> Interestingly crystal has neither sum types or pattern matching (and it doesn't need them) :)

Well, the use cases will be there, and any general-purpose language will need answers for them. One can of course achieve the same effect with other language features (e.g. Java's famous use of the "visitor pattern" for those cases), but there's a cost in verbosity and maintainability.

Crystal can essentially have a sum type by representing it as a union of different record types. It's not common to do in crystal though, at least not with 3+ union entries.

And pattern matching isn't really needed if you have flow typing + OO (at least I haven't found a usecase for pattern matching which crystal doesn't already have an answer to)

> Crystal can essentially have a sum type by representing it as a union of different record types. It's not common to do in crystal though, at least not with 3+ union entries.

Sum types usually only have two or three entries too.

> And pattern matching isn't really needed if you have flow typing + OO (at least I haven't found a usecase for pattern matching which crystal doesn't already have an answer to)

How does "flow typing + OO" solve the problem? You can use OO to emulate pattern matching via the visitor pattern, yes, but that's notoriously cumbersome; I don't see how flow typing makes any difference to that?

Pattern matching, at least in functional languages, is mainly used both as control flow to match on the type of arguments, and also to destructure data structures. In flow typing you replace the control flow part of pattern matching with `if x.is_a? Type` and since everything you can do to an object is just passing a message to it (thanks to ruby's smalltalk origins), you don't really need the destructuring since you can just then use `foo.bar`.

Crystal has method overloading (dynamic dispatch), which is a form of pattern matching (but not destructuring) but that's it. So I guess we have pattern matching but definitely not to the extent say haskell or elixir has it.

Visitor pattern in Crystal is just a class with a bunch of `def visit(node : Type)` and then just call `visit(foo)` and let dynamic dispatch handle the rest. The visited data structure has no knowledge about the visitor.

> In flow typing you replace the control flow part of pattern matching with `if x.is_a? Type`

Hmm, I guess flow typing lets you do an exhaustiveness check of an if-else style set of cases. Still seems like a syntactic shortcut would be nice, just as one misses "switch" in Python.

Yeah, crystal has one of those (and it's about to gain exhaustiveness checking), so I didn't literally always mean `if/else` :)

If you can switch on the type of a variable and have it have the relevant type within each switch arm I'd consider that to be pattern matching, though I guess that's just semantics.