[reactive] Monads in Event streams

[Conal Elliott]

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_______________________________________________
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>
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