[Haskell-beginners] Designing complex Haskell programs

[Bob Ippolito]

I wouldn't recommend going down the path of using IORef or MVar for
everything, it's not easy to build robust systems that way. Do you mind
showing the code that you tried that "fell apart"? I'm sure there's a
slightly different way to structure it that would work just fine, probably
using some kind of message passing.


On Fri, Jan 3, 2014 at 9:57 AM, Courtney Robinson <courtney at crlog.info>wrote:

> Thanks to both of you for your reply.
> I have something similar to your example Bob, wasn't sure if it was a good
> way forward. Plus it fell apart when I tried contacting multiple hosts on
> different threads using forkIO. But with Daniel's response I'll look into
> MVars.
>
> Thanks again
>
>
> On Fri, Jan 3, 2014 at 5:16 PM, Bob Ippolito <bob at redivi.com> wrote:
>
>> Generally speaking, state lives on the call stack in functional
>> programming languages that have tail call elimination. Modification of the
>> state is done by recursion with a new value for the state. This is more or
>> less equivalent to a "do while" loop in imperative programming.
>>
>> myServer :: State -> IO ()
>> myServer state = do
>>   state' <- updateState state
>>   myServer state'
>>
>> For the concurrency, Control.Concurrent or Cloud Haskell (for a higher
>> level Erlang-like approach) is probably the way to go here. Parallel and
>> Concurrent Programming in Haskell is a great resource:
>> http://chimera.labs.oreilly.com/books/1230000000929
>>
>>
>> On Fri, Jan 3, 2014 at 8:45 AM, Courtney Robinson <courtney at crlog.info>wrote:
>>
>>> I'm trying to take the training wheels of and moving more of my code
>>> base to Haskell from C++ but finding it increasingly tricky.
>>>
>>> I have a subset of a gossip protocol written in C++.
>>> When a server comes online it connects to 1 or more nodes already in the
>>> cluster and get data from them about other nodes they know of.
>>>
>>> The new node merges the information and keeps a copy of the merged view.
>>> Every so often it contacts the nodes it knows about and refreshes the
>>> merged view. It also must have the up to date view ready to be sent in
>>> response to a new node joining.
>>>
>>> I currently can't wrap my head around how to maintain this state. How
>>> would a more experienced Haskeller approach this problem? Code is OK if it
>>> demonstrates a particular point but I'm more interested in the line of
>>> thought that would go into designing a solution as I suspect that'll be
>>> more useful as I get further into the migration.
>>>
>>> As a gauge to you for my current level in Haskell. I read and understand
>>> most Haskell programs fine. I write some but currently heavily rely on
>>> hackage/hoogle docs for APIs, even some common ones.
>>>
>>> Thanks
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://www.haskell.org/mailman/listinfo/beginners
>>>
>>>
>>
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>
>
> --
> Courtney Robinson
> courtney at crlog.info
> http://crlog.info
> 07535691628 (No private #s)
>
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