keera-hails-reactivevalues

Reactive Values

Reactive Values are time-varying mutable values that can be read/written and may trigger events when they change. They can be used to define a uniform interface to hardware ports, widget attributes or model fields, and keep them all synchronized at all times (making your controllers just a bunch of synchronization rules).

For instance, in Hails programs, a field of the model that holds a String (for instance, with the name of the file that the user is editing) may be a read-write reactive value. The title of a Gtk window may be a Write-only reactive value.

So we can always keep them in sync in our programs by writing:

    titleOfMainWindow <:= liftR ("My program - " ++) filenameFieldOfModel

This will update the window title every time the file name changes. Because read-only values are functors, you can also use applicative style:

    titleOfMainWindow <:= ("My program - " ++) <$> filenameFieldOfModelRO

(where filenameFieldOfModelRO is a read-only RV.) Note that RW RVs are not Functors (the targets of the morphisms would be read-only variables, even if we originally had an RW, so it does not fulfill the functor laws).

If titleOfMainWindow were instead a RW reactive value, we could have said:

    titleOfMainWindow =:= filenameFieldOfModel

That would also update the file name in the model if the title changed for some reason (although that is not something we'd like to do in this particular example, but we might want to sync a text box with a string field).

You can also lift functions in =:= equations by defining them bijective:

      titleOfMainWindow =:= liftRW f filenameFieldOfModel
    where f = BijectiveFunc (s ++) (drop (len s))
          s = "My program -"

We use a definition of Functor that is parametric also on the morphisms of the source category, which means that you can regard them as functors:

      titleOfMainWindow =:= f <$$> filenameFieldOfModel
    where f = BijectiveFunc (s ++) (drop (len s))
          s = "My program -"

(<$$> is to GFunctor what <$> is to Functor). And if the function being lifted is equal to its inverse, you can declare it an involution:

      titleOfMainWindow =:= (involution reverse) <$$> filenameFieldOfModel

The above line would keep the fields in sync, but one would be the reversed representation of the other. Note that reverse here has nothing to do with inverse functions, it is the normal list reverse operation.

Installation

This package has no direct dependencies other than base and contravariant, so you can install it and use it in your own programs without any of the existing backends.

GHC 7.6, 7.8 and 7.10 are supported. It may work with other versions too.

Support

Open a ticket or send me a message if you use this and have a problem.

Also, I'd be happy to know (and publish a list of) programs that use hails-reactivevalues. Please, let me know if you write something that uses it.