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Issue: DESTRUCTURING-BIND (Version 1)
- To: CL-Cleanup@SAIL.Stanford.EDU
- Subject: Issue: DESTRUCTURING-BIND (Version 1)
- From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
- Date: Tue, 24 Jan 89 17:31 EST
References: The LOOP Facility (X3J13/89-004)
Edit history: 24-Jan-89, Version 1 by Pitman
Status: For Internal Discussion
Programmers have frequently requested an interface to a destructuring
bind facility similar to that available in DEFMACRO.
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.
Destructuring always existed in DEFMACRO, but since forms seen by the
evaluator are generally just lists, rather than arbitrary user-defined
abstract data types, an argument for destructuring of the second kind
in that circumstance seemed like overkill to most people.
Provide a macro called DESTRUCTURING-BIND which behaves like the
destructuring bind in DEFMACRO.
(DESTRUCTURING-BIND pattern datum . body)
evaluates datum, binds the indicated pattern variables, and then
executes the body.
Clarify that LOOP does not permit the use of &keywords in its
destructuring, and that proper lists are implicitly `&REST ignore'
(where the variable is quietly ignored).
(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)
Now that LOOP has been introduced into the language with destructuring
of the first kind, rules of internal consistency could be used to
bypass religious arguments in order to satisfy user common needs.
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
(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.
There is now considerable danger that a lot of people will write
DESTRUCTURING-BIND variants in terms of a LOOP expression that
(DESTRUCTURING-BIND ((A B) C) (FOO) (LIST A B C))
==> (LOOP FOR ((A B) C) ON (FOO) DO (RETURN (LIST A B C)))
We should help to head off the evolution of a number of uselessly
different variants by simply institutionalizing the real support
Symbolics Genera already offers this extension.
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 will reduce subtle deviations, which will
have positive aesthetic impact.
Defining this facility allows other tools like DEFMACRO and LOOP to be
defined in terms of it. That modularization of description is also a
Pitman was on the side of the abstract destructuring and isn't the
slightest bit happy about the kind of destructuring which snuck in
on the `LOOP trojan horse.' However, he thinks something like this
is the rational thing to do at this point. The details are not so
important as coming up with a nice, unified story.