Interestingly, from your description of a binary tree, it could also be defined in Haskell as:
Yawar λmin
1

Yes, that’s true, sub-classing is a fine way to make sum types in an OO language. Where things get awkward, however, is when you try to traverse a structure like that. It seems you either have to check types and cast, or else use something like the visitor pattern.

So I think the difference is that ML languages make it really easy to define sum types, and then really easy to work with them thanks to pattern matching.

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