[erlang-questions] pattern matching registered name

[James Aimonetti]

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On 05/20/2013 10:12 AM, Edmund Sumbar wrote:
> Hello,
> 
> I'm new to erlang and am perplexed by the use of registered names
> when they are passed as function parameters and subsequently used
> in pattern matching.
> 
> For example (taken from Programming Erlang 2/ed by Joe Armstrong),
> 
> -module(kvs). -export([start/0, rpc/1]).
> 
> start() -> register(kvs, spawn(fun() -> loop() end)).
> 
> rpc(Q) -> kvs ! {self(), Q}, receive {kvs, Reply} -> Reply end.
> 
> loop() -> receive {From, {store, Key, Value}} -> put(Key, {ok,
> Value}), From ! {kvs, true}, loop(); {From, {lookup, Key}} -> From
> ! {kvs, get(Key)}, loop() end.
> 
> This module works as expected.
> 
> $ erl Erlang R16B (erts-5.10.1) [source] [64-bit] [smp:8:8]
> [async-threads:10] [hipe] [kernel-poll:false] [dtrace]
> 
> Eshell V5.10.1  (abort with ^G) 1> c(kvs). {ok,kvs} 2>
> kvs:start(). true 3> kvs:rpc({store, name, "Joe"}). true 4>
> kvs:rpc({lookup, name}). {ok,"Joe"}
> 
> Then, I modify the code to remove the registered name.
> 
> -module(kvsx). -export([start/0, rpc/2]).
> 
> start() -> spawn(fun() -> loop() end).
> 
> rpc(Pid, Q) -> Pid ! {self(), Q}, receive {Pid, Reply} -> Reply 
> end.
> 
> loop() -> receive {From, {store, Key, Value}} -> put(Key, {ok,
> Value}), From ! {self(), true}, loop(); {From, {lookup, Key}} -> 
> From ! {self(), get(Key)}, loop() end.
> 
> The modified module also works as expected.
> 
> $ erl Erlang R16B (erts-5.10.1) [source] [64-bit] [smp:8:8]
> [async-threads:10] [hipe] [kernel-poll:false] [dtrace]
> 
> Eshell V5.10.1  (abort with ^G) 1> c(kvsx). {ok,kvsx} 2> Pid =
> kvsx:start(). <0.40.0> 3> kvsx:rpc(Pid, {store, name, "Joe"}). 
> true 4> kvsx:rpc(Pid, {lookup, name}). {ok,"Joe"}
> 
> However, when I register a name for Pid and pass it to the
> function, the process hangs.
> 
> 5> register(abc, Pid). true 6> whereis(abc). <0.40.0> 7>
> kvsx:rpc(abc, {lookup, name}). ^G User switch command --> q
> 
> As a newcomer to the language (but with decades of programming
> experience), I'm confused. Can someone explain registered names and
> when/where they can be used.
> 
> Ed
> 
> 
> 
> _______________________________________________ erlang-questions
> mailing list erlang-questions@REDACTED 
> http://erlang.org/mailman/listinfo/erlang-questions
> 

rpc/2 expects the first arg into the function to be the first element
in the response tuple.
The loop/0 uses self(), which always returns a pid(), not the
registered name.
So you call rpc with an atom, and receive a tuple with a pid(). Those
don't match.
Your call to rpc/2 hangs because the selective receive in rpc/2 never
matches (and you have no timeout clause).

- -- 
James Aimonetti
Distributed Systems Engineer / DJ MC_

2600hz | http://2600hz.com
sip:james@REDACTED
tel: 415.886.7905
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