Why can’t I add a blanket impl on a trait with a type parameter?

by

The problem is that a single type could implement Bar<P> for multiple values of P. If you had a struct Baz that implemented Bar<i32> and Bar<String>, which type should Foo::new use for P?

The only solution is to ensure that a single type cannot implement Bar more than once (if that’s not what you want, then you have a flaw in your design!). To do so, we must replace the P type parameter with an associated type.

pub trait Bar: Foo {
    type Parameter;

    fn with_parameter(arg: u32, parameter: Self::Parameter) -> Self;
}

impl<T> Foo for T
where
    T: Bar,
    T::Parameter: Default,
{
    fn new(arg: u32) -> Self {
        Self::with_parameter(arg, T::Parameter::default())
    }
}

An implementation of Bar would look like this:

struct Baz;

impl Bar for Baz {
    type Parameter = i32;

    fn with_parameter(arg: u32, parameter: Self::Parameter) -> Self {
        unimplemented!()
    }
}

See also:

More Related Contents:

  1. “Expected type parameter” error in the constructor of a generic struct
  2. How do I use integer number literals when using generic types?
  3. How can I avoid a ripple effect from changing a concrete struct to generic?
  4. The trait cannot be made into an object
  5. Conflicting implementations of trait in Rust
  6. Generics Error: expected type parameter, found struct
  7. Can I avoid eager ambiguity resolution for trait implementations with generics?
  8. What is the cited problem with using generic type parameters in trait objects?
  9. How do I require a generic type implement an operation like Add, Sub, Mul, or Div in a generic function?
  10. How to write a trait bound for adding two references of a generic type?
  11. How do I write the lifetimes for references in a type constraint when one of them is a local reference?
  12. Why does a generic method inside a trait require trait object to be sized?
  13. How do I return a Filter iterator from a function?
  14. Matching a generic parameter to an associated type in an impl
  15. Why can’t I use a key function that returns a reference when sorting a vector with sort_by_key?
  16. Why can’t `&(?Sized + Trait)` be cast to `&dyn Trait`?
  17. What does the question mark mean in a type parameter bound?
  18. What does it mean to instantiate a Rust generic with an underscore?
  19. Writing a generic function that takes an iterable container as parameter in Rust
  20. How do I unwrap an arbitrary number of nested Option types?
  21. Unable to create a polymorphic type because the trait cannot be made into an object
  22. Traits and Rust
  23. How do I implement the Add trait for a reference to a struct?
  24. Why is the `Sized` bound necessary in this trait?
  25. Is there any trait that specifies numeric functionality?
  26. How do I pass a closure through raw pointers as an argument to a C function?
  27. Can’t implement a trait I don’t own for all types that implement a trait I do own
  28. How to get generic parameter class in Kotlin
  29. Is it possible to extend a default method implementation of a trait in a struct?
  30. How do I write this member constraint in F#?

Leave a Comment