A Simple XML State Machine Accepting SAX Events to Build xmerl Compitable XML Tree: icalendar demo


xmerl is a full XML functionality in Erlang, with a lot of features like XPATH, XSLT, event_function, acc_function etc. Well, now I just want to get icalendar to be parsed to form of xmerl tree, which will contain #xmlElement, #xmlAttribute, #xmlText etc, and easily to apply XPATH on it.
How about an approach that the parser just generates SAX events, and then, by attaching to a callback state machine to build a JSON or XML tree, or anything else?
I hoped xmerl is something like this, i.e. a parser to generate SAX events, and a state machine to accept the events and build the XML tree. I digged into xmerl's code, but, unfortunately, the parser and state machine are coupled together.
So I wrote a simple state machine which just receives SAX events to build a xmerl compitable XML tree. And, I applied it to icalendar.
I like this idea, by using SAX events as the common interface, I only need to write a another JSON state machine later, then, the result will be JSON of icalendar. I can share the same parser which just generates SAX events.
Here's the code, which is not completed yet, just to show how a SAX interface can serve a lot.
%%% A state machine which receives sax events and builds a xmerl compitable tree


-module(xmlsm).

-include_lib("xmerl/include/xmerl.hrl").

-export([state/2]).

-export([test/0
        ]).

-record(xmlsmState, {
    attributes = [],
    content = [],
    parents = []
}).

receive_events(Events) -> receive_events(Events, undefined).

receive_events([], _States) -> {ok, [], []};
receive_events([Event|T], States) ->
    case state(Event, States) of 
        {ok, TopElement} -> 
            {ok, TopElement, T};
        {error, Reason} -> 
            {error, Reason};
        States1 -> 
            receive_events(T, States1)    
    end.

state({startDocument}, _StateStack) ->
    State = #xmlsmState{attributes = [],
                        content = [], 
                        parents = []},
    [State];
state({endDocument}, StateStack) ->
    io:fwrite(user, "endDocument, states: ~p~n", [StateStack]),
    case StateStack of
        {ok, TopElement} -> {ok, TopElement};
        _ -> {error, "Bad element match"}
    end;
state({startElement, _Uri, LocalName, _QName, Attrs}, StateStack) ->
    %io:fwrite(user, "startElement~n", []),
    %% pop current State
    [State|_StatesPrev] = StateStack,
    #xmlsmState{attributes=_Attributes,
                content=_Content,
                parents=Parents} = State,
    {_Pos, Attributes1} = lists:foldl(
        fun ({Name, Value}, {Pos, AccAttrs}) ->
                Pos1 = Pos + 1,
                Attr = #xmlAttribute{name = Name,
                                     value = Value, 
                                     parents = [{LocalName, Pos1}|Parents]},
                {Pos1, [Attr|AccAttrs]}
        end, {0, []}, Attrs),
    Content1 = [],
    Parents1 = [{LocalName, 0}|Parents],
    %% push new state of Attributes, Content and Parents to StateStack
    NewState = #xmlsmState{attributes = Attributes1,
                           content = Content1,
                           parents = Parents1},
    [NewState|StateStack];
state({endElement, _Uri, LocalName, _QName}, StateStack) ->
    %% pop current State
    [State|StatesPrev] = StateStack,
    #xmlsmState{attributes=Attributes,
                content=Content,
                parents=Parents} = State,
    %% composite a new element            
    Element = #xmlElement{name = LocalName,
                          attributes = lists:reverse(Attributes),
                          content = lists:reverse(Content)},
    %% put Element to parent's content and return new state stack
    case StatesPrev of
        [ParentState|[]] -> %% reached the top now, return final result
            {ok, Element};
        [ParentState|Other] ->
            #xmlsmState{attributes=ParentAttributes,
                        content=ParentContent,
                        parents=ParentParents} = ParentState,
            ParentContent1 = [Element|ParentContent],
            %% update parent state and backward to it:
            ParentState1 = ParentState#xmlsmState{content = ParentContent1},
            %io:fwrite(user, "endElement, state: ~p~n", [State1]),
            [ParentState1|Other]
        end;
state({characters, Characters}, StateStack) ->
    %% pop current State
    [State|StatesPrev] = StateStack,
    #xmlsmState{attributes=_Attributes,
                content=Content,
                parents=Parents} = State,
    [{Parent, Pos}|ParentsPrev] = Parents,
    Pos1 = Pos + 1,
    Text = #xmlText{value = Characters,
                    parents = [{Parent, Pos1}|ParentsPrev]},
    Content1 = [Text|Content],
    Parents1 = [{Parent, Pos1}|ParentsPrev],
    UpdatedState = #xmlsmState{content = Content1,
                               parents = Parents1},
    [UpdatedState|StatesPrev].

