I'm trying to learn how to write Erlang Webmachine resources. One resource throws an error, but I can't tracking it down. The error message in the crash report does not provide enough information.
Is there a way to test these functions in the Erlang shell?
Most of the functions in the resource require request and context parameters. But I don't know how to simulate these parameters in the browser.
Example code below.
Thanks,
LRP
Example code:
I'm thinking specifically of functions like:
content_types_provided(RD, Ctx) ->
Path = wrq:disp_path(RD),
{[{webmachine_util:guess_mime(Path), generate_body}],
RD, Ctx}.
But my current bug is in the init function.
This works...
Dispatch rule:
{["blip"], zzz_resource, []}.
Init:
init([]) -> {ok, undefined}.
to_html(ReqData, State) ->
% {"<html><bodoy>Hello, new world</body></html>", ReqData, State}.
{test:test(), ReqData, State}.
But this throws an error:
Dispatch:
{["static"], static_resource,[]}.
Init:
init(_) ->
DocRoot =
case init:get_argument(doc_root) of
{ok, [[DR]]} -> DR;
error -> "doc_root path error"
end,
{ok, #ctx{docroot=DocRoot}}.
=ERROR REPORT==== 4-Aug-2011::10:54:56 ===
webmachine error: path="/static"
{error,function_clause,
[{filename,join,[[]]},
{static_resource,resource_exists,2},
There are a lot of layers to this answer depending on what you want to see and how deep down the rabbit hole you want to go.
Let's start with the easy stuff:
The error you are getting tells me that a call to static_resource:resource_exists/2 resulted in a call to filename:join/1 which failed because it was passed [] as its argument. That should help you track down the issue.
Recommended reading: errors-and-exceptions
A crude way to track down errors in any language is just to add print statements at strategic loctations. In this case you can use io:format/2 or erlang:display/1 to display whatever you want to the console. For example:
...
erlang:display("I'm inside resource_exists!"),
StuffToJoin = ["foo", "bar"],
erlang:display(StuffToJoin),
filename:join(StuffToJoin),
...
Just reload the page you should see the value printed in the console (assuming the appropriate function was called as part of the reload).
If you want to manually test a resource (like in a unit test) you can do something like the following:
Headers = [{"Host", "mydomain.com"}, {"user-agent", "Firefox"}],
Context = [],
Path = "/static",
ReqData = wrq:create('GET', {1,1}, Path, mochiweb_headers:from_list(Headers)),
static_resource:resource_exists(ReqData, Context)
If you want a deep look at how to debug webmachine, you can read this. You can get pretty far with the above, but doing a full trace can be helpful if you need to see the decision graph.