Traits are widely (ab)used

forth!(
  : factorial (n -- n) 1 swap fact0 ;
  : fact0 (n n -- n) dup 1 = if drop else dup rot * swap pred fact0 then ;
  5 factorial .
);

github.com/Ashymad/fortraith

diesel documentation

bevy discussions

Traits 101

 trait IntoString {
  fn to_string(&self) -> String;
}
 
impl IntoString for (i32, i32) {
  fn to_string(&self) -> String {
    format!(
      "({}, {})",
      self.0, self.1
    )
  }
}
 
impl<T: IntoString>
  IntoString for Vec<T> {
  fn to_string(&self) -> String
  { /* ... */ }
}
 
fn main() {
  let v: Vec<(i32, i32)> = vec![
    (0, 1), (2, 3)
  ];
  println!("{}", v.to_string());
}

A trait definition, establishing the unit of shared behavior.

Traits 101

 trait IntoString {
  fn to_string(&self) -> String;
}
 
impl IntoString for (i32, i32) {
  fn to_string(&self) -> String {
    format!(
      "({}, {})",
      self.0, self.1
    )
  }
}
 
impl<T: IntoString>
  IntoString for Vec<T> {
  fn to_string(&self) -> String
  { /* ... */ }
}
 
fn main() {
  let v: Vec<(i32, i32)> = vec![
    (0, 1), (2, 3)
  ];
  println!("{}", v.to_string());
}

(i32, i32): IntoString.

A trait implementation associates a type with a trait.

Traits 101

 trait IntoString {
  fn to_string(&self) -> String;
}
 
impl IntoString for (i32, i32) {
  fn to_string(&self) -> String {
    format!(
      "({}, {})",
      self.0, self.1
    )
  }
}
 
impl<T: IntoString>
  IntoString for Vec<T> {
  fn to_string(&self) -> String
  { /* ... */ }
}
 
fn main() {
  let v: Vec<(i32, i32)> = vec![
    (0, 1), (2, 3)
  ];
  println!("{}", v.to_string());
}

 (i32, i32): IntoString.
 
Vec<T>: IntoString :-
  T: IntoString.

Implementations can be parametric.

Traits 101

 trait IntoString {
  fn to_string(&self) -> String;
}
 
impl IntoString for (i32, i32) {
  fn to_string(&self) -> String {
    format!(
      "({}, {})",
      self.0, self.1
    )
  }
}
 
impl<T: IntoString>
  IntoString for Vec<T> {
  fn to_string(&self) -> String
  { /* ... */ }
}
 
fn main() {
  let v: Vec<(i32, i32)> = vec![
    (0, 1), (2, 3)
  ];
  println!("{}", v.to_string());
}

(i32, i32): IntoString.

Vec<T>: IntoString :-
  T: IntoString.

?- Vec<(i32, i32)>: IntoString.

Type-checking queries the current facts.

Type-checking is tracing Prolog

 (i32, i32): IntoString.
 
Vec<T>: IntoString :-
  T: IntoString.

?- Vec<(i32, i32)>: IntoString.

Type-checking is tracing Prolog

 (i32, i32): IntoString.
 
Vec<T>: IntoString :-
  T: IntoString.

?- Vec<(i32, i32)>: IntoString.

Type-checking is tracing Prolog

 (i32, i32): IntoString.
 
Vec<T>: IntoString :-
  T: IntoString.

?- Vec<(i32, i32)>: IntoString.

Tracing a failed query

 (i32, i32): IntoString.
 
Vec<T>: IntoString :-
  T: IntoString.

?- Vec<String>: IntoString.

 fn main() {
  let v: Vec<String> = vec![
    String::from("We're"),
    String::from("Strings!")
  ];
  println!("{}", v.to_string());
}

