Is it possible to provide a custom pattern decomposition in Haskell?

Question

In Haskell, I have a module with a partial order type:

data PartialOrder a = PartialOrder [a] [(a, a)]

I don't export the value constructor because that's not how I want the type to be used, but I still want to be able to pattern match the PartialOrder type outside the module; is this possible? In particular, I want to be able to pattern match something that is not the type constructor, but instead, to pattern match something like the following:

f (PartialOrder xs le) = ...

Where le is a function implicitly defining the explicit ordering defined in the value constructor. I know such a facility is available in Scala, is there a way to do the same in Haskell?

Thanks in advance.

Answer 1

It sounds like a use case for ViewPatterns. You could write a type like:

data ViewPartialOrder a = ViewPartialOrder a (a -> a -> Ordering)

Write a function like:

viewOrder :: PartialOrder -> ViewPartialOrder
viewOrder (PartialOrder xs relation) = ...

then use the ViewPatterns extension to write code like this:

f (viewOrder -> ViewPartialOrder xs le) = ...

Of course, you should come up with better names for these things :P!

There's no way to have the same effect implicitly (ie without the viewOrder function), for better or worse. I think it's usually a good thing, making it clear that you're not matching the actual implementation of the type.