[reactive] Monads in Event streams

[Thomas Davie]

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