test() ->
    Events = [
        {startDocument},
        {startElement, [], feed, [], [{link, "http://lightpole.net"}, {author, "Caoyuan"}]},
        {characters, "feed text"},
        {startElement, [], entry, [], [{tag, "Erlang, Function"}]},
        {characters, "Entry1's text"},
        {endElement, [], entry, []},
        {startElement, [], "entry", [], []},
        {characters, "Entry2's text"},
        {endElement, [], entry, []},
        {endElement, [], feed, []},
        {endDocument}
    ],

    %% Streaming:
    {ok, Xml1, _Rest} = receive_events(Events),
    io:fwrite(user, "Streaming Result: ~n~p~n", [Xml1]),
    
    %% Stepped:
    FunCallback = fun xmlsm:state/2,
    FinalStates = lists:foldl(
        fun (Event, States) ->
                FunCallback(Event, States)
        end, undefined, Events),
    {ok, Xml2} = FinalStates,
    XmlText = lists:flatten(xmerl:export_simple([Xml2], xmerl_xml)),
    io:fwrite(user, "Stepped Result: ~n~p~n", [XmlText]).


And the primary icalendar front end:
-module(icalendar).

-include_lib("xmerl/include/xmerl.hrl").

-export([parse/1
        ]).

-export([test/0
        ]).

-define(xmlMachine, fun xmlsm:state/2).

parse(Text) ->
    States1 = ?xmlMachine({startDocument}, undefined),
    States2 = parse_line(skip_ws(Text), 0, States1),
    States3 = ?xmlMachine({endDocument}, States2).

