4 ms·

There are a few more fundemental missing pieces for me: - It's impossible to describe a type that "implements trait A and may or may not implement trait B" -

• tialaramex 4y ago
  > It's impossible to describe a type that "implements trait A and may or may not implement trait B" How is this different from just describing a type that only "implements trait A" ?
  • nicoburns 4y ago
    It would allow you to call a function to check for trait B and downcast to "implements trait A and B" in the case that it does implement the trait.
    • merely-unlikely 4y ago
      I’m still learning the language but couldn’t you use an enum containing two types to accomplish the same thing?
      • nicoburns 4y ago
        You can if you know all of the possible types in advance. But if you want to expose this as an interface from a library that allows users to provide their own custom implementation then you need to use traits.
    • tialaramex 4y ago
      It seems like a way to ask "Can this thing implement X and if so how?" from say the Any trait would be what you want here, I have no idea how hard that would be to deliver but I also don't see how the previous trait thing is relevant, like, why do we need to say up front that maybe we will care whether trait B is implemented?
• insanitybit 4y ago
  > - It's impossible to describe a type that "implements trait A and may or may not implement trait B" So, specialization? Or something else? I haven't found a need for specialization. I remember when I came from C++ I had a hard time adjusting to "no specialization, no variadics" but idk I haven't missed it in years. > - It's impossible to be generic over a trait (not a type that implements a trait, the trait itself) Not sure I understand.
  • nicoburns 4y ago
    > So, specialization? Basically yes. But that works with dynamic dispatch (trait objects) as well as static dispatch (generics). > Not sure I understand. A specific pattern I'd like to be able to represent is: trait AlgorithmAInputData { ... } trait AlgorithmA { trait InputData = AlgorithmAInputData; ... } trait DataStorage<trait AlgorithmA> { type InputData : Algorithm::InputData; fn get_input_data() -> InputData; } fn compute_algorithm_a<Storage: DataStorage<AlgorithmA>>() { ... }