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#### 6.6.3.1 Document Server Internals

Adding new document types to be recognized by `nndoc` isn’t difficult. You just have to whip up a definition of what the document looks like, write a predicate function to recognize that document type, and then hook into `nndoc`.

First, here’s an example document type definition:

    (mmdf
     (article-begin .  "^\^A\^A\^A\^A\n")
     (body-end .  "^\^A\^A\^A\^A\n"))

The definition is simply a unique name followed by a series of regexp pseudo-variable settings. Below are the possible variables—don’t be daunted by the number of variables; most document types can be defined with very few settings:

`first-article`

  If present, `nndoc` will skip past all text until it finds something that match this regexp. All text before this will be totally ignored.

`article-begin`

  This setting has to be present in all document type definitions. It says what the beginning of each article looks like. To do more complicated things that cannot be dealt with a simple regexp, you can use `article-begin-function` instead of this.

`article-begin-function`

  If present, this should be a function that moves point to the beginning of each article. This setting overrides `article-begin`.

`head-begin`

  If present, this should be a regexp that matches the head of the article. To do more complicated things that cannot be dealt with a simple regexp, you can use `head-begin-function` instead of this.

`head-begin-function`

  If present, this should be a function that moves point to the head of the article. This setting overrides `head-begin`.

`head-end`

  This should match the end of the head of the article. It defaults to ‘`^$`’—the empty line.

`body-begin`

  This should match the beginning of the body of the article. It defaults to ‘`^\n`’. To do more complicated things that cannot be dealt with a simple regexp, you can use `body-begin-function` instead of this.

`body-begin-function`

  If present, this function should move point to the beginning of the body of the article. This setting overrides `body-begin`.

`body-end`

  If present, this should match the end of the body of the article. To do more complicated things that cannot be dealt with a simple regexp, you can use `body-end-function` instead of this.

`body-end-function`

  If present, this function should move point to the end of the body of the article. This setting overrides `body-end`.

`file-begin`

  If present, this should match the beginning of the file. All text before this regexp will be totally ignored.

`file-end`

  If present, this should match the end of the file. All text after this regexp will be totally ignored.

So, using these variables `nndoc` is able to dissect a document file into a series of articles, each with a head and a body. However, a few more variables are needed since not all document types are all that news-like—variables needed to transform the head or the body into something that’s palatable for Gnus:

`prepare-body-function`

  If present, this function will be called when requesting an article. It will be called with point at the start of the body, and is useful if the document has encoded some parts of its contents.

`article-transform-function`

  If present, this function is called when requesting an article. It’s meant to be used for more wide-ranging transformation of both head and body of the article.

`generate-head-function`

  If present, this function is called to generate a head that Gnus can understand. It is called with the article number as a parameter, and is expected to generate a nice head for the article in question. It is called when requesting the headers of all articles.

`generate-article-function`

  If present, this function is called to generate an entire article that Gnus can understand. It is called with the article number as a parameter when requesting all articles.

`dissection-function`

  If present, this function is called to dissect a document by itself, overriding `first-article`, `article-begin`, `article-begin-function`, `head-begin`, `head-begin-function`, `head-end`, `body-begin`, `body-begin-function`, `body-end`, `body-end-function`, `file-begin`, and `file-end`.

Let’s look at the most complicated example I can come up with—standard digests:

    (standard-digest
     (first-article . ,(concat "^" (make-string 70 ?-) "\n\n+"))
     (article-begin . ,(concat "\n\n" (make-string 30 ?-) "\n\n+"))
     (prepare-body-function . nndoc-unquote-dashes)
     (body-end-function . nndoc-digest-body-end)
     (head-end . "^ ?$")
     (body-begin . "^ ?\n")
     (file-end . "^End of .*digest.*[0-9].*\n\\*\\*\\|^End of.*Digest *$")
     (subtype digest guess))

We see that all text before a 70-width line of dashes is ignored; all text after a line that starts with that ‘`^End of`’ is also ignored; each article begins with a 30-width line of dashes; the line separating the head from the body may contain a single space; and that the body is run through `nndoc-unquote-dashes` before being delivered.

To hook your own document definition into `nndoc`, use the `nndoc-add-type` function. It takes two parameters—the first is the definition itself and the second (optional) parameter says where in the document type definition alist to put this definition. The alist is traversed sequentially, and `nndoc-type-type-p` is called for a given type type. So `nndoc-mmdf-type-p` is called to see whether a document is of `mmdf` type, and so on. These type predicates should return `nil` if the document is not of the correct type; `t` if it is of the correct type; and a number if the document might be of the correct type. A high number means high probability; a low number means low probability with ‘`0`’ being the lowest valid number.

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