parse_line([], _Line, States) -> States;
parse_line([$\s|T], Line, States) -> parse_line(T, Line, States);
parse_line([$\t|T], Line, States) -> parse_line(T, Line, States);
parse_line([$\r|T], Line, States) -> parse_line(T, Line, States);
parse_line([$
|T], Line, States) -> parse_line(T, Line + 1, States); parse_line("BEGIN"++T, Line, States) -> case skip_ws(T) of [$:|T1] -> {Rest, Line1, Name} = parse_component_name(skip_ws(T1), Line, States, []), %io:fwrite(user, "Component started: ~p~n", [Name]), States1 = ?xmlMachine({startElement, [], Name, [], []}, States), parse_line(skip_ws(Rest), Line1, States1); _ -> error end; parse_line("END"++T, Line, States) -> case skip_ws(T) of [$:|T1] -> {Rest, Line1, Name} = parse_component_name(skip_ws(T1), Line, States, []), States1 = ?xmlMachine({endElement, [], Name, []}, States), parse_line(skip_ws(Rest), Line1, States1); _ -> error end; parse_line(Text, Line, States) -> {Rest, Line1, {Name, Params}, Value} = parse_prop(skip_ws(Text), Line, States, {[], []}), States1 = ?xmlMachine({startElement, [], Name, [], Params}, States), States2 = ?xmlMachine({characters, Value}, States1), States3 = ?xmlMachine({endElement, [], Name, []}, States2), parse_line(skip_ws(Rest), Line1, States3). parse_component_name([$\r|T], Line, States, Name) -> parse_component_name(T, Line, States, Name); parse_component_name([$
|T], Line, States, Name) -> case unfolding_line(T) of {true, Rest} -> parse_component_name(Rest, Line, States, Name); {false, Rest} -> {Rest, Line + 1, list_to_atom(string:to_lower(lists:reverse(Name)))} end; parse_component_name([H|T], Line, States, Name) -> parse_component_name(skip_ws(T), Line, States, [H|Name]). parse_prop([$:|T], Line, States, {Name, NameParams}) -> PropName = list_to_atom(string:to_lower(lists:reverse(Name))), PropNameParams = lists:reverse(NameParams), %io:fwrite(user, "parsed prop name: ~p, with params: ~p~n", [PropName, NameParams]), {Rest, Line1, Value} = parse_prop_value(T, Line, States, []), %io:fwrite(user, "parsed prop : ~p~n", [{PropName, NameParams, Value}]), {Rest, Line1, {PropName, PropNameParams}, Value}; parse_prop([$;|T], Line, States, {Name, NameParams}) -> {Rest, Line1, ParamName, ParamValue} = parse_param(T, Line, States, []), parse_prop(Rest, Line1, States, {Name, [{ParamName, ParamValue}|NameParams]}); parse_prop([H|T], Line, States, {Name, NameParams}) -> parse_prop(skip_ws(T), Line, States, {[H|Name], NameParams}). parse_prop_value([$\r|T], Line, States, Value) -> parse_prop_value(T, Line, States, Value); parse_prop_value([$
|T], Line, States, Value) -> case unfolding_line(T) of {true, Rest} -> parse_prop_value(Rest, Line, States, Value); {false, Rest} -> {Rest, Line + 1, lists:reverse(Value)} end; parse_prop_value([H|T], Line, States, Value) -> parse_prop_value(T, Line, States, [H|Value]). parse_param([$=|T], Line, States, Name) -> ParamName = list_to_atom(string:to_lower(lists:reverse(Name))), {Rest, Line1, Value} = parse_param_value(T, Line, States, []), {Rest, Line1, ParamName, Value}; parse_param([H|T], Line, States, Name) -> parse_param(skip_ws(T), Line, States, [H|Name]). parse_param_value([$;|T], Line, States, Value) -> {T, Line, lists:reverse(Value)}; parse_param_value([$:|T], Line, States, Value) -> %% keep $: for end of prop name {[$:|T], Line, lists:reverse(Value)}; parse_param_value([H|T], Line, States, Value) -> parse_param_value(T, Line, States, [H|Value]). unfolding_line([$\s|T]) -> {true, T}; %% space unfolding_line([$\t|T]) -> {true, T}; %% htab unfolding_line(Chars) -> {false, Chars}. skip_ws([$\s|T]) -> skip_ws(T); skip_ws([$\t|T]) -> skip_ws(T); skip_ws(Text) -> Text. test() -> Text = " BEGIN:VCALENDAR METHOD:PUBLISH X-WR-CALNAME:Mi's Calendar VERSION:2.0 PRODID:Spongecell CALSCALE:GREGORIAN BEGIN:VEVENT DTSTART;TZID=America/Los_Angeles:20061206T120000 DTSTAMP:20070728T004842 LOCATION:Gordon Biersch, 640 Emerson St, Palo Alto, CA URL: UID:295803:spongecell.com SUMMARY:All hands meeting RRULE:FREQ=WEEKLY;INTERVAL=1 DTEND;TZID=America/Los_Angeles:20061206T130000 DESCRIPTION: END:VEVENT BEGIN:VEVENT DTSTART;TZID=America/Los_Angeles:20061207T120000 DTSTAMP:20070728T004842 LOCATION:395 ano nuevo ave\, sunnyvale\, ca URL: UID:295802:spongecell.com SUMMARY:Company lunch RRULE:FREQ=WEEKLY;INTERVAL=1 DTEND;TZID=America/Los_Angeles:20061207T130000 DESCRIPTION:Let's have lots of beer!! (well\, and some code review :) END:VEVENT BEGIN:VEVENT DTSTART;TZID=America/Los_Angeles:20061213T123000 DTSTAMP:20070728T004842 LOCATION:369 S California Ave\, Palo Alto\, CA URL: UID:295714:spongecell.com SUMMARY:Ben is back.. want to meet again DTEND;TZID=America/Los_Angeles:20061213T133000 DESCRIPTION:Re: Ben is back.. want to meet again
Marc END:VEVENT BEGIN:VEVENT DTSTART;TZID=America/Los_Angeles:20070110T200000 DTSTAMP:20070728T004842 LOCATION: URL: UID:304529:spongecell.com SUMMARY:flight back home DTEND;TZID=America/Los_Angeles:20070110T210000 DESCRIPTION: END:VEVENT BEGIN:VTIMEZONE TZID:America/Los_Angeles BEGIN:STANDARD DTSTART:20071104T000000 TZNAME:PST RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU TZOFFSETFROM:-0700 TZOFFSETTO:-0800 END:STANDARD BEGIN:DAYLIGHT DTSTART:20070311T000000 TZNAME:PDT RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=1SU TZOFFSETFROM:-0800 TZOFFSETTO:-0700 END:DAYLIGHT END:VTIMEZONE END:VCALENDAR ", io:fwrite(user, "Text: ~s~n", [Text]), {ok, Xml} = parse(Text), XmlText = lists:flatten(xmerl:export_simple([Xml], xmerl_xml)), io:fwrite(user, "Parsed: ~p~n", [XmlText]).

You may have noticed, the ?stateMachine can be pointed to a json_machine:state/2 some day, and we can get a JSON result without modification of icalendar.erl.
This also can be applied on JSON<->XML transform. Actually, I think SAX events is a good interface for various formats transform of data object. It's also a bit Erlang Style (Event passing). The parser/state-machine can communicate via SAX events as two separate processes and live with send/receive.