Issue: NTH-VALUE (Version 1)

[Dan L. Pierson <pierson%mist@multimax.ARPA>]

Issue:         NTH-VALUE
References:    Multiple values, pp. 133-139
Category:      ADDITION
Edit history:  Version 1 by Pierson 8/16/88
Status:        For Internal Discussion

Problem description:

The set of actions on multiple values in Common Lisp is incomplete.

Proposal (NTH-VALUE:ADD):

Add a new macro:

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 compatability with NTH and NTHCDR.

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 fills a hole in the initial definition of Common Lisp and permits
clearer code in some circumstances.

Current practice:

I don't know of any implementation that provides this feature.

Cost to Implementors:

A simple new macro needs to be written.  In addition, most
implementations will choose to implement compiler hooks so that code
written with NTH-VALUE will be as efficient as possible.

Cost to Users:

None, this is an upward-compatible change.

Cost of non-Adoption:

This feature won't be available to portable programs.

Benefits:

This makes some code that uses multiple values easier to read and
write.  In addition, it may be easier for implementations to optimize
expressions which use NTH-VALUE instead of using MULTIPLE-VALUE-BIND
and ignoring the unwanted values.

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.


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