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type parameters of trait functions and impl selection combine in confusing ways #5221
Description
It started with this error:
pub trait VideoDriver {
static fn init() -> Self;
}
pub trait Platform {
static fn init<D: VideoDriver>() -> Self;
static fn foo() -> bool;
}
pub fn run<P: Platform, D: VideoDriver>() {
let platform: P = Platform::init::<D>();
}
---------------------------------------------------------------------------------------------
trait_generic_method.rs:11:19: 11:38 error: not enough type parameters provided for this item
trait_generic_method.rs:11 let platform: P = Platform::init::<D>();
^~~~~~~~~~~~~~~~~~~~~~
trait_generic_method.rs:11:19: 11:38 error: cannot determine a type for this bounded type parameter: unconstrained type
trait_generic_method.rs:11 let platform: P = Platform::init::<D>();
^~~~~~~~~~~~~~~~~~~~~~
Which is confusing because the init() method is just defined on one type parameter.
However if I change the line to let platform: P = Platform::init::<P, D>(); it compiles, which is confusing for the same reasons.
The cause for this is that you can specify the type of an trait impl by appending an ::<T> to the call of an function like this: let is_foo = Platform::foo::<int>(), which is fine as long as the function itself has no specific trait bounds. Combined though it's a mess, as it's not even clear in which order the parameters go.
As the type selection really belongs to the trait, and not to an function of that trait, I propose that the syntax should get changed to let platform = Platform::<P>::init::<D>().
More linenoise, but at least it's not confusing.
Activity
bstrie commentedon Mar 4, 2013
Or, since Rust has already diverged from C++ in this area, perhaps it's time to think of a less-hideous way to specify types at use sites. :)
Kimundi commentedon Mar 4, 2013
@bstrie sure, but how? ^^
bstrie commentedon Mar 4, 2013
Putting aside syntax for the moment, I still don't understand the error here. In the line:
what, exactly, is
P, and why is it necessary?Kimundi commentedon Mar 4, 2013
@bstrie: The
Pthere means 'use the implementation of Platform for type P'. It should be inferred becausePis already given as type of the variable, but apparently it is required if you want to give any other specific types.nikomatsakis commentedon Mar 5, 2013
Yes, we need to integrate type parameters into paths in a better way.
catamorphism commentedon May 23, 2013
Still a bug. Since this potentially involves changing syntax, I'm nominating for milestone 1, well-defined.
graydon commentedon May 23, 2013
accepted for well-defined milestone
graydon commentedon May 23, 2013
accepted for backwards-compatible milestone
graydon commentedon May 23, 2013
relative of #5527
emberian commentedon Jul 21, 2013
triage bump
catamorphism commentedon Oct 10, 2013
This is done.
Auto merge of rust-lang#5221 - JohnTitor:rustup, r=phansch
Merge pull request rust-lang#5221 from niooss-ledger/readme-Ctarget-f…
Merge rust-lang#5221