Issue THE-VALUES Writeup

Issue:                 THE-VALUES

References:            THE special form, VALUES type specifier

Related issues:        THE-AMBIGUITY

Category:              CLARIFICATION

Edit history:          v1, 24 Feb 1991, Sandra Loosemore

Status: X3J13 passed v1:RETURN-NUMBER-RECEIVED on vote of 10-1, Mar-91


Problem description:


  Are the following forms valid, and if so how many values are returned 

  in each case?


  #1: (truncate 7 3)

  #2: (the integer (truncate 7 3))

  #3: (the (values integer) (truncate 7 3))

  #4: (the (values integer integer) (truncate 7 3))

  #5: (the (values integer integer symbol) (truncate 7 3))


There are two proposals, RETURN-NUMBER-RECEIVED and STRICT-MATCHING.


Proposal (THE-VALUES:RETURN-NUMBER-RECEIVED):


  (1) Clarify that the THE special form returns the exact values as 

      its subform.


  (2) Clarify that it is not an error if the subform returns more values 

      than what is specified by the type specifier, provided that the

      values for which types are declared are indeed of those types.

      (In particular, if the type specifier is not a VALUES type 

      specifier, multiple values may still be returned.)


  (3) Clarify that it is not an error if the subform returns fewer values

      than what is specified by the type specifier.  Missing values are

      treated as NIL for the purposes of checking their types.


  Rationale for proposal RETURN-NUMBER-RECEIVED:


    Point by point,

  

    (1) THE is supposed to act purely as a declaration.  It should not

        change the number of values that are returned by its subform.

  

    (2) This is consistent with the way extra values are simply ignored

        by MULTIPLE-VALUE-BIND and friends.

  

    (3) This is consistent with the way missing values are treated as NIL

        by MULTIPLE-VALUE-BIND and friends.



Proposal (THE-VALUES:STRICT-MATCHING):


  (1) Clarify that the THE special form returns the exact values as 

      its subform.


  (2) Clarify that it is an error if the number and syntax of values 

      returned by the subform do not match the given type specifier

      exactly.  In particular, if the type specifier is not a VALUES

      type specifier then only a single value may be returned.


  Rationale for proposal STRICT-MATCHING:

  

    Point by point,

  

    (1) THE is supposed to act purely as a declaration.  It should not

        change the number of values that are returned by its subform.


    (2) This is consistent with the description of the VALUES type 

        specifier as specifying the arguments of a function which could

        capture the values via MULTIPLE-VALUE-CALL.  Since argument

        mismatches are clearly "is an error" situations, this should be

        too.  This is also apparently closer to current practice than

        proposal RETURN-NUMBER-RECEIVED.


Examples:


  Under proposal RETURN-NUMBER-RECEIVED, all of the examples given in 

  the problem description section are valid and return the values 2 and 1.


  Under proposal STRICT-MATCHING, tests 2, 3, and 5 are in error.  Tests

  1 and 4 are valid and return the values 2 and 1.

  

Current Practice:


  Lucid CL version 4.0 signals an error in cases 3 and 5 (at least in the

  interpreter).  It returns the values 2 and 1 in case 2.


  Allegro CL version 3.1 signals an error in cases 2, 3, and 5.


Cost to Implementors:


  Probably small.  Neither proposal requires checking to be performed in

  any situation.  However under proposal RETURN-NUMBER-RECEIVED, some 

  implementations will have to be changed to not do some checks they 

  currently do.


Cost to Users:


  Users may find the stricter error checking permitted by proposal 

  STRICT-MATCHING helpful.  However, if implementations that currently

  do not check these errors begin to do so, some programs that currently

  work in those implementations to stop working.


Cost of non-adoption:


  Confusion about what the THE type specifier really means.


Performance impact:


  Probably small, but proposal STRICT-MATCHING might permit compilers

  to be more aggressive about some kinds of type and multiple-value

  optimizations.


Benefits:


  The cost of non-adoption is avoided.


Esthetics:


  Seems to depend on who you talk to.  In the best of all possible

  worlds, MULTIPLE-VALUE-BIND and friends would have ordinary lambda-list

  syntax and follow the same rules as for ordinary lambda-binding, so there 

  wouldn't be the an inconsistency with MULTIPLE-VALUE-CALL to have to

  resolve one way or the other.


Discussion:



  This issue was discussed on the common-lisp mailing list some time ago,

  but never written up.  Here's the last message from that discussion:


    Date: Thu, 20 Apr 1989 17:53 EDT

    From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>

    

        Date: Thu, 20 Apr 89 16:59:15 EDT

        From: Guy Steele <gls@Think.COM>

           ...

           PROPOSAL:

    

           What it boils down to, is that THE should check only as many types

           as requested (and pass back only as many).

    

        No, this is not cool.  THE is supposed to act purely as a declaration,

        but you are changing it to require it to pass on only as many values

        as the type specifer indicates.  This could change the semantics of

        a suitably devious program.

    

        Better to say that it checks as many types as requsted, but passes on

        exactly the values it receives.

        --Guy

    

    Even though I agree with your position, I think it's worth our writing up

    a clarification issue to make sure we're all agreed and that it's 100% 

    clear in the ANSI spec.

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