Hello,

Currently, a back-end is a symbol which may point to an entry in
`org-export-registered-backends' variable. Therefore a back-end must be
registered (with a unique name) before one can call it. Or, to put it
differently, it is impossible to use an anonymous and temporary
back-end.

This is not satisfying for developers, as there are situations when you
need to use a one-shot back-end but don't want to clutter registered
back-ends list. You can of course let-bind
`org-export-registered-backends' to something else, but it leads to
clunky code. The process should be abstracted a bit more.

The following (long) patches address this by defining back-ends as
structures (see `defstruct'), possibly anonymous and by separating
creation from registration process. It allows to quickly create and use
temporary back-ends. In the example below, we quickly export a string
using a temporary back-end:

  (org-export-string-as
   "* H1\n** H2\nSome string"
   (org-export-create-backend
    :transcoders
    '((headline . (lambda (h contents i)
                    (let ((m (make-string (org-export-get-relative-level h i) ?=)))
                      (concat m " " (org-element-property :raw-value h) " " m "\n"
                              contents))))
      ;; Contents only.
      (section . (lambda (e c i) c))
      (paragraph . (lambda (e c i) c)))))

It is also possible to create a temporary derived back-end. The
following export will use registered `latex' back-end, excepted for
`bold' type objects.

  (org-export-string-as
     "Some *bold* /string/"
     (org-export-create-backend
      :parent 'latex
      :transcoders '((italic . (lambda (o c i) (format "\\texit{%s}" c)))))
     'body-only)

Besides `org-export-create-backend', tools provided are:
 - `org-export-get-backend'
 - `org-export-register-backend'
 - `org-export-get-all-transcoders' (handles inheritance)
 - `org-export-get-all-options' (handles inheritance)
 - `org-export-get-all-filters' (handles inheritance)

At a higher level, `org-export-define-backend' and
`org-export-define-derived-backend' do not change (they are equivalent
to create and register in a row). So this change only matters for
back-end developers who used advanced features like
`org-export-with-translations' (which should now be
`org-export-with-backend' coupled with an anonymous back-end).

Also, it leads to a cleaner implementation as it removes the confusion
between a back-end and its name.

The next step after applying this patch will be to make
orgtbl-to-BACKEND functions use anonymous functions in order to
support :splice property, which is tedious with the new export
framework.

Feedback welcome.


Regards,

-- 
Nicolas Goaziou