[Haskell-cafe] Re: [Haskell] Dynamic binding

[Andreas Rossberg]

Ralf Lammel wrote:
> 
> Can you just tell how *you* would favor encoding the shapes example
> that was posed by poster? (It might just be that your code would be
> very close to Lennart's proposal?)

There is no universal answer. The shape example obviously is just a toy 
example. As long as I have no idea what *concrete* problem the original 
poster is trying to solve, I cannot really tell which approach is 
preferable for his endeavour. It seems more appropriate to describe the 
options and their trade-offs.

First of all, note that very often, you do not need a heterogeneous 
collection at all. Then plain polymorphism with type classes is more 
than enough, and more than OO provides in that situation.

In many cases where you need some kind of heterogenicity (is that a 
word?), the standard datatype approach shown by Lennart is perfectly 
suitable - in fact, often much more suitable than the OO solution. You 
know about the "expression problem", and the two dual notions of 
extensibility? OO supports one dimension, datatypes the other. It 
depends on the problem which one is more important.

When you really need OO-style intensional kind of behaviour selection 
then first-class functions can bring you quite a long way, and often 
with much less amount of boilerplate than typical OO solutions.

When the behaviour you have to encapsulate in heterogeneous collection 
becomes more complex - say, more then just one or two functions - 
first-class functions can be tedious. Existential types represent a more 
coarse-grained and structured variant of the first-class function 
approach. They combine the power of first-class functions with the 
convenience of type classes, very similar to class-based objects. In 
these cases, they are the most appropriate solution.

Note again that with the latter two solutions, as with the OO approach, 
you do not have extensibility on the operations dimension.

Cheers,

   - Andreas

-- 
Andreas Rossberg, rossberg at ps.uni-sb.de

Let's get rid of those possible thingies!  -- TB