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


From: Yuri Khan
Subject: Re: Skeleton angle brackets '<' and '>'
Date: Thu, 7 Jan 2016 15:07:15 +0600

On Wed, Jan 6, 2016 at 11:59 PM, Alan Mackenzie <address@hidden> wrote:

>> 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".

With some careful coding convention planning, the impossible becomes easy.

> For example, how many parameters are passed to foo in the following:
>
>     foo (a < b, c > d);
>
> ?

As written, it is clearly a function call, because there are spaces on
both sides of the operators.

> 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>.

In this case, there will be no spaces on the inner side of the
brackets, and in some coding conventions, neither before the opening
bracket: f(x, y); a<b, c>. So

    Foo foo(a<b, c> d);

declares a function foo taking a single parameter named d of type a<b,
c> and returning Foo; while

    Foo foo(a < b, c > d);

declares a variable foo of type Foo passing a < b as the first
constructor argument and c > d as the second.

(The original example is invalid because C++ does not have implicit int.)

(There is one technical exception to the “no space on the inner side
of a bracket” rule: we have to put an intervening space when closing
two template argument lists in code that has to preserve C++03
compatibility.)


So, returning to the original question, I do it with smartparens this way:

    (sp-local-pair 'c++-mode "<" ">" :when '(sp-point-after-word-p))

This way, < is treated as a bracket only when used immediately after
an identifier with no intervening space. This also solves the case of
the << operator — the first angle is not paired because it is preceded
by a space, and the second angle because the immediately preceding
token (the first angle) is not a word.

(I do, however, get pairing when writing operator< or operator<<, and
no pairing when writing #include <>. Should probably code a couple of
exceptions for those.)



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