DESTRUCTURING-BINDDEFMACRO 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.
(DESTRUCTURING-BIND ((A B) C) (FOO) (LIST A B C))
==> (LOOP FOR ((A B) C) ON (FOO) DO (RETURN (LIST A B C)))
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.
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.
DEFUN IOTA (N) (LOOP FOR I FROM 1 TO N COLLECT I)) ;helper
(DESTRUCTURING-BIND ((A &OPTIONAL (B 'BEE)) ONE TWO THREE)
`((ALPHA) ,@(IOTA 3))
(LIST A B THREE TWO ONE))
=> (ALPHA BEE 3 2 1)
The intent of the specification is to make DESTRUCTURING-BIND lambda-lists compatible with inner-list elements of a macro lambda-list.
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.
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.
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 writing (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 destructuring.
The advantage of the latter approach is that it allows interesting extensions that accomodate data-hiding, such as: (DEFMACRO MAKE-FOO (&REST ELEMENTS) `(LIST ,@ELEMENTS)) (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.