Hi,

We have been discussing with Artur an issue that will appear with seq.el
once we'll have Emacs 25.1 out.

Quoting Artur:

  Hi Nico, I just realised a problem that we're about to face. Once Emacs
  25 is released,there'll be a version of seq.el (2.x) in the wild, with a
  fixed set of features.  Later on, if you add new features to the seq.el
  that's on Elpa (whose version is 1.y) these features will be impossible
  to use without issue.
  
  For instance, let's say you add seq-map-indexed on version 1.15, and I
  want to use it on a package. For that, I would require seq.el version
  1.15 on my package.
  
  However, I have no guarantee that I'll get that. If the user is running
  Emacs 25.1, they'll have a seq.el installed with version 2.something
  (which satisfies the dependency) but does not contain map indexed.
  
  The same will happen for all newly added features. And seriously reduces
  the usefulness of seq as dependency package.


One solution would be to have a single seq package on Elpa (carrying a
2.x version number), that offered both implementations. It would have 3
files:

- seq.el
- seq-25.el
- seq-24.el

The file seq-24.el would have the exact same contents as the current
seq-1.x we have on Elpa except:

- The file-header and the provide statement would say seq-24

- It wouldn't have the package pseudo-headers (no version number, no
  keywords, etc).

The file seq-25.el would have the exact same contents as the current
seq-2.x we have in Emacs except:

- The file-header and the provide statement would say seq-25

- It wouldn't have the package pseudo-headers (no version number, no
  keywords, etc).

This way there is only one version of the seq package. Packages that
want some recent functionality added in minor-version 12 can simply
depend on 2.12, regardless of the Emacs version.  Also, packages that
want the generics functionality can simply depend on (seq "2.12") and on
(emacs "25").

The problem with this solution is that we'd get tons of byte-compilation
warnings, becaus even if seq-25.el is never used with Emacs 24.x, it
will get byte-compiled.

We were wondering if there would be a way to prevent byte-compilation
only on some emacs versions, or if someone have a better solution to the
problem.

Cheers,
Nico
-- 
Nicolas Petton
http://nicolas-petton.fr