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# 8.3. Erlang

Note

The Erlang query server is disabled by default. Read configuration guide about reasons why and how to enable it.

`Emit`(Id, Value)

  Emits key-value pairs to view indexer process.

      fun({Doc}) ->
        <<K,_/binary>> = proplists:get_value(<<"_rev">>, Doc, null),
        V = proplists:get_value(<<"_id">>, Doc, null),
        Emit(<<K>>, V)
      end.

`FoldRows`(Fun, Acc)

  Helper to iterate over all rows in a list function.

  Arguments: | Fun – Function object. Acc – The value previously returned by Fun.

      fun(Head, {Req}) ->
        Fun = fun({Row}, Acc) ->
          Id = couch_util:get_value(<<"id">>, Row),
          Send(list_to_binary(io_lib:format("Previous doc id: ~p~n", [Acc]))),
          Send(list_to_binary(io_lib:format("Current  doc id: ~p~n", [Id]))),
          {ok, Id}
        end,
        FoldRows(Fun, nil),
        ""
      end.

`GetRow`()

  Retrieves the next row from a related view result.

      %% FoldRows background implementation.
      %% https://git-wip-us.apache.org/repos/asf?p=couchdb.git;a=blob;f=src/couchdb/couch_native_process.erl;hb=HEAD#l368
      %%
      foldrows(GetRow, ProcRow, Acc) ->
        case GetRow() of
          nil ->
            {ok, Acc};
          Row ->
            case (catch ProcRow(Row, Acc)) of
              {ok, Acc2} ->
                foldrows(GetRow, ProcRow, Acc2);
              {stop, Acc2} ->
                {ok, Acc2}
            end
      end.

`Log`(Msg)

  Arguments: | Msg – Log a message at the INFO level.

      fun({Doc}) ->
        <<K,_/binary>> = proplists:get_value(<<"_rev">>, Doc, null),
        V = proplists:get_value(<<"_id">>, Doc, null),
        Log(lists:flatten(io_lib:format("Hello from ~s doc!", [V]))),
        Emit(<<K>>, V)
      end.

  After the map function has run, the following line can be found in CouchDB logs (e.g. at /var/log/couchdb/couch.log):

      [Sun, 04 Nov 2012 11:33:58 GMT] [info] [<0.9144.2>] Hello from 8d300b86622d67953d102165dbe99467 doc!

`Send`(Chunk)

  Sends a single string Chunk in response.

      fun(Head, {Req}) ->
        Send("Hello,"),
        Send(" "),
        Send("Couch"),
        "!"
      end.

  The function above produces the following response:

      Hello, Couch!

`Start`(Headers)

  Arguments: | Headers – Proplist of response object.

  Initialize List functions response. At this point, response code and headers may be defined. For example, this function redirects to the CouchDB web site:

      fun(Head, {Req}) ->
        Start({[{<<"code">>, 302},
                {<<"headers">>, {[
                  {<<"Location">>, <<"http://couchdb.apache.org">>}]
                }}
              ]}),
        "Relax!"
      end.

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