# XMerL - XML processing tools for Erlang

Reference Manual

Ulf Wiger
1.0
2003-02-04
Converted xml from html

XMerL tools contains xmerl_scan; a non-validating XML processor, xmerl_xpath; a XPath implementation, xmerl for export of XML trees to HTML, XML or text and xmerl_xs for XSLT like transforms in erlang.

## xmerl_scan - the XML processor

The (non-validating) XML processor is activated through `xmerl_scan:string/[1,2]` or `xmerl_scan:file/[1,2]`. It returns records of the type defined in xmerl.hrl.

As far as I can tell, xmerl_scan implements the complete XML 1.0 spec, including:

- entity expansion

- fetching and parsing external DTDs

- contitional processing

- UniCode

- XML Names

    xmerl_scan:string(Text [ , Options ]) -> #xmlElement{}.
    xmerl_scan:file(Filename [ , Options ]) -> #xmlElement{}.

The Options are basically to specify the behaviour of the scanner. See the source code for details, but you can specify funs to handle scanner events (event_fun), process the document entities once identified (hook_fun), and decide what to do if the scanner runs into eof before the document is complete (continuation_fun).

You can also specify a path (fetch_path) as a list of directories to search when fetching files. If the file in question is not in the fetch_path, the URI will be used as a file name.

### Customization functions

The XML processor offers a number of hooks for customization. These hooks are defined as function objects, and can be provided by the caller.

The following customization functions are available. If they also have access to their own state variable, the access function for this state is identified within parentheses:

- event function (`xmerl_scan:event_state/[1,2]` )

- hook function (`xmerl_scan:hook_state/[1,2]` )

- fetch function (`xmerl_scan:fetch_state/[1,2]` )

- continuation function (`xmerl_scan:cont_state/[1,2]` )

- rules function (`xmerl_scan:rules_state/[1,2]` )

- accumulator function

- close function

For all of the above state access functions, the function with one argument (e.g. `event_fun(GlobalState)`) will read the state variable, while the function with two arguments (e.g.: `event_fun(NewStateData, GlobalState)`) will modify it.

For each function, the description starts with the syntax for specifying the function in the `Options` list. The general forms are `{Tag, Fun}`, or `{Tag, Fun, LocalState}`. The second form can be used to initialize the state variable in question.

#### User State

All customization functions are free to access a "User state" variable. Care must of course be taken to coordinate the use of this state. It is recommended that functions, which do not really have anything to contribute to the "global" user state, use their own state variable instead. Another option (used in e.g. `xmerl_eventp.erl`) is for customization functions to share one of the local states (in `xmerl_eventp.erl`, the continuation function and the fetch function both acces the `cont_state`.)

Functions to access user state:

- `xmerl_scan:user_state(GlobalState)`

- `xmerl_scan:user_state(UserState', GlobalState)`

#### Event Function

`{event_fun, fun()} | {event_fun, fun(), LocalState}`

The event function is called at the beginning and at the end of a parsed entity. It has the following format and semantics:

    fun(Event, GlobalState) ->
       EventState = xmerl_scan:event_state(GlobalState),
       EventState' = foo(Event, EventState),
       GlobalState' = xmerl_scan:event_state(EventState', GlobalState)
    end.

#### Hook Function

`{hook_fun, fun()} | {hook_fun, fun(), LocalState}`

The hook function is called when the processor has parsed a complete entity. Format and semantics:

    fun(Entity, GlobalState) ->
       HookState = xmerl_scan:hook_state(GlobalState),
       {TransformedEntity, HookState'} = foo(Entity, HookState),
       GlobalState' = xmerl_scan:hook_state(HookState', GlobalState),
       {TransformedEntity, GlobalState'}
    end.

The relationship between the event function, the hook function and the accumulator function is as follows:

1. The event function is first called with an 'ended' event for the parsed entity.

2. The hook function is called, possibly re-formatting the entity.

3. The acc function is called in order to (optionally) add the re-formatted entity to the contents of its parent element.

#### Fetch Function

`{fetch_fun, fun()} | {fetch_fun, fun(), LocalState}`

The fetch function is called in order to fetch an external resource (e.g. a DTD).

The fetch function can respond with three different return values:

        Result ::=
          {ok, GlobalState'} |
          {ok, {file, Filename}, GlobalState'} |
          {ok, {string, String}, GlobalState'}

Format and semantics:

    fun(URI, GlobalState) ->
       FetchState = xmerl_scan:fetch_state(GlobalState),
       Result = foo(URI, FetchState).  % Result being one of the above
    end.

#### Continuation Function

`{continuation_fun, fun()} | {continuation_fun, fun(), LocalState}`

The continuation function is called when the parser encounters the end of the byte stream. Format and semantics:

    fun(Continue, Exception, GlobalState) ->
       ContState = xmerl_scan:cont_state(GlobalState),
       {Result, ContState'} = get_more_bytes(ContState),
       GlobalState' = xmerl_scan:cont_state(ContState', GlobalState),
       case Result of
          [] ->
             GlobalState' = xmerl_scan:cont_state(ContState', GlobalState),
             Exception(GlobalState');
          MoreBytes ->
             {MoreBytes', Rest} = end_on_whitespace_char(MoreBytes),
             ContState'' = update_cont_state(Rest, ContState'),
             GlobalState' = xmerl_scan:cont_state(ContState'', GlobalState),
             Continue(MoreBytes', GlobalState')
       end
    end.

