Erlang (programming language)/Tutorials/Yecc: Difference between revisions
imported>Eric Evers mNo edit summary |
imported>Tom Morris m (Erlang programming language/Tutorials/Yecc moved to Erlang (programming language)/Tutorials/Yecc) |
||
| (One intermediate revision by one other user not shown) | |||
| Line 9: | Line 9: | ||
We can parse a simple xhtml file using yecc. Actually, | We can parse a simple xhtml file using yecc. Actually, | ||
we use yecc to turn html.yrl into a parser, html_parser.erl. | we use yecc to turn html.yrl into a parser, html_parser.erl. | ||
Next we use html_parser.erl to parse our xhtml data. | Next we use html_parser.erl to parse our xhtml data, in this case, the xhtml data is in a string. | ||
yecc:yecc("html.yrl","html_parser.erl"). | yecc:yecc("html.yrl","html_parser.erl"). | ||
c(html_parser). | c(html_parser). | ||
| Line 47: | Line 47: | ||
build and run the parser for us. We compile and run the test | build and run the parser for us. We compile and run the test | ||
program which builds the parser and tests it for | program which builds the parser and tests it for | ||
us on some document. | us on some document, or in this example, the xhtml data is in a string. | ||
-module(html_test). | -module(html_test). | ||
Latest revision as of 06:07, 8 August 2009
| The metadata subpage is missing. You can start it via filling in this form or by following the instructions that come up after clicking on the [show] link to the right. | |||
|---|---|---|---|
|
Making Parsers with yecc
Yecc is an erlang version of yacc.
We have a BNF(Backus-Naur_form) grammar in a source file ending in .yrl yrl means yecc rule list. We can parse a simple xhtml file using yecc. Actually, we use yecc to turn html.yrl into a parser, html_parser.erl. Next we use html_parser.erl to parse our xhtml data, in this case, the xhtml data is in a string.
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).
Note: all tags must match with open and close. (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'.
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 program to build and run the parser for us. We compile and run the test program which builds the parser and tests it for us on some document, or in this example, the xhtml data is in a string.
-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().
% [greating]
% [hello,there,world,what,is,up]
% and events.