[zogrodea]

I think "sum types" and "pattern matching" in functional languages are great and expressive.

A sum type is basically an enum type you can associate arbitrary (but still strongly typed) data with (like associating two strings and an integer for one case, and nothing at all for the second case).

Pattern matching is basically like a switch statement on these sum types, destructuring the data and branching on the different cases. It comes with compile-time checking making sure you've covered all the cases if your business requirements change which is helpful too.

They are useful for implementing state machines which go from one discrete state to another. They've also been used to implement data structures like balanced binary trees, but I think that's a less "everyday" use for them.

I hope I explained them at least sort-of clearly. I think their functionality can be replicated in other ways (like defining an abstract Animal class and implementing Cat and Dog classes, instead of defining an Animal "enum" and a function that branches on each case), but this feels more natural and expressive to me personally.