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Y Combinators and Monads - How?

From: "andrew cooke" <andrew@...>

Date: Wed, 2 May 2007 13:38:17 -0400 (CLT)

Search for "unix pipe erlang" and it seems like everyone and their dog has
written code something like that below (they allows functions to be
chained together as parallel processes).

But what if my functions need to manage some state?  I have a rather large
(and useful) (quasi-)monad library.  How do I integrate that?

Andrew

PS "Response" doesn't make much sense out of context, but provides some
information for logging.

-module(chain).

-import(choopa_util, [res_info/2, res_info/3]).
-export([new_end/3, new_link/4, echo/2]).


bootstrap(Fun) ->
    Pid = spawn(fun() -> Fun(Fun) end),
    fun(Message) -> Pid ! Message end.

new_end(Response, Filter, Name) ->
    bootstrap(fun(Self) ->
                      receive
                          {data, Data} ->
                              Filter(Data),
                              Self(Self);
                          exit ->
                              res_info(Response, "Exiting ~s", [Name]);
                          Other ->
                              res_info(Response, "~s: unexpected ~p ",
                                       [Name, Other]),
                              Self(Self)
                      end
              end).

new_link(Response, Filter, Name, Next) ->
    bootstrap(fun(Self) ->
                      receive
                          {data, Data} ->
                              Next({data, Filter(Data)}),
                              Self(Self);
                          exit ->
                              Next(exit),
                              res_info(Response, "Exiting ~s", [Name]);
                          Other ->
                              res_info(Response, "~s: unexpected ~p ",
                                       [Name, Other]),
                              Self(Self)
                      end
              end).

echo(Response, Tag) ->
    fun(Message) -> res_info(Response, Tag),
                    Message
    end.

Bananas etc

From: "andrew cooke" <andrew@...>

Date: Wed, 2 May 2007 13:45:56 -0400 (CLT)

Some answers after sleeping on this (original post was sent yesterday;
reposted today due to svn shenanigans - not having a very good day,
svn-wise).

1 - What I keep calling monads are really bananas and folds and all that
stuff - ana-thing and cata-wotsist - and I really need to get those clear
(and their relation to monads).  For example, the most popular of my
"monad" functions is really a fold that cleans up after a A -> B -> (C *
A) converts [B] to [C] (and what is that thing called? - it's the A that
is my "monad").

2 - This is kind-of irrelevant to what I was stuck on because the levels
are different.  My monads/folds/whatevers are processing (individual items
in) lists of data, while the processes are processing (passing around)
chunks of data (sub-lists).

3 - (2) is kind-of related to streams and protocols.  Working on Mule has
made me more interested in how you define and separate protocols at
different layers (in theory easy, but in practice you seem to need to make
certain compromises).

Andrew

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