Difference ok and end [Erlang] - erlang

Question

What is the difference in ending a function with end and ok in Erlang? I've been trying to grasp the meaning in the following code:
-module(esOne).
-export([start/1, func/1]).
start(Par) ->
io:format("Client: I am ~p, spawned by the server: ~p~n",[self(),Par]),
spawn(esOne, func, [self()]),
Par ! {onPid, self()},
serverEsOne ! {onName, self()},
receiveMessage(),
ok.
receiveMessage() ->
receive
{reply, N} ->
io:format("Client: I received a message: ~p~n",[N])
after
5000->
io:format("Client: I received no message, i quit~n",[])
end.
func(Parent)->
io:format("Child: I am ~p, spawned from ~p~n",[self(),Parent]).
This code works in conjunction with another .erl file that acts as server. I managed to write this only through analyzing the given server file and copying it's behavior. First I thought ok was used to end every function, but that is not the case as I can't end receiveMessage() with ok. Then I thought I could maybe end every function with end, but start(Par) will give an error if I replace ok by end. Not only that, but in the server file I see that ok and end are used within functions to end loops. The way they're used look the same to me, yet they clearly fulfill a separate function as one cannot be replaced by another. Some clarification would be much appreciated.

Answer 1

Two points to understand:
Some code block types in Erlang are closed with an "end". So if ... end, case ... end, receive ... [after N] ... end and so on. It is certainly possible to use "end" as its own atom in place of OK, but that is not what is happening above.
Every function in Erlang returns some value. If you aren't explicit about it, it returns the value of the last expression. The "=" operator isn't assignment to a variable like in other languages, it is assignment to a symbol as in math, meaning that reassigning is effectively a logical assertion. If the assertion fails, the process throws an exception (meaning it crashes, usually).
When you end something with "ok" or any other atom you are providing a known final value that will be returned. You don't have to do anything with it, but if you want the calling process to assert that the function completed or crash if anything unusual happened then you can:
do_stuff() ->
ok = some_func().
instead of
do_stuff() ->
some_func().
If some_func() may have had a side effect that can fail, it will usually return either ok or {error, Reason} (or something similar). By checking that the return was ok we prevent the calling process from continuing execution if something bad happened. That is central to the Erlang concept of "let it crash". The basic idea is that if you call a function that has a side-effect and it does anything unexpected at all, you should crash immediately, because proceeding with bad data is worse than not proceeding at all. The crash will be cleaned up by the supervisor, and the system will be restored to a known state instead of being in whatever random condition was left after the failure of the side-effect.
A variation on the bit above is to have the "ok" part appear in a tuple if the purpose of a function is to return a value. You can see this in any dict-type handling library, for example. The reason some data returning functions have a return type of {ok, Value} | {error, Reason} instead of just Value | {error, Reason} is to make pattern matching more natural.
Consider the following case clauses:
case dict:find(Key, Dict) of
{ok, Value} ->
Value;
{error, Reason} ->
log(error, Reason),
error
end.
And:
case finder(Key, Struct) of
Value ->
Value;
{error, Reason}
log(error, Reason),
error
end.
In the first example we match the success condition first. In the second version, though, this is impossible because the error clause could never match; any return at all would always be represented by Value. Oops.
Most of the time (but not quite always) functions that return a value or crash will return just the value. This is especially true of pure functions that carry no state but what you pass in and have no side effects (for example, dict:fetch/2 gives the value directly, or crashes the calling process, giving you an easy choice which way you want to do things). Functions that return a value or signal an error usually wrap a valid response in {ok, Value} so it is easy to match.