# [erlang-questions] how the lists:seq/3 implemented, confused when reading the source code of lists:seq/3

Graham Hay <>
Tue Nov 4 12:01:54 CET 2014

```That's a guard sequence, not a block:

http://erlang.org/doc/reference_manual/expressions.html#id81869

i.e. if (Inc > 0, First - Inc =< Last) OR (Inc < 0, First - Inc >= Last).

On 4 November 2014 10:32, Taotao.Li <> wrote:

>
> ----Source Code--------
>
> -spec seq(From, To, Incr) -> Seq when
>       From :: integer(),
>       To :: integer(),
>       Incr :: integer(),
>       Seq :: [integer()].
>
> seq(First, Last, Inc)
>     when is_integer(First), is_integer(Last), is_integer(Inc) ->
>     if
>         Inc > 0, First - Inc =< Last;
>         Inc < 0, First - Inc >= Last ->
>             N = (Last - First + Inc) div Inc,
>             seq_loop(N, Inc*(N-1)+First, Inc, []);
>         Inc =:= 0, First =:= Last ->
>             seq_loop(1, First, Inc, [])
>     end.
>
> seq_loop(N, X, D, L) when N >= 4 ->
>      Y = X-D, Z = Y-D, W = Z-D,
>      seq_loop(N-4, W-D, D, [W,Z,Y,X|L]);
> seq_loop(N, X, D, L) when N >= 2 ->
>      Y = X-D,
>      seq_loop(N-2, Y-D, D, [Y,X|L]);
> seq_loop(1, X, _, L) ->
>      [X|L];
> seq_loop(0, _, _, L) ->
>      L.
>
> -----------------------
>
> -----Problem-----------
> when I entered lists:seq(-10, 10, 2).
> what happened?
> I think it should execute 'Inc > 0, First-Inc =< Last;' and in this way it
> should just returning nothing.
>
>
>
> -----------------------
>
>
> ----- 原始邮件 -----
> 发件人:
> 收件人:
> 发送时间: 星期一, 2014年 11 月 03日 下午 7:00:00
> 主题: erlang-questions Digest, Vol 190, Issue 1
>
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>
> than "Re: Contents of erlang-questions digest..."
>
>
> Today's Topics:
>
>    1. Re:  DTLS/SRTP for WebRTC (Ingela Andin)
>    2. Re:  spawn and controlling_process (Hynek Vychodil)
>    3. Re:  spawn and controlling_process (Jachym Holecek)
>    4. Re:  spawn and controlling_process (Gadi Srebnik)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 2 Nov 2014 20:03:28 +0100
> From: Ingela Andin <>
> To: pablo platt <>
> Cc: erlang questions <>
> Subject: Re: [erlang-questions] DTLS/SRTP for WebRTC
> Message-ID:
>         <CAFj9NSTjtsh9u+LmKm-Pv4YM5OJ8Fz=_-
> >
> Content-Type: text/plain; charset="utf-8"
>
> Hi!
>
> 2014-10-30 22:33 GMT+01:00 pablo platt <>:
>
> > Hi,
> >
> > What is the state of DTLS in OTP?
> >
>
> Alas Ericsson has prioritised quite a few other things higher than DTLS
> just lately so the implementation has been delayed.
> I hope it will get back on track soon but I am afraid you can no count on
> it in a short term perspective.
>
> Regards Ingela Erlang/OTP team - Ericsson AB
>
>
>
> >
> > I'm interested in using DTLS/SRTP in Erlang for WebRTC.
> >
> > I've found two implementations:
> >
> > Clean code but I couldn't make it work because it's missing SRTP support.
> >
> > This works quite well. I'm able to connect clients to the server:
> >
> > The problem is that some clients can't connect and I'm getting HANDSHAKE
> > packets while in the CIPHER state in the following order:
> > HANDSHAKE, SERVER_HELLO
> > HANDSHAKE, CERTIFICATE
> > HANDSHAKE, SERVER_KEY_EXCHANGE
> > HANDSHAKE, CERTIFICATE_REQUEST
> > HANDSHAKE, SERVER_HELLO_DONE
> >
> > I'm assuming that is related to wrong state change or retransmission.
> > The code only implement retransmission for two packet types and have the
> > following comment:
> > %% problem remainning: different strategy should be adopted while
> > state==hello or cipher.
> >
> > Does anyone use DTLS/SRTP in Erlang and help solve this issue or
> recommend
> > another solution?
> >
> > Thanks
> >
> >
> > _______________________________________________
> > erlang-questions mailing list
> >
> > http://erlang.org/mailman/listinfo/erlang-questions
> >
> >
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>
> ------------------------------
>
> Message: 2
> Date: Mon, 3 Nov 2014 10:06:28 +0100
> From: Hynek Vychodil <>
> Cc: erlang-questions <>
> Subject: Re: [erlang-questions] spawn and controlling_process
> Message-ID:
>         <
> >
> Content-Type: text/plain; charset="utf-8"
>
> No, there is not any difference. Both versions of code is compiled exactly
> same:
>
> fun1(Socket,Mod, Fun, Args) ->
>     Pid = erlang:spawn(Mod, Fun, Args),
>     gen_tcp:controlling_process(Socket,Pid).
>
> fun2(Socket,Mod, Fun, Args) ->
>     gen_tcp:controlling_process(Socket,erlang:spawn(Mod, Fun, Args)).
>
> Then you can compile it with 'S' and get "assembler":
>
> {function, fun1, 4, 2}.
>   {label,1}.
>     {line,[{location,"test2.erl",5}]}.
