[cdr-discuss] What about CDR 3, then?

[Christophe Rhodes]

Pascal Costanza <pc at p-cos.net> writes:

> My suggestion to resolve this is as follows: Instead of just removing
> the "must signal an error of type type-error" requirement, it should
> be made more specific what the consequences are when the types are
> not list or vector, roughly as follows.
>
> - If the given type is not a recognizable subtype of sequence, then
> an error of type type-error is signaled.
> - If the given type is a recognizable subtype of sequence, but
> neither a recognizable subtype of list, nor a recognizable subtype of
> vector, the consequences are unspecified.

I think there are some more corner cases of what I think is the
original intent that I would like to preserve.  I'd replace the first
three paragraphs of the proposal proper with:

\paragraph{Proposal \texttt{(CONCATENATE-SEQUENCE-AGAIN:GENERALIZE)}:}
\begin{itemize}
\item Remove from \cl{make-sequence}, \cl{merge}, \cl{map} and
  \cl{concatenate} the requirement that ``An error [of type
  \cl{type-error}] must be signaled if the result-type is neither a
  recognizable subtype of \cl{list}, nor a recognizable subtype of
  \cl{vector}.''

\item Specify that if a type specifier is not a \recsub{} of
  \cl{sequence}, or is a \recsub{} of \texttt{(or list vector)} but
  not of either \cl{list} or \cl{vector}, then an error [of type
  \cl{type-error}] must be signaled.

\item Specify that if a type specifier is a \recsub{} of
  \cl{sequence}, but not a \recsub{} of any of the types \texttt{(or
    list vector)}, \cl{list} or \cl{vector}, then the consequences are
  unspecified.
\end{itemize}
The square brackets above reflect the slightly different error
requirements on the five functions affected; the intent of this
proposal is to preserve the original requirements.

\paragraph{Rationale:} This allows implementors to make extensions of
\cl{sequence}, as seems to have been the original intent.

\paragraph{Test Case:} No portable test case.  In implementations
extending the \cl{sequence} class, 
\begin{verbatim}
(make-sequence '<sequence-class> 8)
\end{verbatim}
need not signal an error under this proposal.

(Apologies for the use of TeX; I've put a formatted copy of the
document with this and some minor typographical changes up at
<http://www-jcsu.jesus.cam.ac.uk/~csr21/tmp/concatenate-sequence-again.pdf>)

Pascal, does that address your concern?  Does anyone else have a
comment?  I'm minded to make that document version two of CDR 3, if I
don't hear adverse commentary.

Cheers,

Christophe