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11.7.2.2 Optional Back End Functions
(nnchoke-retrieve-groups GROUPS &optional SERVER)
groups is a list of groups, and this function should request data on all those groups. How it does it is of no concern to Gnus, but it should attempt to do this in a speedy fashion.

The return value of this function can be either active or group, which says what the format of the result data is. The former is in the same format as the data from nnchoke-request-list, while the latter is a buffer full of lines in the same format as nnchoke-request-group gives.

          group-buffer = *active-line / *group-status

(nnchoke-request-update-info GROUP INFO &optional SERVER)
A Gnus group info (see Group Info) is handed to the back end for alterations. This comes in handy if the back end really carries all the information (as is the case with virtual and imap groups). This function should destructively alter the info to suit its needs, and should return a non-nil value (exceptionally, nntp-request-update-info always returns nil not to waste the network resources).

There should be no result data from this function.

(nnchoke-request-type GROUP &optional ARTICLE)
When the user issues commands for “sending news” (F in the summary buffer, for instance), Gnus has to know whether the article the user is following up on is news or mail. This function should return news if article in group is news, mail if it is mail and unknown if the type can't be decided. (The article parameter is necessary in nnvirtual groups which might very well combine mail groups and news groups.) Both group and article may be nil.

There should be no result data from this function.

(nnchoke-request-set-mark GROUP ACTION &optional SERVER)
Set/remove/add marks on articles. Normally Gnus handles the article marks (such as read, ticked, expired etc.) internally, and store them in ~/.newsrc.eld. Some back ends (such as IMAP) however carry all information about the articles on the server, so Gnus need to propagate the mark information to the server.

action is a list of mark setting requests, having this format:

          (RANGE ACTION MARK)

range is a range of articles you wish to update marks on. action is add or del, used to add marks or remove marks (preserving all marks not mentioned). mark is a list of marks; where each mark is a symbol. Currently used marks are read, tick, reply, expire, killed, dormant, save, download, unsend, and forward, but your back end should, if possible, not limit itself to these.

Given contradictory actions, the last action in the list should be the effective one. That is, if your action contains a request to add the tick mark on article 1 and, later in the list, a request to remove the mark on the same article, the mark should in fact be removed.

An example action list:

          (((5 12 30) 'del '(tick))
           ((10 . 90) 'add '(read expire))
           ((92 94) 'del '(read)))

The function should return a range of articles it wasn't able to set the mark on (currently not used for anything).

There should be no result data from this function.

(nnchoke-request-update-mark GROUP ARTICLE MARK)
If the user tries to set a mark that the back end doesn't like, this function may change the mark. Gnus will use whatever this function returns as the mark for article instead of the original mark. If the back end doesn't care, it must return the original mark, and not nil or any other type of garbage.

The only use for this I can see is what nnvirtual does with it—if a component group is auto-expirable, marking an article as read in the virtual group should result in the article being marked as expirable.

There should be no result data from this function.

(nnchoke-request-scan &optional GROUP SERVER)
This function may be called at any time (by Gnus or anything else) to request that the back end check for incoming articles, in one way or another. A mail back end will typically read the spool file or query the POP server when this function is invoked. The group doesn't have to be heeded—if the back end decides that it is too much work just scanning for a single group, it may do a total scan of all groups. It would be nice, however, to keep things local if that's practical.

There should be no result data from this function.

(nnchoke-request-group-description GROUP &optional SERVER)
The result data from this function should be a description of group.
          description-line = name <TAB> description eol
          name             = <string>
          description      = <text>

(nnchoke-request-list-newsgroups &optional SERVER)
The result data from this function should be the description of all groups available on the server.
          description-buffer = *description-line

(nnchoke-request-newgroups DATE &optional SERVER)
The result data from this function should be all groups that were created after ‘date’, which is in normal human-readable date format (i.e., the date format used in mail and news headers, and returned by the function message-make-date by default). The data should be in the active buffer format.

It is okay for this function to return “too many” groups; some back ends might find it cheaper to return the full list of groups, rather than just the new groups. But don't do this for back ends with many groups. Normally, if the user creates the groups herself, there won't be too many groups, so nnml and the like are probably safe. But for back ends like nntp, where the groups have been created by the server, it is quite likely that there can be many groups.

(nnchoke-request-create-group GROUP &optional SERVER)
This function should create an empty group with name group.

There should be no return data.

(nnchoke-request-expire-articles ARTICLES &optional GROUP SERVER FORCE)
This function should run the expiry process on all articles in the articles range (which is currently a simple list of article numbers.) It is left up to the back end to decide how old articles should be before they are removed by this function. If force is non-nil, all articles should be deleted, no matter how new they are.

This function should return a list of articles that it did not/was not able to delete.

There should be no result data returned.

(nnchoke-request-move-article ARTICLE GROUP SERVER ACCEPT-FORM &optional LAST)
This function should move article (which is a number) from group by calling accept-form.

This function should ready the article in question for moving by removing any header lines it has added to the article, and generally should “tidy up” the article. Then it should eval accept-form in the buffer where the “tidy” article is. This will do the actual copying. If this eval returns a non-nil value, the article should be removed.

If last is nil, that means that there is a high likelihood that there will be more requests issued shortly, so that allows some optimizations.

The function should return a cons where the car is the group name and the cdr is the article number that the article was entered as.

There should be no data returned.

(nnchoke-request-accept-article GROUP &optional SERVER LAST)
This function takes the current buffer and inserts it into group. If last in nil, that means that there will be more calls to this function in short order.

The function should return a cons where the car is the group name and the cdr is the article number that the article was entered as.

The group should exist before the back end is asked to accept the article for that group.

There should be no data returned.

(nnchoke-request-replace-article ARTICLE GROUP BUFFER)
This function should remove article (which is a number) from group and insert buffer there instead.

There should be no data returned.

(nnchoke-request-delete-group GROUP FORCE &optional SERVER)
This function should delete group. If force, it should really delete all the articles in the group, and then delete the group itself. (If there is such a thing as “the group itself”.)

There should be no data returned.

(nnchoke-request-rename-group GROUP NEW-NAME &optional SERVER)
This function should rename group into new-name. All articles in group should move to new-name.

There should be no data returned.