Issue DESTRUCTURING-BIND Writeup

Issue:        DESTRUCTURING-BIND

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.

    (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.


Proposal (DESTRUCTURING-BIND:NEW-MACRO):


  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:


  (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)


Rationale:


  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.


Benefits:


  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.


Aesthetics:


  Defining this macro centrally for the Common Lisp community will reduce

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


Discussion:


  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

  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.