[erlang-questions] pattern matching registered name

[Edmund Sumbar]

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
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