Issue NTH-VALUE Writeup

Status: Passed, as amended, Jan 89 X3J13

Issue:         NTH-VALUE

References:    Multiple values, pp. 133-139

Category:      ADDITION

Edit history:  16-Aug-88, Version 1 by Pierson

               01-Oct-88, Version 2 by Pitman (minor edits)

               5-Oct-88, Version 3 by Masinter

			(add Current Practice as per Gray)

                8-Dec-88, Version 4 by Masinter

			(add comments to discussion)

                17-Mar-89, Version 5, Masinter (as amended)


Problem description:


  The set of operations on multiple values in Common Lisp is incomplete:


  The only ways to retrieve just one of several return values (other than

  the first) are:


   - Bind several variables and then ignore all but one.

     eg, (MULTIPLE-VALUE-BIND (X Y Z) <exp> (DECLARE (IGNORE X Y)) Z)

     This is somewhat cumbersome to write and not perspicuous.


   - Get a list of all return values and select from that.

     eg, (THIRD (MULTIPLE-VALUE-LIST <exp>))

     This is somewhat cumbersome, not perspicuous, and creates

     needless garbage.


Proposal (NTH-VALUE:ADD):


  Add a new macro NTH-VALUE, described as


  NTH-VALUE n form                                               [Macro]


  N must be a non-negative integer.

  Evaluates the FORM and returns the Nth value returned by the form as

  a single value.  N is 0-based, i.e. the first returned value is 

  value 0 (for consistency with NTH and NTHCDR). Both N and FORM are

  evaluated, in left-to-right order.


  NTH-VALUE returns NIL if N is greater than or equal to the number

  of values returned by FORM.


Examples:


  With this proposal MOD could be defined as:


  (DEFUN MOD (NUMBER DIVISOR)

    (NTH-VALUE 1 (FLOOR NUMBER DIVISOR)))


  The same code would currently be:


  (DEFUN MOD (NUMBER DIVISOR)

    (MULTIPLE-VALUE-BIND (DIVIDEND REMAINDER)

        (FLOOR NUMBER DIVISOR)

      (DECLARE (IGNORE DIVIDEND))

      REMAINDER))


Rationale:


  This corrects the stated problem.


Current practice:


  The TI Explorer and LMI Lambda have this feature.

 

Cost to Implementors:


  Writing the macro version is fairly straightforward.


  Some will choose to implement compiler hooks so that code written with

  NTH-VALUE will be as efficient as possible. This may involve some

  additional work, but presumably nothing major.


Cost to Users:


  This is an upward-compatible change.


Cost of non-Adoption:


  The occassional code where this comes up may be one or more of 

  clumsier to write, more difficult to read, or less efficient.

  (The feature is rarely used in implementations that have it.)


Benefits:


  The cost of non-adoption is avoided.


Aesthetics:


  While it does add another function to the language it removes

  some need for the hairier multiple-value forms.


Discussion:


  Pitman proposed this in the very late pre-CLtL days. It was

  rejected then because it was too late in the cycle.


  There was not strong sentiment for including this feature

  in Common Lisp, but no active opposition.


Comments at the October 1988 X3J13 meeting:


"Trivial, gratuitous."


"Not trivial -- allows index computation. Hard to do this

 in a portable, efficient way."


"Says he has an NTH-VALUE macro for a portable system that he

 uses (which exploits the computed index feature) and that it's

 a gross kludge in one implementation to make it efficient."