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Re: Skeleton angle brackets '<' and '>'


From: Alan Mackenzie
Subject: Re: Skeleton angle brackets '<' and '>'
Date: Wed, 6 Jan 2016 17:59:39 +0000 (UTC)
User-agent: tin/2.3.1-20141224 ("Tallant") (UNIX) (FreeBSD/10.2-RELEASE-p7 (amd64))

B.V. Raghav <address@hidden> wrote:
> Hi,

> There are smart tools for the purpose, but I prefer to stick to the old
> school for auto pairing.

> Electric Pair mode and Skeleton pair, like this:
> (electric-pair-mode t)
> (setq skeleton-pair t)

> While in the c++ mode, (hopefully true for all c-modes)
> I am able to successfully pair (), {}, and [], with a single key press,
> i.e. of the preceding character of the pair.

> I also want to pair the angle brackets, i.e. `<' and `>' characters.

Be careful what you want!  Do you also want automatically to pair up
"less than" with "greater than"?  It could get pretty tedious if, every
time you wanted to write "i < 10", you got "i <>", forcing you to delete
the ">" before writing in the "10".

This overloading of < and > with two meanings (actually, three when you
count "#include <stdio.h>", but that causes few problems) causes great
difficulties for C++ Mode.  Only after extensive analysis can the mode
determine that a < and a > are a pair of template delimiters, and even
then, can't do it with 100% certainty.

For example, how many parameters are passed to foo in the following:

    foo (a < b, c > d);

?  It might be a function call with two relational expressions, it might
be a declaration with the single parameter d of templated type a <b , c>.
Such was the wisdom of C++'s designers.

When it is determined there is a pair of delimiters, they are marked as
such with syntax-table text properties, but this marking is always done
on both delimiters at once.

In short, there is little chance of ever being able to insert angle
brackets as a pair in C++ Mode.  Sorry!

> The documentation for the variable sekeleton-pair-alist says,

> [...]
> Each alist element, which looks like (ELEMENT ...), is passed to
> `skeleton-insert' with no interactor. Variable `str' does nothing.

> Elements might be (?` ?` _ "''"), (?\( ? _ ")") or(?{ \n > _ \n ?} >).
> [...]

> I am sorry to say, I cant make a head or tail of this cryptic value
> suggested for the variable.

Sorry to say this, but neither can I.  I don't know what an "interactor"
is, variable `str' is obscure, and it is totally unclear what the various
example elements might mean.

> Help solicited.

Can I suggest you submit a bug report for this dreadful doc string?

> Thanks,
> r
> -- 
> (B.V. Raghav)

-- 
Alan Mackenzie (Nuremberg, Germany).



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