CLHS: Issue DESTRUCTURING-BIND Writeup

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## 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.

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