMBDA-LIST to the corresponding

values in the tree structure resulting from evaluating EXPRESSION,

then evaluates the FORMS in the body.

Anywhere in the LAMBDA-LIST where a parameter name may appear, and

where ordinary lambda-list syntax (as described in CLtL section 5.2.2)

does not otherwise allow a list, a lambda-list may appear in place of

the parameter name. When this is done, then the argument form that

would match the parameter is treated as a (possibly dotted) list, to

be used as an argument forms list for satisfying the parameters in

the embedded lambda-list.

If any of the lambda list keywords &OPTIONAL, &REST, &KEY,

&ALLOW-OTHER-KEYS and &AUX appears in the lambda list, it is treated

as with any other lambda-list.

If the lambda list keyword &BODY appears, it is treated as a synonym

for &REST.

The lambda list keyword &ENVIRONMENT is not allowed.

If the lambda list keyword &WHOLE appears, it must be followed by a

single variable that is bound to the entire expression at the current

level. &WHOLE and its following variable should appear first in the

list, before any other parameter or lambda-list keyword.

It is also permissible for any level of the LAMBDA-LIST to be dotted,

ending in a parameter name. This situation is treaed exactly as if

the aprameter name that ends the list had appeared preceded by &REST

in a proper list. For example, the notation (X Y . Z) is equivalent

to (X Y &REST Z).

If the result of evaluating the expression does not match the

destructuring pattern, an error should be signaled.

Test Case:



`((ALPHA) ,@(IOTA 3))


=> (ALPHA BEE 3 2 1)


The proposal directly addresses the stated problem, and is current practice

in numerous implementations. Our charter effectively dictates that where

feasible we should try to head off the widespread development of uselessly

different variants of commonplace tools.

The intent of the specification is to make DESTRUCTURING-BIND lambda-lists

compatible with inner-list elements of a macro lambda-list.

Current Practice:

Symbolics Genera, Envos Medley, TI Explorer, and Lucid CL all offer

DESTRUCTURING-BIND, though the details vary slightly.

The DESTRUCTURING-BIND offered by Symbolics Genera signals an error if

the pattern is not matched. The TI Explorer version does not.

Cost to Implementors:

Very small. In most cases, it's a matter of renaming and/or exporting an

already existing symbol. In a few cases, a very small amount of

`program interface' code would have to be written.

Cost to Users:

None. This is an upward compatible change.

Cost of Non-Adoption:

Loss of the Benefits and Aesthetics cited below.


Users will get a powerful feature they have asked for on many occassions.

In implementations which `autoload' code, it would be better for this

support to be separable so that people could do DESTRUCTURING-BIND

without demand loading all other LOOP support.


Defining this macro centrally for the Common Lisp community will reduce

subtle deviations, which will in turn have positive aesthetic impact.


JonL observes that although LOOP does destructuring, it can't directly

make use of the DESTRUCTURING-BIND interface suggested here.

Pitman and Gray think a facility of this sort is a good idea, though

obviously the details may still need a little fleshing out before the

proposal is ready for vote.

To date, the excuse for not satisfying this request has been a

religious war between factions who want to destructure lists by


(DESTRUCTURING-BIND (var1 var2 var3) exp . body)

and those who want to destructure lists by writing

(DESTRUCTURING-BIND (LIST var1 var2 var3) exp . body)

The advantage of the former approach is that it is notationally

concise for the common case of destructuring a list. The disadvantage

is that it is not extensible to accomodate abstract kinds of


The advantage of the latter approach is that it allows interesting

extensions that accomodate data-hiding, such as:


(DESTRUCTURING-BIND (MAKE-FOO var1 var2 var3) exp . body)

and later the ability to change the representation of a FOO without

updating the associated binding forms. The disadvantage is that it

is more verbose in the common case of destructuring a list, and still

even more verbose for nested lists.

Although destructuring has always existed in DEFMACRO, this has not

been adequate precedence for deciding the outcome of the religious war

because DEFMACRO only needs to destructure programs, and programs are

generally made up only of lists -- not arbitrary user-defined abstract

data types.

The lambda-list form of DESTRUCTURING-BIND in this version is

not completely compatible with the destructuring done by LOOP

in three areas: LOOP allows NIL elements of a list to be ignored,

LOOP does not allow &-keywords, and LOOP destructuring ignores

extra elements in the list being matched.

[Starting Points][Contents][Index][Symbols][Glossary][Issues]
Copyright 1996-2005, LispWorks Ltd. All rights reserved.