[Haskell-cafe] exceptions, errors, random

[Ivan Lazar Miljenovic]

On 23 June 2017 at 14:54, Dennis Raddle <dennis.raddle at gmail.com> wrote:
> Thanks for pointing me to mwc-random. I like that they provide a normal
> distribution; that is handy.
>
> I think by asking me parallelism or concurrency, you mean do I have
> algorithms that interact as they run in separate threads?
>
> This is a backtracking search algorithm, so each thread can explore a subset
> of the search space with no need to communicate with other threads. Does
> that mean "concurrency"?
>
> This is a pretty simple task. I hope that I don't have to delve deeply into
> concurrency/parallelism.

Parallelism: split data set up, run the same operation on each sub-set.
Concurrency: do multiple things at once, possibly unrelated.

Many languages use concurrency to implement parallelism; Haskell
doesn't: see https://wiki.haskell.org/Parallelism_vs._Concurrency and
https://wiki.haskell.org/Parallel

For a start, have a look at the parallel package:
http://hackage.haskell.org/package/parallel ; in particular, par can
be used to fork off a new thread, and pseq can be used to ensure a
thread finishes before you continue.

>
> D
>
> On Tue, Jun 20, 2017 at 3:59 PM, Ivan Lazar Miljenovic
> <ivan.miljenovic at gmail.com> wrote:
>>
>> On 21 June 2017 at 03:19, Dennis Raddle <dennis.raddle at gmail.com> wrote:
>> > Looking at my application needs, such as access to pseudorandom numbers
>> > and
>> > threads, does this suggest a particular course of action?
>>
>> For randomness, probably mwc-random, carrying the seed around in a
>> StateT with IO on the bottom of the stack.
>>
>> For multi-threaded: parallelism or concurrency?  Simon Marlow's
>> Parallel and Concurrent Programming in Haskell is an excellent book on
>> the topic: http://shop.oreilly.com/product/0636920026365.do
>>
>> >
>> > D
>> >
>> >
>> > On Tue, Jun 20, 2017 at 1:40 AM, Ivan Lazar Miljenovic
>> > <ivan.miljenovic at gmail.com> wrote:
>> >>
>> >> There are two general schools of thought as to how to deal with
>> >> exceptions:
>> >>
>> >> * ExceptT/MonadError from transformers/mtl:
>> >>
>> >>
>> >> http://www.mega-nerd.com/erikd/Blog/CodeHacking/Haskell/what_do_you_mean.html
>> >>
>> >> * The exceptions package (which is a lifted variant of
>> >> Control.Exception in base):
>> >>
>> >> https://www.fpcomplete.com/blog/2016/11/exceptions-best-practices-haskell
>> >>
>> >> On 20 June 2017 at 15:53, Dennis Raddle <dennis.raddle at gmail.com>
>> >> wrote:
>> >> > Just want to bump this request as I have not gotten a reply.
>> >> >
>> >> > On Fri, Jun 16, 2017 at 1:45 PM, Dennis Raddle
>> >> > <dennis.raddle at gmail.com>
>> >> > wrote:
>> >> >>
>> >> >> I am wondering what exception/error and random classes I should use
>> >> >> for
>> >> >> my
>> >> >> application.
>> >> >>
>> >> >> The application will be doing Monte Carlo backtracking search. I
>> >> >> want
>> >> >> it
>> >> >> to be multithreaded and run on all four cores on my MacBook.
>> >> >>
>> >> >> I need detailed error messages in order to investigate errors. So I
>> >> >> will
>> >> >> throw errors or exceptions, then catch them in lower functions and
>> >> >> annotate
>> >> >> them with the parameters or computations within the lower function,
>> >> >> then
>> >> >> re-throw until finally the exception pops into the terminal.
>> >> >>
>> >> >> Because it's multi-threaded, at least some parts will needs to be in
>> >> >> IO.
>> >> >> Also, because it's a Monte Carlo algorithm it will need to make
>> >> >> pseudorandom
>> >> >> choices.
>> >> >>
>> >> >> Perhaps I should put everything in the IO monad? This will give me
>> >> >> access
>> >> >> to threads and the standard generator seed. But how about throwing,
>> >> >> catching/rethrowing exceptions in IO? I have done this in pure code
>> >> >> before:
>> >> >> not sure if it's better to do it that way, in which case I could put
>> >> >> some
>> >> >> kind of state monad to hold the random seed together with an error
>> >> >> monad and
>> >> >> IO at the core of the monad stack.
>> >> >>
>> >> >> Any advice welcome.
>> >> >> D
>> >> >>
>> >> >
>> >> >
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>> >>
>> >>
>> >>
>> >> --
>> >> Ivan Lazar Miljenovic
>> >> Ivan.Miljenovic at gmail.com
>> >> http://IvanMiljenovic.wordpress.com
>> >
>> >
>>
>>
>>
>> --
>> Ivan Lazar Miljenovic
>> Ivan.Miljenovic at gmail.com
>> http://IvanMiljenovic.wordpress.com
>
>



-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com