Issue EXTRA-RETURN-VALUES Writeup

Version 6 (not shown here) is what passed.

It was modified by amendment to strike everything in the 

proposal section except for the first paragraph. 

v6 is the one that passed.

 -kmp (Aug 89)


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Issue:        EXTRA-RETURN-VALUES

References:   Chapter 1, Section 1.5, Working draft of standard

Category:     Clarification

Edit history: 8-JAN-89, Version 1 by Masinter

	      3-FEB-89, Version 2 by Chapman

	      10-MAR-89, Version 3 by Chapman

              30-MAY-89, Version 4 by Pierson

              20-JUN-89, Version 5 by Chapman

 

 

Problem: Is it OK to return extra values from Common Lisp functions?

 

Proposal: EXTRA-RETURN-VALUES:NO

 

A function that is specified by the standard must return EXACTLY the number 

of return values specified by the standard.  

 

The following functions are explicited permitted to have additional

return values: 

 

  gethash

  remhash

  get-macro-character  

  get-dispatch-macro-character

  read

  read-preserving-whitespace

  read-delimited-list

  read-line

  read-char

  unread-char

  peek-char

  listen

  read-char-no-hang

  read-from-string

  parse-integer

  read-byte

  write-to-string

  prin1-to-string

  princ-to-string

  write-string

  format

  y-or-n-p

  yes-or-no-p

  open

  rename-file

  probe-file

  directory

  trace

  untrace

  apropos-list

  describe 


 

 

Rationale:

 

The reason is that additional arguments are visible to otherwise

portable programs. For instance, (multiple-value-call #'cons (floor x

y)) looks portable, but it will try to pass the wrong number of

arguments to CONS if FLOOR returns an extra value.


The order of the above list is by page number in CLtL.

 

Current Practice:

 

At least one implementation returns extra values for certain functions

not currently specified to return those values.

 

Adoption Cost:

 

Implementations and their associated documentation that now contain 

functions that return extra values will have to change.

 

Benefits:

 

Programs will potentially become more portable.

 

Conversion Cost:

 

See Adoption Cost.

 

Aesthetics:

 

None.

 

Discussion:

Pierson created the original list, Moon revised the list.

 

Moon says:

"The ones that I care the most about are gethash and read-line."