A good Rust diagnostic

 error[E0599]: the method `to_string` exists for struct `Vec<String>`,
  but its trait bounds were not satisfied
  --> src/main.rs:24:20
   |
24 |   println!("{}", v.to_string());
   |                    ^^^^^^^^^
   |   method cannot be called on `Vec<String>` due to unsatisfied trait bounds
   |
   = note: doesn't satisfy `String: IntoString`
   = note: doesn't satisfy `Vec<String>: IntoString`
   |
note: trait bound `String: IntoString` was not satisfied
  --> src/main.rs:14:9
   |
14 | impl<T: IntoString>
   |         ^^^^^^^^^^ unsatisfied trait bound introduced here
15 |   IntoString for Vec<T> {
   |   ----------     ------ error[E0599]: the method `to_string` exists for struct `Vec<String>`,
  but its trait bounds were not satisfied
  --> src/main.rs:24:20
   |
24 |   println!("{}", v.to_string());
   |                    ^^^^^^^^^
   |   method cannot be called on `Vec<String>` due to unsatisfied trait bounds
   |
   = note: doesn't satisfy `String: IntoString`
   = note: doesn't satisfy `Vec<String>: IntoString`
   |
note: trait bound `String: IntoString` was not satisfied
  --> src/main.rs:14:9
   |
14 | impl<T: IntoString>
   |         ^^^^^^^^^^ unsatisfied trait bound introduced here
15 |   IntoString for Vec<T> {
   |   ----------     ------ error[E0599]: the method `to_string` exists for struct `Vec<String>`,
  but its trait bounds were not satisfied
  --> src/main.rs:24:20
   |
24 |   println!("{}", v.to_string());
   |                    ^^^^^^^^^
   |   method cannot be called on `Vec<String>` due to unsatisfied trait bounds
   |
   = note: doesn't satisfy `String: IntoString`
   = note: doesn't satisfy `Vec<String>: IntoString`
   |
note: trait bound `String: IntoString` was not satisfied
  --> src/main.rs:14:9
   |
14 | impl<T: IntoString>
   |         ^^^^^^^^^^ unsatisfied trait bound introduced here
15 |   IntoString for Vec<T> {
   |   ----------     ------

Trait-heavy crates across domains

bevy: game development.
axum: web applications.
nalgebra: linear algebra.
diesel: extensible ORM and query builder.
︙

bevyengine.org

How simple is it really?

Hello World! from Bevy

use bevy::prelude::*;

#[derive(Resource)]
struct Timer(usize);

fn run_timer(timer: Res<Timer>) {
  /* ... */
}

fn main() {
  App::new()
    .insert_resource(Timer(0))
    .add_system(run_timer)
    .run();
}

Hello World! from Bevy

use bevy::prelude::*;

#[derive(Resource)]
struct Timer(usize);

fn run_timer(timer: Timer) {
  /* ... */
}

fn main() {
  App::new()
    .insert_resource(Timer(0))
    .add_system(run_timer)
    .run();
}

A poor Rust diagnostic

We’re looking for the information about Timer: SystemParam.

Timer nor SystemParam is mentioned in the ~30 line diagnostic.

The trait bound `fn(Timer) {run_timer}: IntoSystem<(), (), _>` is not satisfied
  --> src/main.rs:6:24
   |
6  | fn main() { use_system(run_timer) }
   |             ---------- ^^^^^^^^^ the trait `IntoSystem<(), (), _>`
   |             | is not implemented for fn item `fn(Timer) {run_timer}`
   |             |
   |             required by a bound introduced by this call
   |
note: required for `fn(Timer) {run_timer}` to implement `bevy::prelude::IntoSystemConfig<_>`
  --> src/main.rs:41:9
   |
41 |         IntoSystemConfig<Marker> for F
   |         ^^^^^^^^^^^^^^^^^^^^^^^^     ^
42 |         where F: IntoSystem<(), (), Marker> {}
   |                  ------------------ unsatisfied trait bound introduced here
note: required by a bound in `use_system`
  --> src/main.rs:4:26
   |
4  | fn use_system<M>(_: impl IntoSystemConfig<M>) {}
   |                          ^^^^^^^^^^^^^^^^^^^ required by this bound in `use_system`

Why?

But…documentation!

Shouldn’t documentation aid developers in understanding diagnostics?

Information spans across:

3 pages
92 trait implementations!

Tracing the type-checker for our Bevy example

Why does Bevy produce a worse diagnostic?

 Timer: Resource.
Res<T>: SystemParam :-
    T: Resource.
 
fn(P0): SystemParamFunc :-
  P0: SystemParam.
 
fn(TheWorld): ExclSystemParamFunc.
 
F: IntoSystem :-
  F: SystemParamFunc.
 
F: IntoSystem :-
  F: ExclSystemParamFunc.
 
F: IntoSystemConfigs :-
  F: IntoSystem.

?- fn(Timer): IntoSystem.

Why does Bevy produce a worse diagnostic?

 Timer: Resource.
Res<T>: SystemParam :-
    T: Resource.
 
fn(P0): SystemParamFunc :-
  P0: SystemParam.
 
fn(TheWorld): ExclSystemParamFunc.
 
