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Comment by aerovistae | Hacker News Reader

That's super informative, thank you! Can you also add a word of explanation as to how the `sig` call gets associated with the method that follows it? What ties them together, is there some static parser?

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Sorbet has its own written-in-C++ parser that does the actual parsing. At runtime, the sig call basically sets a flag for the next method that is defined which hooks into it to validate that the parameters passed in are as defined, and that the return value is as expected. I believe they're delving into the dark magic in the interpreter directly, the docs are here: https://sorbet.org/docs/runtime

For the runtime, there is not much dark magic. Each type that has an `extend T::Sig` has `method_added` hooks registered, which notifies Sorbet runtime whenever a method is defined on the type. When that `method_added` hook is called, Sorbet runtime uses the sig flag that you mention to associate the `sig` with the method definition that follows it.

It is, more or less, an implementation of this idea: https://yehudakatz.com/2009/07/11/python-decorators-in-ruby/

Yes indeedy, I'd consider `method_added` to be pretty dark magic though :)

The actual signature checking is complicated, but associating the sig call with the method is pretty simple in Ruby — you just hook `method_added`. (Of course the actual implementation in something like Sorbet is a lot more sophisticated than just `def method_added`, but that's the basics of how you make a method call alter the next method definition.)

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