[erlang-questions] What is the point of Spawn(Node, Fun) if Node has to have the same module loadable as a client node?

Matthias Lang <>
Fri Sep 2 09:09:05 CEST 2016


Hi,

Thanks Nathaniel for pointing out that this is a dupe of a
stackoverflow question. It's good form to indicate when you're
aware of previous discussion and, perhaps, why you're not satisifed.

My guess is OP isn't satisifed because

  a) The two top-voted stack overflow answers gloss over the
     difference between funs defined in the shell and funs
     defined in code.

  b) Neither of the top-voted answers deals with the question the
     OP opens with.

Kresten's blog post, which is linked from the stack overflow comments,
is good, but it causes confusion for people who don't read it
carefully. One source of confusion is that it gives an example of a
fun which _does_ include the definition, says that the definition
isn't included and then, seven paragraphs later, explains that the
first example did actually include the definition.

Similarly, Joe has a blog post which was up on Hacker News (again?)
just a few days ago with an example which borders on being overly cute:

  http://joearms.github.io/2013/11/21/My-favorite-erlang-program.html

this similarly creates the impression that all you have to do is send
a fun to server and you're done, not even giving a nod to the problem
that you need to get the code to all 1171 nodes he wants to run the
code on.

---


I'd like to wrap up with a proper answer, the sort that Richard
Carlsson might write, but I don't have time for that. It would:

  - cover the difference between funs defined in the shell, funs which
    contain actual code and/or variables and funs which are pure
    references, e.g. fun io:fwrite/2

  - comment on the OP's mis-use of the word "definition" and cleaning
    up other language sloppiness to avoid disagreements and confusion
    caused by misunderstanding.

  - talk about how it's nice that the concept of funs is the same across
    lots of different uses, i.e. passing to lists:filter/2, sending
    to a local process, sending to a remote process and spawning, and
    that the cost of getting a fairly harmless surprise or two in the
    distributed case is low compared to the benefit of the concept being
    so simple.

  - speculate on the possibility of an Erlang system which
    automaticaly distributes code when you send a fun. It seems
    possible, the fun includes information about where it came from.

    My guess is that there's a simple, sane, useful case of
    distributed Erlang, which the case where you make sure all nodes
    have the same code from the start.

    Then there's the case where nodes don't all have the same code,
    and that case is messy and complicated. There are problems such as
    "what do you do if the fun refers to a different version of a module
    which is already loaded on the receiving node?". This feels like
    research, not "getting things done", and I'm into the latter.

Matthias

----------------------------------------------------------------------

Date: 01. September 2016
From: nesvarbu Pavarde <>
To 
Subject: [erlang-questions] What is the point of Spawn(Node, Fun) if Node has to have the same module loadable as a client node?


> Why create illusion that you are sending a Fun to remote node to execute in
> a new process? If client node has to have same module loadable with the Fun
> defined as a server node anyway. Why not only spawn(Node, M, F, A) then,
> which makes it clear that you are sending a definition of a function call,
> not Fun itself.
> I am experimenting with distributed Erlang and want it to pass clojures
> around "willy nilly", even through network. "Code is data" kinda thing.

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