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> Arguably, c is a Color object. But actually, c must be either Color::Green or Color::Blue. The compiler understands this and uses it in lots of little ways. B

> Arguably, c is a Color object. But actually, c must be either Color::Green or Color::Blue. The compiler understands this and uses it in lots of little ways. But unfortunately we can't actually name the restricted type in the program. I think that's less a question whether you can, but rather whether you should or shouldn't design it that way... (I'll use TypeScript here for the simpler syntax) It'd be perfectly fine to do something like this: type ColorR = 'red'; type ColorG = 'green'; type ColorB = 'blue'; type ColorRGB = ColorR | ColorG | ColorB; But which constraint should your new type ColorGB (your variable c) adhere to? // constraint A type ColorGB = ColorG | ColorB; // constraint B type ColorGB = Exclude<ColorRGB, ColorR>; I'd argue if the type ColorGB is only needed in the derived form from ColorRGB within a single scope, then just let the compiler do its control flow analysis, yes - it'll infer the type as constraint B. But if you really need to reuse the type ColorGB (probably some categorization other than all the colors), then you'd need to pay close attention to your designed constraint.

josephg 2y ago
I really wish I could do in rust what you're doing here in typescript. Say I have some enum like ColorRGB here. In some contexts, only a limited subset of variants are valid - say, ColorGB. There's a few ways to code this - but they're all - in different ways - horrible: 1. Use ColorRGB in all contexts. Use asserts or something to verify that the value is one of the expected variants. This fails at expressing what I want in the type system - and the code is longer, slower and more error prone as a result. 2. Have two enums, ColorRGB and ColorGB. ColorRGB is defined as enum ColorRGB { Red, Restricted(ColorGB) }. This lets me encode the constraint - since I can use ColorGB explicitly. But it makes Color harder to use - since I need to match out the inner value all over the place. 3. Have two enums, ColorRGB and ColorGB which both have variants for Green and Blue. Implement conversion methods (impl From) between the two types. Now I have two types instead of one. I have conversions between them. And I'll probably end up with duplicate methods & trait impls for ColorRGB and ColorGB. Luckily this doesn't come up that often. But - as your typescript example shows - it can just be expressed directly in the type system. And LLVM already tracks which variants are possible throughout a function for optimisations' sake. I wish rust had a way to express something like Exclude<ColorRGB, ColorR>.

josephg 2y ago

I really wish I could do in rust what you're doing here in typescript. Say I have some enum like ColorRGB here. In some contexts, only a limited subset of variants are valid - say, ColorGB. There's a few ways to code this - but they're all - in different ways - horrible: 1. Use ColorRGB in all contexts. Use asserts or something to verify that the value is one of the expected variants. This fails at expressing what I want in the type system - and the code is longer, slower and more error prone as a result. 2. Have two enums, ColorRGB and ColorGB. ColorRGB is defined as enum ColorRGB { Red, Restricted(ColorGB) }. This lets me encode the constraint - since I can use ColorGB explicitly. But it makes Color harder to use - since I need to match out the inner value all over the place. 3. Have two enums, ColorRGB and ColorGB which both have variants for Green and Blue. Implement conversion methods (impl From) between the two types. Now I have two types instead of one. I have conversions between them. And I'll probably end up with duplicate methods & trait impls for ColorRGB and ColorGB. Luckily this doesn't come up that often. But - as your typescript example shows - it can just be expressed directly in the type system. And LLVM already tracks which variants are possible throughout a function for optimisations' sake. I wish rust had a way to express something like Exclude<ColorRGB, ColorR>.