Erlang (programming language)/Tutorials/Yecc: Difference between revisions
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% "<html><head></head><body>hello_world</body></html>"). | % "<html><head></head><body>hello_world</body></html>"). | ||
% html_parser:parse(B). | % html_parser:parse(B). | ||
It can be a pain to build and run a parser each time we edit the source yrl file. To speed things up, we can use a build and run the parser for us. html_test.erl: | |||
-module(html_test). | |||
-compile(export_all). | |||
start() -> | |||
yecc:yecc("html.yrl","html_parser.erl"), | |||
cover:compile(html_parser), | |||
{_,List_of_symbols,_}=erl_scan:string( | |||
"<html><head><title>greating</title></head> | |||
<body> | |||
<h1>hello there world what is up</h1> | |||
</body> | |||
</html>"), | |||
{ok,L} = html_parser:parse(List_of_symbols), | |||
register(do_event, spawn(html_test,event_loop,[])), | |||
Events = lists:flatten(L), | |||
send_events(Events), | |||
Events. | |||
send_events([]) -> do_event ! {exit}; | |||
send_events([H|T]) -> | |||
do_event ! H, | |||
%io:format(" ~w ~n",[H]), | |||
send_events(T). | |||
event_loop() -> | |||
receive | |||
{open,{atom,_Line_Number,html}} -> | |||
io:format("~n start scan ~n", []), | |||
event_loop(); | |||
{contents,List} -> | |||
Contents = get_contents(List,[]), | |||
io:format("~n contents: ~w ~n", [Contents]); | |||
{exit} -> exit(normal) | |||
end, | |||
event_loop(). | |||
get_contents([],Items) -> Items; | |||
get_contents([H|T],Items)-> | |||
if | |||
length(T) > 0 -> | |||
NT = hd(T); | |||
true -> | |||
NT = T | |||
end, | |||
{atom,_N,Item} = H, | |||
NItems = Items++[Item], | |||
% io:format(" ~w ",[Item]), | |||
get_contents(NT,NItems). | |||
% 6> c(html_test). | |||
% {ok,html_test} | |||
% 7> html_test:start(). | |||
% [hello,there,world,what,is,up] | |||
% and events. | |||
Revision as of 12:52, 1 May 2008
Making Parsers with yecc
Yecc is an erlang version of yacc.
We have a BNF grammar in a source file ending in .yrl yrl means yecc rule list. We can parse simple a simple xhtml file using yecc. (Of course a more powerful way to do xml in erlang is xmerl)
html.yrl source:
Nonterminals tag elements element start_tag end_tag .
Terminals 'atom' '<' '>' '/'.
Rootsymbol tag.
tag ->
start_tag tag end_tag :
['$1', '$2', '$3'].
tag ->
start_tag tag tag end_tag :
['$1', '$2', '$3', '$4'].
tag ->
start_tag elements end_tag :
['$1', {'contents','$2'}, '$3'].
tag ->
start_tag end_tag :
['$1','$2'].
start_tag -> '<' 'atom' '>' : {'open','$2'}.
end_tag -> '<' '/' 'atom' '>' : {'close','$3'}.
elements -> element : ['$1'].
elements -> element elements : ['$1', '$2'].
element -> atom : '$1'.
% yecc:yecc("html.yrl","html_parser.erl").
% c(html_parser).
% f(B), {_,B,_} =
% erl_scan:string(
% "<html><head></head><body>hello_world</body></html>").
% html_parser:parse(B).
It can be a pain to build and run a parser each time we edit the source yrl file. To speed things up, we can use a build and run the parser for us. html_test.erl:
-module(html_test).
-compile(export_all).
start() ->
yecc:yecc("html.yrl","html_parser.erl"),
cover:compile(html_parser),
{_,List_of_symbols,_}=erl_scan:string(
"<html><head><title>greating</title></head>
<body>
hello there world what is up
</body>
</html>"),
{ok,L} = html_parser:parse(List_of_symbols),
register(do_event, spawn(html_test,event_loop,[])),
Events = lists:flatten(L),
send_events(Events),
Events.
send_events([]) -> do_event ! {exit};
send_events([H|T]) ->
do_event ! H,
%io:format(" ~w ~n",[H]),
send_events(T).
event_loop() ->
receive
{open,{atom,_Line_Number,html}} ->
io:format("~n start scan ~n", []),
event_loop();
{contents,List} ->
Contents = get_contents(List,[]),
io:format("~n contents: ~w ~n", [Contents]);
{exit} -> exit(normal)
end,
event_loop().
get_contents([],Items) -> Items;
get_contents([H|T],Items)->
if
length(T) > 0 ->
NT = hd(T);
true ->
NT = T
end,
{atom,_N,Item} = H,
NItems = Items++[Item],
% io:format(" ~w ",[Item]),
get_contents(NT,NItems).
% 6> c(html_test).
% {ok,html_test}
% 7> html_test:start().
% [hello,there,world,what,is,up]
% and events.