Re: [reactive] Monads in Event streams

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

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 wrote:

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On 23 Mar 2009, at 18:38, Freddie Manners wrote:

I've been worrying about what happens when monads end up caught inside an

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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

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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.

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