Error reports in Erlang

Question

I've taken the cowboy example code and brooken it.

The code for the default request handler looked like this:

-module(default_handler).
-behaviour(cowboy_http_handler).
-export([init/3, handle/2, terminate/2]).

init({_Any, http}, Req, []) ->
    {ok, Req, undefined}.

handle(Req, State) ->
    {ok, Req2} = cowboy_http_req:reply(200, [], <<"Hello world!">>, Req),
    {ok, Req2, State}.

terminate(_Req, _State) ->
    ok.

its straight forward, but I wanted to make return files so I changed it to:

-module(default_handler).
-behaviour(cowboy_http_handler).
-export([init/3, handle/2, terminate/2]).

init({_Any, http}, Req, []) ->
    {ok, Req, undefined}.

handle(Req, State) ->
    try 
    {Path, Req1} = cowboy_http_req:path(Req),
    {ok, File} = file:read_file(Path),
    cowboy_http_req:reply(200, [], File, Req1)
    of 
    {ok, Req2} ->
        {ok, Req2, State}
    catch
    _ -> 
        {ok, Req3} = cowboy_http_req:reply(200, [], <<"Hello world!">>, Req),
        {ok, Req3, State}
    end.

terminate(_Req, _State) ->
    ok.

The try-catch thing should handle the fact that there might not be a file, but it does not. Why is that?

When I try to fetch a file that is not there I get a large error report in the console, can anyone tell me why?

=ERROR REPORT==== 15-Jun-2012::14:24:54 ===
** Handler default_handler terminating in handle/2
   for the reason error:{badmatch,{error,badarg}}
** Options were []
** Handler state was undefined
** Request was [{socket,#Port<0.1515>},
                {transport,cowboy_tcp_transport},
                {connection,keepalive},
                {pid,<0.1175.0>},
                {method,'GET'},
                {version,{1,1}},
                {peer,undefined},
                {host,[<<"localhost">>]},
                {host_info,undefined},
                {raw_host,<<"localhost">>},
                {port,8080},
                {path,[<<"favicon.ico">>]},
                {path_info,undefined},
                {raw_path,<<"/favicon.ico">>},
                {qs_vals,undefined},
                {raw_qs,<<>>},
                {bindings,[]},
                {headers,
                    [{'Accept-Charset',<<"ISO-8859-1,utf-8;q=0.7,*;q=0.3">>},
                     {'Accept-Language',<<"en-US,en;q=0.8">>},
                     {'Accept-Encoding',<<"gzip,deflate,sdch">>},
                     {'User-Agent',
                         <<"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/535.19 (KHTML, like Gecko) Ubuntu/10.10 Chromium/18.0.1025.151 Chrome/18.0.1025.151 Safari/535.19">>},
                     {'Accept',<<"*/*">>},
                     {'Connection',<<"keep-alive">>},
                     {'Host',<<"localhost">>}]},
                {p_headers,[{'Connection',[<<"keep-alive">>]}]},
                {cookies,undefined},
                {meta,[]},
                {body_state,waiting},
                {buffer,<<>>},
                {resp_state,waiting},
                {resp_headers,[]},
                {resp_body,<<>>},
                {onresponse,undefined},
                {urldecode,{#Fun<cowboy_http.urldecode.2>,crash}}]
** Stacktrace: [{default_handler,handle,2,
                                 [{file,"src/default_handler.erl"},{line,13}]},
                {cowboy_http_protocol,handler_handle,3,
                                      [{file,"src/cowboy_http_protocol.erl"},
                                       {line,298}]}]

Answer 1

This line:

{Path, Req1} = cowboy_http_req:path(Req),

Actually returns a list of binaries, like [<<"path">>,<<"path2">>] instead of something like "/path/path2" which should be what you're actually looking for.

So, to form the filesystem path:

{Path, Req1} = cowboy_http_req:path(Req),
FsPath = lists:foldl(
    fun(PathComponent, Acc) ->
        string:join([Acc, erlang:binary_to_list(PathComponent)], "/")
    end,
    "",
    Path
),
{ok, File} = file:read_file(FsPath),

(The badarg error you're getting is because the argument to file:read_file/1 is not a string (a list) but a list of binaries, which is not the expected argument.

And the catch needs a _:_ clause, just like Harald answer states.

Cheers.

Answer 2

probably because of how it evaluates the catch clause, see http://www.erlang.org/doc/reference_manual/expressions.html#try

If an exception occurs during evaluation of Exprs but there is no matching ExceptionPattern of the right Class with a true guard sequence, the exception is passed on as if Exprs had not been enclosed in a try expression.

You need to specify an error class (error, throw or exit) if not looking for the default, which is throw.

try Exprs of
   Pattern1 [when GuardSeq1] ->
       Body1;
    ...;
   PatternN [when GuardSeqN] ->
        BodyN
catch
    [Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    ...;
   [ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
        ExceptionBodyN

a catch-all-errors would be written as

catch
    _:_ -> 

notice how you specify both class and ExpressionPattern as "don't care".

Hope this helps because I only 'dabbled' in erlang until now. :)