[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: THE VALUES
Date: Wed, 23 Jul 86 12:54 EDT
From: David C. Plummer <DCP@QUABBIN.SCRC.Symbolics.COM>
To: Rob MacLachlan <RAM@C.CS.CMU.EDU>, common-lisp@SU-AI.ARPA
Subject: THE VALUES
Date: Wed, 23 Jul 1986 09:50 EDT
From: Rob MacLachlan <RAM@C.CS.CMU.EDU>
Well, I think everyone is wrong about this one. (THE (VALUES ...) ...)
specifies the functional type of the continuation. This is in effect
what CLTL already says: "...indicates the parameter list of a function
that, when given to multiple-value-call along with the values, would
be suitable for receiving those values."
I strongly disagree with any proposal which gives THE mandatory
run-time semantics. THE is a declaration --- its presence should not
change the meaning of a correct program. If the form returns too many
values, then the program is in error and the result is undefined.
I agree that it is highly desirable to be able to specify the
type of the first value without worrying about the actual number of
values. To this end, I propose that (THE FOO ...) should be
synonomous with the (THE (VALUES FOO &REST T) ...). This allows one
to say things like (THE FIXNUM (TRUNCATE ...)).
OK, I can buy that. I still want a form for run-time semantics for the
purposes of multiple-value-call and multiple-value-list. Something like
where <something> is not evaluated and can be either a non-negative integer
which is the number of values, or a list of type specifiers. (An
integer is shorthand for (t t ...).) The take-values form returns
exactly the number of specified values, by truncating or padding the
values returned by <form> as necessary. This can be naively implemented
as a macro given what is currently in CLtL, but I think it is useful
enough for some compilers to take advantage of it that it should be
considered part of the language.
Hmmm... I'll think about this one. I dont like the name or the
integer stuff, but I think there is a need for such a form.
It should also be clarified that the VALUES type specifier may
- THE VALUES
- From: David C. Plummer <DCP@QUABBIN.SCRC.Symbolics.COM>