Hi,

When talking about the Observer pattern I was thinking in the case that appears in the reactive programming entry in Wikipedia (http://en.wikipedia.org/wiki/Reactive_programming).

If we have a reactive setting then the sentence
a := b + c
could mean that there is a dynamic data flow from the values b and c to the value a, so whenever b or c change, a is automatically changed.

This has similarities to the OO Observer pattern (in fact, you can implement it using the pattern) and is also supported in some new scripting languages as JavaFX.

It seems that some goals of reactive programming are shared with the goals of the Observer pattern. Are they actually related? Maybe this relationship is between the pattern and reactive programming in general, not FRP. Is this so or is Wikipedia just wrong about that? Anyway, is it possible to implement something alike the a := b + c case using Reactive or any oher FRP package? Can someone show any example?

Thanks,

Alvaro.

---------- Forwarded message ----------
From: Patai Gergely <patai_gergely@fastmail.fm>
Date: 2009/6/9
Subject: Re: [reactive] FRP, continuous time and concurrency
To: Álvaro García Pérez <agarcia@babel.ls.fi.upm.es>, reactive@haskell.org


Hi Alvaro,

> Observer patern using the standard FRP conventions? Are these situations
> inside the aims of FRP?
There are several flavours of FRP that approach reactivity from
different angles. I'd say Grapefruit is the one most relevant to the
Observer pattern, since it models complex systems as a network of
interconnected circuits, where circuits are effectful entities, and they
can communicate through both discrete and continuous signals. The other
system that might be relevant is Yampa, since you model entities as
stateful signal functions (but unlike in Grapefruit they cannot perform
side effects), and connect them however you want. Incidentally, both of
these approaches are arrow based.

In contrast, Reactive aims to describe the (output over the) whole
lifetime of an entity as a pure value. Dependencies between entities are
established simply by defining one as a function of the other, and
mutual dependencies are naturally allowed. I don't think there's any
meaningful way to connect the Observer pattern to that. In fact, the
basic OO design patterns are often meaningless in functional
programming, because it's a completely different world.

Gergely

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