Issue: NTH-VALUE (Version 3)

[masinter.pa@Xerox.COM]

change Current Practice as per Gray. I'm no longer sure if
I'm in favor of the proposal; I'd guess I'm neutral. 

I just re-read McCarthy's note about Lisp standards and
"intellectual logrolling", and it makes me pause. (I'll forward
the note to 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)

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]

  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.

Test Cases/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.