Why can't I add a blanket impl on a trait with a type parameter?
Consider these two traits:
pub trait Foo {
fn new(arg: u32) -> Self;
}
pub trait Bar<P>: Foo {
fn with_parameter(arg: u32, parameter: P) -> Self;
}
I'd like to add the blanket impl:
impl<T: Bar<P>, P: Default> Foo for T {
fn new(arg: u32) -> Self {
Self::with_parameter(arg, P::default())
}
}
But I get the compiler error:
error[E0207]: the type parameter `P` is not constrained by the impl trait, self type, or predicates
I think I get this error because I'm violating trait coherence rules, but I don't understand exactly what rule this would break. Why is this pattern not allowed? And, more importantly, can I achieve what I want without getting an error?
generics rust traits type-parameter
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
add a comment |
Consider these two traits:
pub trait Foo {
fn new(arg: u32) -> Self;
}
pub trait Bar<P>: Foo {
fn with_parameter(arg: u32, parameter: P) -> Self;
}
I'd like to add the blanket impl:
impl<T: Bar<P>, P: Default> Foo for T {
fn new(arg: u32) -> Self {
Self::with_parameter(arg, P::default())
}
}
But I get the compiler error:
error[E0207]: the type parameter `P` is not constrained by the impl trait, self type, or predicates
I think I get this error because I'm violating trait coherence rules, but I don't understand exactly what rule this would break. Why is this pattern not allowed? And, more importantly, can I achieve what I want without getting an error?
generics rust traits type-parameter
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
add a comment |
Consider these two traits:
pub trait Foo {
fn new(arg: u32) -> Self;
}
pub trait Bar<P>: Foo {
fn with_parameter(arg: u32, parameter: P) -> Self;
}
I'd like to add the blanket impl:
impl<T: Bar<P>, P: Default> Foo for T {
fn new(arg: u32) -> Self {
Self::with_parameter(arg, P::default())
}
}
But I get the compiler error:
error[E0207]: the type parameter `P` is not constrained by the impl trait, self type, or predicates
I think I get this error because I'm violating trait coherence rules, but I don't understand exactly what rule this would break. Why is this pattern not allowed? And, more importantly, can I achieve what I want without getting an error?
generics rust traits type-parameter
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
Consider these two traits:
pub trait Foo {
fn new(arg: u32) -> Self;
}
pub trait Bar<P>: Foo {
fn with_parameter(arg: u32, parameter: P) -> Self;
}
I'd like to add the blanket impl:
impl<T: Bar<P>, P: Default> Foo for T {
fn new(arg: u32) -> Self {
Self::with_parameter(arg, P::default())
}
}
But I get the compiler error:
error[E0207]: the type parameter `P` is not constrained by the impl trait, self type, or predicates
I think I get this error because I'm violating trait coherence rules, but I don't understand exactly what rule this would break. Why is this pattern not allowed? And, more importantly, can I achieve what I want without getting an error?
generics rust traits type-parameter
generics rust traits type-parameter
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
edited Mar 5 '17 at 21:52
Shepmaster
153k14301439
153k14301439
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
asked Mar 5 '17 at 21:03
OthersOthers
1,3871939
1,3871939
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
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!()
}
}
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
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!()
}
}
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
add a comment |
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!()
}
}
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
add a comment |
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!()
}
}
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
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!()
}
}
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
answered Mar 5 '17 at 22:09
Francis GagnéFrancis Gagné
32.6k26981
32.6k26981
add a comment |
add a comment |
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