Elixir in Action - ch07 - implementing a cache help

Questions / Help
1

Pretty new to Elixir and I’ve been going through Elixir in Action. I sense this issue could be solved if I drop down to a previous Elixir version but any advice to be able to do this on 1.11 would be very nice.

In 7.2.1 it talks about implementing a cache to manage multiple to-do lists.

Code from the book’s repo: https://github.com/sasa1977/elixir-in-action/tree/master/code_samples/ch07/todo_cache/lib/todo

The problem is, when I run these commands after iex -S mix

{:ok, cache} = Todo.Cache.start()
{:ok, #PID<0.154.0>}
Todo.Cache.server_process(cache, "Bob's List")
#PID<0.156.0>
Todo.Cache.server_process(cache, "Alice's Lists")

I receive the following error:

21:28:33.353 [error] GenServer #PID<0.154.0> terminating
** (MatchError) no match of right hand side value: {:error, {:already_started, #PID<0.156.0>}}
    (todo_cache 0.1.0) lib/todo/cache.ex:29: Todo.Cache.handle_call/3
    (stdlib 3.13.2) gen_server.erl:706: :gen_server.try_handle_call/4
    (stdlib 3.13.2) gen_server.erl:735: :gen_server.handle_msg/6
    (stdlib 3.13.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Last message (from #PID<0.152.0>): {:server_process, "Bob's Lists"}
State: %{"Bob's List" => #PID<0.156.0>}
Client #PID<0.152.0> is alive

    (stdlib 3.13.2) gen.erl:208: :gen.do_call/4
    (elixir 1.11.2) lib/gen_server.ex:1024: GenServer.call/3
    (stdlib 3.13.2) erl_eval.erl:680: :erl_eval.do_apply/6
    (elixir 1.11.2) src/elixir.erl:280: :elixir.recur_eval/3
    (elixir 1.11.2) src/elixir.erl:265: :elixir.eval_forms/3
    (iex 1.11.2) lib/iex/evaluator.ex:261: IEx.Evaluator.handle_eval/5
    (iex 1.11.2) lib/iex/evaluator.ex:242: IEx.Evaluator.do_eval/3
    (iex 1.11.2) lib/iex/evaluator.ex:220: IEx.Evaluator.eval/3
** (exit) exited in: GenServer.call(#PID<0.154.0>, {:server_process, "Bob's Lists"}, 5000)
    ** (EXIT) an exception was raised:
        ** (MatchError) no match of right hand side value: {:error, {:already_started, #PID<0.156.0>}}
            (todo_cache 0.1.0) lib/todo/cache.ex:29: Todo.Cache.handle_call/3
            (stdlib 3.13.2) gen_server.erl:706: :gen_server.try_handle_call/4
            (stdlib 3.13.2) gen_server.erl:735: :gen_server.handle_msg/6
            (stdlib 3.13.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
    (elixir 1.11.2) lib/gen_server.ex:1027: GenServer.call/3

This seems to only happen when I change the string in the second parameter. Adding another clause for the case statement didn’t stop the error. How do I get past this?

2

I just downloaded the files and compiled them with Elixir 1.11.3.

iex(1)> c("server.ex")
warning: ...
[Todo.Server]
iex(2)> c("cache.ex")
[Todo.Cache]
iex(3)> c("list.ex")
warning: ...
[Todo.List]
iex(4)> {:ok, cache} = Todo.Cache.start
{:ok, #PID<0.129.0>}
iex(5)> Todo.Cache.server_process(cache, "Bob's List")
#PID<0.132.0>
iex(6)> Todo.Cache.server_process(cache, "Alice's Lists")
#PID<0.134.0>

No errors. So there must be some differences, either between your code and the repo code, or in the commands you’re running. Glancing at the code, that string in the second parameter is only used as a map key. You should be able to pass any string as the name.

1 Like
3

I cloned the code, and didn’t got any error. Maybe some code has changed?

iex(3)> {:ok, cache} = Todo.Cache.start()
{:ok, #PID<0.189.0>}
iex(4)> Todo.Cache.server_process(cache, "Bob's List")
#PID<0.191.0>
iex(5)> Todo.Cache.server_process(cache, "Alice's Lists")
#PID<0.193.0>

my elixir version is 1.11.3, erlang 23

4

It did work when I cloned the code over. So I took another look, and then it turns out my server.ex had its name parameter filled in. And the files I was meticulously looking over might have been in a different folder.

This is embarrassing.

Thank you all for replying.

2 Likes