Subject: bug#22101: Emacs-25: inaccuracy in documentation of `mapconcat' in .../lispref/functions.texi

From: Alan Mackenzie <[email-masked]>
Date: Sun, 6 Dec 2015 21:41:42 +0000

Hello, Eli.

On Sun, Dec 06, 2015 at 06:04:46PM +0200, Eli Zaretskii wrote:
> > Date: Sun, 6 Dec 2015 10:26:22 +0000
> > From: Alan Mackenzie <[email-masked]>

[ .... ]

> > diff --git a/doc/lispref/functions.texi b/doc/lispref/functions.texi
> > index 8835667..1b949f2 100644
> > --- a/doc/lispref/functions.texi
> > +++ b/doc/lispref/functions.texi
> > @@ -861,13 +861,15 @@ Mapping Functions

> >  @defun mapconcat function sequence separator
> >  @code{mapconcat} applies @var{function} to each element of
> > -@var{sequence}: the results, which must be strings, are concatenated.
> > -Between each pair of result strings, @code{mapconcat} inserts the string
> > +@var{sequence}: the results, which must be sequences, are
> > +concatenated.  These result sequences are usually strings, but may
> > +also be lists of numbers or vectors of numbers.  Between each pair of
> > +result sequences, @code{mapconcat} inserts the sequence
> >  @var{separator}.  Usually @var{separator} contains a space or comma or
> >  other suitable punctuation.

> IMO, this errs on the other side: it seems to allow sequences that
> will be rejected by mapconcat or by concat that it calls.  I suggest
> the following alternative wording:

>  @code{mapconcat} applies @var{function} to each element of
>  @var{sequence}; the results, which must be sequences of characters
>  (strings, vectors, or lists), are concatenated into a single string
>  return value.  Between each pair of result sequences,
>  @code{mapconcat} inserts the characters from @var{separator}, which
>  also must be a string, or a vector or list of characters.

>  The argument @var{function} must be a function that can take one
>  argument and return a sequence of characters: a string, a vector, or
>  a list.  The argument @var{sequence} can be any kind of sequence
>  except a char-table; that is, a list, a vector, a bool-vector, or a
>  string.

> WDYT?

I like very much the way the omission of a comma in "or a vector or list
of characters" attaches the "of characters" to both  "vector" and "list".
:-)

I have an uneasy feeling that the two paragraphs might be a bit dense for
a newish Lisp programmer, who's just come across `mapconcat' for the
first time, and needs to know what it does.  I have an urge to insert a
"(@pxref{Sequence Type})" somewhere, but can't really see where - maybe
right at the end - or maybe at the end of the first paragraph.  The
problem is "string" is concrete and means what it means, but "sequence"
is a rather vague sounding abstract word (even though Emacs gives it a
precise meaning).

Yes, I think "(@pxref{Sequence Type}) should be inserted at the end of
the first paragraph.

What do you think of that?

-- 
Alan Mackenzie (Nuremberg, Germany).