>     {func_info,{atom,test2},{atom,fun1},4}.
>   {label,2}.
>     {allocate,1,4}.
>     {move,{x,0},{y,0}}.
>     {move,{x,1},{x,0}}.
>     {move,{x,2},{x,1}}.
>     {move,{x,3},{x,2}}.
>     {line,[{location,"test2.erl",6}]}.
>     {call_ext,3,{extfunc,erlang,spawn,3}}.
>     {move,{x,0},{x,1}}.
>     {move,{y,0},{x,0}}.
>     {line,[{location,"test2.erl",7}]}.
>     {call_ext_last,2,{extfunc,gen_tcp,controlling_process,2},1}.
>
>
> {function, fun2, 4, 4}.
>   {label,3}.
>     {line,[{location,"test2.erl",9}]}.
>     {func_info,{atom,test2},{atom,fun2},4}.
>   {label,4}.
>     {allocate,1,4}.
>     {move,{x,0},{y,0}}.
>     {move,{x,1},{x,0}}.
>     {move,{x,2},{x,1}}.
>     {move,{x,3},{x,2}}.
>     {line,[{location,"test2.erl",10}]}.
>     {call_ext,3,{extfunc,erlang,spawn,3}}.
>     {move,{x,0},{x,1}}.
>     {move,{y,0},{x,0}}.
>     {line,[{location,"test2.erl",10}]}.
>     {call_ext_last,2,{extfunc,gen_tcp,controlling_process,2},1}.
>
> As you can see, both functions are exactly same. Why should be there any
> difference.
>
> Hynek Vychodil
>
> On Sat, Nov 1, 2014 at 11:34 AM, Gadi Srebnik <> wrote:
>
> > Hi all, my first question here.
> >
> > How safe it is to use controlling_process after spawn? I am using
> > additional gen_tcp:controlling_process inside the Mod:Fun itself after
> few
> > additional commands. Is it possible that process will not be owner by
> then?
> >
> >
> > Pid = erlang:spawn(Mod, Fun, Args),
> >
> >  gen_tcp:controlling_process(Socket,Pid),
> >
> > Is one-liner safer?
> >
> > gen_tcp:controlling_process(Socket,erlang:spawn(Mod, Fun, Args)),
> >
> >
> > Thanks,
> >
> >
> >
> > _______________________________________________
> > erlang-questions mailing list
> >
> > http://erlang.org/mailman/listinfo/erlang-questions
> >
> >
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> ------------------------------
>
> Message: 3
> Date: Mon, 3 Nov 2014 04:30:26 -0500
> From: Jachym Holecek <>
> Cc:
> Subject: Re: [erlang-questions] spawn and controlling_process
> Message-ID: <>
> Content-Type: text/plain; charset=us-ascii
>
> > How safe it is to use controlling_process after spawn? I am using
> > additional gen_tcp:controlling_process inside the Mod:Fun itself after
> few
> > additional commands. Is it possible that process will not be owner by
> then?
>
> Yes, there's a race condition there, you need explicit synchronization to
> be safe:
>
>   start_worker(Sock) ->
>       Pid = erlang:spawn(?MODULE, worker_init, [Sock]),
>       gen_tcp:controlling_process(Sock, Pid),
>       Pid ! proceed.
>
>   worker_init(Sock) ->
>           proceed ->
>               %% Okay now we actually own it.
>               worker_loop(Sock)
>       after 1000 ->
>           exit(things_seem_awfully_slow_today)
>       end.
>
> I wish there were a variant of controlling_process that would advise the
> new owner automatically by sending {tcp_handover, Sock, Info} to it, the
> last item being arbitrary term passed by previous owner. No idea how hard
> would it be to implement.
>
> BR,
>         -- Jachym
>
>
> ------------------------------
>
> Message: 4
> Date: Mon, 3 Nov 2014 11:34:31 +0200
> To:
> Subject: Re: [erlang-questions] spawn and controlling_process
> Message-ID:
>         <CAFjX2eC2y8oE4O7COpiwpwhExFv-Zo0Gm+iF_fHexXRZOkh=
> >
> Content-Type: text/plain; charset="utf-8"
>
> Thank you, Jachym and Hynek.
>
> On Mon, Nov 3, 2014 at 11:30 AM, Jachym Holecek <>
> wrote:
>
> > # Gadi Srebnik 2014-11-01:
> > > How safe it is to use controlling_process after spawn? I am using
> > > additional gen_tcp:controlling_process inside the Mod:Fun itself after
> > few
> > > additional commands. Is it possible that process will not be owner by
> > then?
> >
> > Yes, there's a race condition there, you need explicit synchronization to
> > be safe:
> >
> >   start_worker(Sock) ->
> >       Pid = erlang:spawn(?MODULE, worker_init, [Sock]),
> >       gen_tcp:controlling_process(Sock, Pid),
> >       Pid ! proceed.
> >
> >   worker_init(Sock) ->
> >           proceed ->
> >               %% Okay now we actually own it.
> >               worker_loop(Sock)
> >       after 1000 ->
> >           exit(things_seem_awfully_slow_today)
> >       end.
> >
> > I wish there were a variant of controlling_process that would advise the
> > new owner automatically by sending {tcp_handover, Sock, Info} to it, the
> > last item being arbitrary term passed by previous owner. No idea how hard
> > would it be to implement.
> >
> > BR,
> >         -- Jachym
> >
>
>
>
> --
> VP Communications
>  m: +972 54 5929261
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>
> 李涛涛 Taotao · Li | Fixed  | Software Engineer
>
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