Add sum types to the language #46
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byorgey
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This was referenced Oct 3, 2021
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Oct 16, 2021
Adds sum types (e.g. `int + bool`) along with constants `inl : a -> a + b`, `inr : b -> a + b`, and `case : a + b -> (a -> c) -> (b -> c) -> c`.
I considered whether to automatically wrap the arguments to `case` in `delay`, like the arguments to `if`, but they are already functions anyway so we get a lot of laziness for free. It would only make a difference if you wrote something like `case foo (let x = blah in \y. ...) (...)` in which case the `let x = blah` is going to be evaluated eagerly, *before* we know which branch we are going to take. I don't think it's worth bothering about. In the normal case that you write `case foo (\y. ...) (\z. ...)` then only one of the two functions gets run at all, without us having to do anything special with `delay`.
I also considered whether to make special syntax for case expressions, like `case foo of { inl x -> ... ; inr y -> ... }` but doing it as a simple built-in function is both easier to implement and feels like it fits better with the aesthetic of the language so far.
Closes #46 .
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Somehow I added product types but haven't gotten around to adding sum types yet!
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