## 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.

(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.

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

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.

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