Erlang: supervisor:start_child/2 error has me baffled

Question

I'm making slight modifications to Logan/Merritt/Carlson's simple cache, Chapter 6, pp 149-169, Erlang and OTP in Action. So far, no code changes, just renaming the modules.

I start the application:

application:start(gridz).
ok

I insert an item:

gridz_maker:insert(blip, blop).

I get this error:

** exception error: no match of right hand side value 
                {error,
                    {function_clause,
                        [{gridz_edit,init,
                             [{blop,86400}],
                             [{file,"src/gridz_edit.erl"},{line,51}]},
                         {gen_server,init_it,6,
                             [{file,"gen_server.erl"},{line,304}]},
                         {proc_lib,init_p_do_apply,3,
                             [{file,"proc_lib.erl"},{line,227}]}]}}
 in function  gridz_maker:insert/2 (src/gridz_maker.erl, line 15)

Here's the code:

insert(Key, Value) ->
   case gridz_store:lookup(Key) of
      {ok, Pid}  -> gridz_edit:replace(Pid, Value);
      {error, _} -> {ok, Pid} = gridz_edit:create(Value),   %% line 15
                    gridz_store:insert(Key, Pid)
   end.

I look at line 15:

  {error, _} -> {ok, Pid} = gridz_edit:create(Value),

I expect the error because this is a new item. gridz:edit is a gen_server (sc_element in Logan et/al.) Here's the code for create/1:

create(Value) ->
  create(Value, ?DEFAULT_LEASE_TIME).

create(Value, LeaseTime) ->
   gridz_sup:start_child(Value, LeaseTime).

And here's the code for gridz_sup:start_child/2:

start_child(Value, LeaseTime) ->
   supervisor:start_child(?SERVER, [Value, LeaseTime]).

init([]) ->
   Grid            = {gridz_edit, {gridz_edit, start_link, []},
                     temporary, brutal_kill, worker, [gridz_edit]},
   Children        =  [Grid],
   RestartStrategy = {simple_one_for_one, 0, 1},
                     {ok, {RestartStrategy, Children}}.

If I execute supervisor:start_child/2 directly, here's what I get:

{error,{function_clause,[{gridz_edit,init,
                                 [{blop,50400}],
                                 [{file,"src/gridz_edit.erl"},{line,51}]},
                     {gen_server,init_it,6,
                                 [{file,"gen_server.erl"},{line,304}]},
                     {proc_lib,init_p_do_apply,3,
                               [{file,"proc_lib.erl"},{line,227}]}]}}

Line 51 in gridz_edit is an init function:

init([Value, LeaseTime]) ->
   Now = calendar:local_time(),
   StartTime = calendar:datetime_to_gregorian_seconds(Now),
   {ok,
   #state{value = Value,
          lease_time = LeaseTime,
          start_time = StartTime},
   time_left(StartTime, LeaseTime)}.

If I execute it directly, it works:

120> gridz_edit:init([blop, (60 * 60 * 24)]).
{ok,{state,blop,86400,63537666408},86400000}

So now I'm baffled. What am I missing? Why does supervisor:start_child/2 throw an error?

Thanks,

LRP

Answer 1

The error says you are passing in a tuple with 2 members, {blop,86400}, when you seem to be expecting a list of 2 members: [Value, LeaseTime]. In your direct execution, you are also using a list, so it works. You should figure out where the tuple is being created, and create a list instead.