[reactive] Monads in Event streams

[Freddie Manners]

Thanks.  The (non-contrived) motivation was related to trying to link
external databases into reactive code; my attempts to make legacy adapters
for database calls seemed very unnatural, so Event (IO a) everywhere looked
like an option.  I will give the pure approach another hack.

Freddie

2009/3/24 Conal Elliott <conal at conal.net>

> Thanks, Bob.  I agree entirely.  FRP is a replacement for the imperative
> (IO) way of thinking.  The IO exposed in Reactive is solely for making
> legacy adapters.  Applications would have no visible IO.  Why?  Because
> (non-toy) imperative programming has intractable denotation and hence is
> troublesome to compose and reason about.  - Conal
>
> On Mon, Mar 23, 2009 at 3:48 PM, Thomas Davie <tom.davie at gmail.com> wrote:
>
>>
>> On 23 Mar 2009, at 18:38, Freddie Manners wrote:
>>
>>  I've been worrying about what happens when monads end up caught inside an
>>> Event stream, as they inevitably do.  Usually this is the IO monad, &
>>> usually it's not a problem because adaptE deals with Event (IO a) types
>>> eventually.  But look at the following (contrived) example:
>>>
>>> I read lots of people's mail.  I have a load of buttons which add a
>>> person's mail to the set I receive, summarised by:
>>> addPerson :: Event Person
>>> I want an event of all the mail I get.  Given:
>>> getMail :: Person -> Event Mail
>>> I can do:
>>> allMail :: Event Mail
>>> allMail = addPerson >>= getMail
>>> But realistically, I might have to create a new event for each person, or
>>> look the event up in a database or something.  Then I have only:
>>> getMailM :: Person -> IO (Event Mail)
>>>
>>
>> Why might that be the type?  This sounds like the getMail function, only
>> with a chunk of legacy adapter exposed in your pure code.
>>
>>  Also, these fixes will only stand a chance with IO (or ST, maybe); what
>>> if getMailM :: Person -> State Foo (Event Mail) ?
>>>
>>
>>
>> The simple answer is – don't!  IO and State are methods of describing time
>> based computation, but we already have a lovely pure functional description
>> of time based computation – Reactive.  Your IO should end in the legacy
>> adapter.
>>
>> Bob_______________________________________________
>> Reactive mailing list
>> Reactive at haskell.org
>> http://www.haskell.org/mailman/listinfo/reactive
>>
>
>
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