#### Rules Functions

`{rules, ReadFun : fun(), WriteFun : fun(), LocalState} | {rules, Table : ets()}`

The rules functions take care of storing scanner information in a rules database. User-provided rules functions may opt to store the information in mnesia, or perhaps in the user_state(LocalState).

The following modes exist:

- If the user doesn't specify an option, the scanner creates an ets table, and uses built-in functions to read and write data to it. When the scanner is done, the ets table is deleted.

- If the user specifies an ets table via the `{rules, Table}` option, the scanner uses this table. When the scanner is done, it does not delete the table.

- If the user specifies read and write functions, the scanner will use them instead.

The format for the read and write functions are as follows:

    WriteFun(Context, Name, Definition, ScannerState) -> NewScannerState.
    ReadFun(Context, Name, ScannerState) -> Definition | undefined.

Here is a summary of the data objects currently being written by the scanner:

Scanner data objects

Context | Key Value | Definition
--- | --- | ---
notation | NotationName | `{system, SL} | {public, PIDL, SL}`
elem_def | ElementName | `#xmlElement{content = ContentSpec}`
parameter_entity | PEName | `PEDef`
entity | EntityName | `EntityDef`

    ContentSpec ::= empty | any | ElemContent
    ElemContent ::= {Mode, Elems}
    Mode        ::= seq | choice
    Elems       ::= [Elem]
    Elem        ::= '#PCDATA' | Name | ElemContent | {Occurrence, Elems}
    Occurrence  ::= '*' | '?' | '+'

When <Elem> is not wrapped with <Occurrence>, (Occurrence = once) is implied.

#### Accumulator Function

`{acc_fun, fun()} | {acc_fun, fun(), LocalState}`

The accumulator function is called to accumulate the contents of an entity.When parsing very large files, it may not be desireable to do so.In this case, an acc function can be provided that simply doesn't accumulate.

Note that it is possible to even modify the parsed entity before accumulating it, but this must be done with care. `xmerl_scan` performs post-processing of the element for namespace management. Thus, the element must keep its original structure for this to work.

The acc function has the following format and semantics:

    %% default accumulating acc fun
    fun(ParsedEntity, Acc, GlobalState) ->
       {[X|Acc], GlobalState}.

    %% non-accumulating acc fun
    fun(ParsedEntity, Acc, GlobalState) ->
       {Acc, GlobalState}.

#### Close Function

The close function is called when a document (either the main document or an external DTD) has been completely parsed. When xmerl_scan was started using `xmerl_scan:file/[1,2]`, the file will be read in full, and closed immediately, before the parsing starts, so when the close function is called, it will not need to actually close the file. In this case, the close function will be a good place to modify the state variables.

Format and semantics:

    fun(GlobalState) ->
       GlobalState' = ....  % state variables may be altered

## XPATH

    xmerl_xpath:string(QueryString, #xmlElement{}) ->
    	[DocEntity]

    DocEntity :	#xmlElement{}
    		| #xmlAttribute{}
    		| #xmlText{}
    		| #xmlPI{}
    		| #xmlComment{}

The xmerl_xpath module does seem to handle the entire XPATH 1.0 spec, but I haven't tested that much yet. The grammar is defined in `xmerl_xpath_parse.yrl`. The core functions are defined in `xmerl_xpath_pred.erl`.

## Some useful shell commands for debugging the XPath parser

`c(xmerl_xpath_scan). yecc:yecc("xmerl_xpath_parse.yrl", "xmerl_xpath_parse", true, []). c(xmerl_xpath_parse). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("position() > -1")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("5 * 6 div 2")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("5 + 6 mod 2")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("5 * 6")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("5 * 6")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("-----6")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("parent::node()")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("descendant-or-self::node()")). xmerl_xpath_parse:parse(xmerl_xpath_scan:tokens("parent::processing-instruction('foo')")).`

## Erlang Data Structure Export

The idea as follows:

The Erlang data structure should look like this:

    Element:	{Tag, Attributes, Content}
    Tag :		atom()
    Attributes:	[{Key, Value}]
    Content:	[String | Element]
    String:		[char() | binary() | String]

Some short forms are allowed:

    {Tag, Content}	-> {Tag, [], Content}
    Tag		-> {Tag, [], []}

Note that content lists must be flat, but strings can be deep.

It is also allowed to include normal `#xml...` elements in the simple format.

`xmerl:export_simple(Data, Callback)` takes the above data structure and exports it, using the callback module `Callback`.

The callback module should contain hook functions for all tags present in the data structure. The hook function must have the format:

`Tag(Data, Attrs, Parents, E)`

where E is an `#xmlElement{}` record (see `xmerl.hrl`).

Attrs is converted from the simple `[{Key, Value}]` to `[#xmlAttribute{}]`

Parents is a list of `[{ParentTag, ParentTagPosition}]`.

The hook function should return either the Data to be exported, or the tuple `{'#xml-redefine#', NewStructure}`, where `NewStructure` is an element (which can be simple), or a (simple-) content list wrapped in a 1-tuple as `{NewContent}`.

The callback module can inherit definitions from other callback modules, through the required function `'#xml-interitance#() -> [ModuleName]`.

As long as a tag is represented in one of the callback modules, things will work. It is of course also possible to redefine a tag.

### XSLT like transforms

See separate document xmerl_xs.html

.