Error in Prosses <proc_num> with exit value: - parallel-processing

Question

I'm trying to parallelize a function but I seem to be having problems (in Erlang).
I have code similar to this:
-module(examples).
-import(lists, [sublist/2, sublist/3]).
motherFunc(Ls) -> childFunc(Ls, false).
childFunc(Ls, false) ->
Pid1 = spawn(example, mergesortP, [lists:sublist(Ls, length(Ls) div 2, self()]),
Pid2 = spawn(example, mergesortP, [lists:sublist(Ls, length(Ls) div 2, self()]),
FirstHalf = [],
SecondHalf = [],
receive
{Pid1, Msg} -> FirstHalf = Msg;
{Pid2, Msg} -> SecondHalf = Msg
end,
SecondHalf ++ FirstHalf;
childFunc([], Pid) -> Pid ! {self(), []};
childFunc([L], Pid) -> Pid ! {self(), [L]};
childFunc(Ls, Pid) ->
Pid1 = spawn(examples, childFunc, [lists:sublist(Ls, length(Ls) div 2, self()]),
Pid2 = spawn(examples, childFunc, [lists:sublist(Ls, length(Ls) div 2 + 1, length(Ls) div 2 + 1), self()]),
FirstHalf = [],
SecondHalf = [],
receive
{Pid1, Msg} -> FirstHalf = Msg;
{Pid2, Msg} -> SecondHalf = Msg
end,
Pid ! {self(), SecondHalf ++ FirstHalf}.
When I run this I'm getting this error message for both threads, and then it does nothing else: Error in process <0.31.0> with exit value: {undef,[{examples,childFunc,[[1,2,3,4],<0.2.0>],[]}]}

Answer 1

The function spawn/3 with the module and the function name as the first and second argument requires that the function is exported.
In fact, any explicit qualified call:
module:function(...)
or implicit qualified call:
apply(module, function, ...)
requires that the function be exported. Whether it's supposedly from the same module or not is irrelevant (1).
In your code, you should either use spawn/1 or export the function:
-export([childFunc/2, mergesortP/2]).
% ...
spawn(?MODULE, mergesortP, [lists:sublist(Ls, length(Ls) div 2), self()])
or
Self = self(),
spawn(fun() -> mergesortP(lists:sublist(Ls, length(Ls) div 2), Self) end.
Note the difference when passing self(). With spawn/3 invocation, self() is evaluated by the spawning process and therefore is its pid. If you call self() within the anonymous function passed to spawn/1, it will evaluate to the pid of the spawned process. So you need to call it before and pass it in a variable.
Additionally, your code is unlikely to work. Indeed, the following section does not do what you mean:
FirstHalf = [],
SecondHalf = [],
receive
{Pid1, Msg} -> FirstHalf = Msg;
{Pid2, Msg} -> SecondHalf = Msg
end,
This will first assign [] to FirstHalf and SecondHalf variables, and then try to match this with the first message that arrived, as variables in Erlang cannot be re-assigned. The second message is ignored as received is only used once. The first result probably isn't an empty list and therefore it will fail with a badmatch error. Instead, it seems you mean to perform parallel execution of the merge an collect the results afterwards. You could write:
FirstHalf = receive {Pid1, Msg1} -> Msg1 end,
SecondHalf = receive {Pid2, Msg2} -> Msg2 end,
This will collect the first half and then the second half.
(1) Actually, a qualified call can in fact be executed by another module, or more precisely a newer version of the module. Qualifying calls to (a loop) function within the same module is a common method to continue execution in a newer version during code upgrades.