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