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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Issue THE-AMBIGUITY Writeup</TITLE>
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<H2>Issue THE-AMBIGUITY Writeup</H2>
<PRE><B>Forum:</B> cleanup<P>
<B>Issue:</B> <A HREF="iss346.htm">THE-AMBIGUITY</A><P>
<B>References:</B> <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> (page 161)<P>
<B>Category:</B> CLARIFICATION<P>
<B>Edit history:</B> 21-Oct-88, version 1 by Rees<P>
11-Jan-89, version 2 by Masinter (fix typos)<P>
<P>
<B>Problem description:<P>
</B><P>
CLtL does not explicitly say whether the type specifier in a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A><P>
form may be any type specifier or must be a type specifier suitable<P>
for discrimination. Although <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> is decsribed as a &quot;declaration&quot;<P>
form, some CL implementations have assumed that the specifier must<P>
be for discrimination, and disallow e.g.,<P>
<P>
(<A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> (T T) <A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A>) #'<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A>)<P>
<P>
We should either say that the implementations are right, or<P>
explicitly say that they are wrong, since this case is easily<P>
overlooked.<P>
<P>
<B>Proposal (THE-AMBIGUITY:FOR-DECLARATION):<P>
</B><P>
Clarify that the type specifier in<P>
<P>
(<A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> type exp)<P>
<P>
may be any valid type specifier. In the case that exp returns one<P>
value and type is not a <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> type specifier, (<A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> type exp) is<P>
equivalent to<P>
<P>
(<A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>LET</B></A> ((g exp))<P>
(<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> (<A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>TYPE</B></A> type g))<P>
g)<P>
<P>
where &quot;g&quot; is a <A REL=DEFINITION HREF="../Body/f_gensym.htm#gensym"><B>gensym</B></A>.<P>
<P>
<B>Proposal (THE-AMBIGUITY:FOR-DISCRIMINATION):<P>
</B><P>
Clarify that the type specifier in<P>
<P>
(<A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> type exp)<P>
<P>
must be a valid type for discrimination, as for <A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A>, or it must<P>
be of the form (<A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> type*) where type* are all valid for discrimination.<P>
<P>
<B>Current practice:<P>
</B><P>
The Symbolics Genera and VAX LISP V2.2 interpreters signal errors for<P>
<P>
(<A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> (T T) <A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A>) #'<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A>),<P>
<P>
but this may not be intentional. CLtL would seem to allow it.<P>
<P>
<B>Test case:<P>
</B><P>
(<A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> (T T) <A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A>) #'<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A>),<P>
<P>
should return the <A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A> function under FOR-DECLARATION,<P>
and should be an error under FOR-DISCRIMINATION.<P>
<P>
<B>Cost to implementors:<P>
</B><P>
Trivial cost for THE-AMBIGUITY:FOR-DISCRIMINATION; this is a compatible<P>
restriction.<P>
<P>
For <A HREF="iss346.htm">THE-AMBIGUITY:FOR-DECLARATION</A>, implementations that do not<P>
already allow arbitrary type specifiers but which want to check that<P>
the type in a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> is satisfied would have to create an internal<P>
version of <A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> which could manage not to signal invalid-type-specifier<P>
errors in those situations where <A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> would because the type is a<P>
declaration-only one.<P>
<P>
<B>Cost to users:<P>
</B><P>
Users of implementations that support <A HREF="iss346.htm">THE-AMBIGUITY:FOR-DECLARATION</A><P>
might have to remove or change some uses of <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> in their code if the<P>
opposing alternative is adopted.<P>
<P>
<B>Benefits:<P>
</B><P>
Either way, an ambiguity in the language specification would be clarified.<P>
<P>
<B>Aesthetics:<P>
</B><P>
<A HREF="iss346.htm">THE-AMBIGUITY:FOR-DECLARATION</A> would seem to be more consistent with<P>
<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> and with the intent of <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A>, which is supposed to be a way to<P>
<A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> information for documentation and for the benefit of compilation.<P>
<P>
<B>Discussion:<P>
</B><P>
Rees supports <A HREF="iss346.htm">THE-AMBIGUITY:FOR-DECLARATION</A>.<P>
<P>
Appropriate error situation terminology must be chosen for the<P>
situation that a <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> declaration (or other declaration) is<P>
unsatisfied, but that must be done regardless of this proposal.<P>
<P>
This proposal would suggest that a function should be added to CL to<P>
do the checking that <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> would want to do:<P>
<P>
(PROBABLY-TYPEP object type-spec)<P>
<P>
[terrible name of course] returns multiple values a la SUBTYPEP: T T<P>
if the object definitely has the type, <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> T if it definitely<P>
doesn't, and T <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> (or <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> NIL?) otherwise. Assuming that an<P>
interpreted THE-expression actually checks types, you could almost<P>
define this function using the condition system and <A REL=DEFINITION HREF="../Body/f_eval.htm#eval"><B>EVAL</B></A>. (Ugh!)<P>
Without PROBABLY-TYPEP, a meta-circular interpreter is more<P>
difficult to write.<P>
<P>
If a suitable name was found for this function, the additional<P>
functionality could be suggested as an independent proposal, since<P>
regardless of the outcome of this issue, the functionality is still<P>
useful for checking <A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A>'s.<P>
<P>
Various implementation mechanisms were discussed for dealing<P>
with <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> checking.<P>
<P>
Are there any remaining type specifiers beyond the list form<P>
of the <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> type that differ between &quot;declaration&quot; and<P>
&quot;discrimination&quot;?<P>
<P>
&quot;I support FOR-DECLARATION. Lucid CL has the same bug in<P>
the interpreter as the others (a &quot;bug&quot; assuming FOR-DECLARATION).<P>
<A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> is used to check the legality of the type specifier in <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A>.&quot;<P>
<P>
In considering possible ways in which the type-checking logic<P>
for <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>THE</B></A> and <A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> might work, don't forget things like<P>
(the (not (function (t t) integer)) 7),<P>
which you would want to signal an error. I don't think this can be<P>
done with only <A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> and conditions.<P>
<P>
</PRE>
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