Drop Emacs 23 support

[gracjan]

We have introduced lexical binding, currently we use it in backward compatible manner. This requires us to jump through some hops to keep the appearances. An example can be found here:

https://github.com/gracjan/haskell-mode/blob/9a6fa5073ac505c41f085d492f39e8ff415a7d13/haskell-indentation.el#L899-L904

  (haskell-indentation-with-starter
   `(lambda ()
      (let ((implicit-layout-active nil))
        (haskell-indentation-separated
         #',parser ,sep ,stmt-sep)))
   end))

On top of this Emacs 23 lacks build in cl-lib and does not support package.el out of the box. Emacs 23 is old: 23.1 was released 2009-07-29, 23.4 was released 2012-01-29.

Emacs 24 and coming Emacs 25 are much better and are easy to install on every supported platform.

Steps to take:

• make sure we do not alienate anybody by dropping Emacs 23
• make last release supporting Emacs 23
• remove TravisCI build for Emacs 23
• make haskell-mode fail if enabled under Emacs less than 24
• add a note in documentation that release 13.18 was the last one to support Emacs 23
• remove other mentions of Emacs 23 from the documentation
• remove Emacs 23 from README
• remove Emacs 23 from wiki

Comments?

[mrkkrp]

Of course, let's drop it. I can't image any devoted Emacs user still uses Emacs 23.

[gracjan]

There might be people stuck on not-their-own machines without a possibility to upgrade.

But then those would be more of Emacs 22 kind and that is already as good as dead.

[geraldus]

There was some activity recently from people using Emacs 23, so let's wait a bit for feedback.

[gracjan]

I was thinking about dropping emacs 23 in the beginning of year 2016.

[gracjan]

What about XEmacs? Is it active and used?

[purcell]

FWIW, cl-lib from ELPA works fine with Emacs 23, and plenty of people use package.el with Emacs 23. So those shouldn't be relevant factors here. Lexical binding may be sufficient justification, but I don't personally find it a compelling argument.

[gracjan]

I know it works. It just isn't there by default.

[gracjan]

One more reason to use Emacs 24 is syntax-propertize-function that is not available in Emacs 23 but is mandatory to properly support Haskell's token lexing for font-lock.