F: IntoSystem :-
  F: SystemParamFunc.
 
F: IntoSystem :-
  F: ExclSystemParamFunc.
 
F: IntoSystemConfigs :-
  F: IntoSystem.

?- fn(Timer): IntoSystem.

Why does Bevy produce a worse diagnostic?

 Timer: Resource.
Res<T>: SystemParam :-
    T: Resource.
 
fn(P0): SystemParamFunc :-
  P0: SystemParam.
 
fn(TheWorld): ExclSystemParamFunc.
 
F: IntoSystem :-
  F: SystemParamFunc.
 
F: IntoSystem :-
  F: ExclSystemParamFunc.
 
F: IntoSystemConfigs :-
  F: IntoSystem.

?- fn(Timer): IntoSystem.

Why does Bevy produce a worse diagnostic?

 Timer: Resource.
Res<T>: SystemParam :-
    T: Resource.
 
fn(P0): SystemParamFunc :-
  P0: SystemParam.
 
fn(TheWorld): ExclSystemParamFunc.
 
F: IntoSystem :-
  F: SystemParamFunc.
 
F: IntoSystem :-
  F: ExclSystemParamFunc.
 
F: IntoSystemConfigs :-
  F: IntoSystem.

?- fn(Timer): IntoSystem.

Why does Bevy produce a worse diagnostic?

 Timer: Resource.
Res<T>: SystemParam :-
    T: Resource.
 
fn(P0): SystemParamFunc :-
  P0: SystemParam.
 
fn(TheWorld): ExclSystemParamFunc.
 
F: IntoSystem :-
  F: SystemParamFunc.
 
F: IntoSystem :-
  F: ExclSystemParamFunc.
 
F: IntoSystemConfigs :-
  F: IntoSystem.

?- fn(Timer): IntoSystem.

Why does Bevy produce a worse diagnostic?

 Timer: Resource.
Res<T>: SystemParam :-
    T: Resource.
 
fn(P0): SystemParamFunc :-
  P0: SystemParam.
 
fn(TheWorld): ExclSystemParamFunc.
 
F: IntoSystem :-
  F: SystemParamFunc.
 
F: IntoSystem :-
  F: ExclSystemParamFunc.
 
F: IntoSystemConfigs :-
  F: IntoSystem.

?- fn(Timer): IntoSystem.

error[E0599]: the method `to_string` exists for struct `Vec<String>`,
  but its trait bounds were not satisfied
  --> src/main.rs:24:20
   |
24 |   println!("{}", v.to_string());
   |                    ^^^^^^^^^
   |   method cannot be called on `Vec<String>` due to unsatisfied trait bounds
   |
   = note: doesn't satisfy `String: IntoString`
   = note: doesn't satisfy `Vec<String>: IntoString`
   |
note: trait bound `String: IntoString` was not satisfied
  --> src/main.rs:14:9
   |
14 | impl<T: IntoString>
   |         ^^^^^^^^^^ unsatisfied trait bound introduced here
15 |   IntoString for Vec<T> {
   |   ----------     ------

The trait bound `fn(Timer) {run_timer}: IntoSystem<(), (), _>` is not satisfied
  --> src/main.rs:6:24
   |
6  | fn main() { use_system(run_timer) }
   |             ---------- ^^^^^^^^^ the trait `IntoSystem<(), (), _>`
   |             | is not implemented for fn item `fn(Timer) {run_timer}`
   |             |
   |             required by a bound introduced by this call
   |
note: required for `fn(Timer) {run_timer}` to implement `bevy::prelude::IntoSystemConfig<_>`
  --> src/main.rs:41:9
   |
41 |         IntoSystemConfig<Marker> for F
   |         ^^^^^^^^^^^^^^^^^^^^^^^^     ^
42 |         where F: IntoSystem<(), (), Marker> {}
   |                  ------------------ unsatisfied trait bound introduced here
note: required by a bound in `use_system`
  --> src/main.rs:4:26
   |
4  | fn use_system<M>(_: impl IntoSystemConfig<M>) {}
   |                          ^^^^^^^^^^^^^^^^^^^ required by this bound in `use_system`

We see this as an interface problem.

Debugging traits is debugging an arbitrary program!

overflow evaluating the requirement `_: diesel::Expression`
  --> src/main.rs:15:10
   |
15 |         .group_by(users::id)
   |          ^^^^^^^^

Demo

Dealing with large data

requirements on the impl of
  `LoadQuery<'_, _, (i32, std::string::String)>`
for
  `SelectStatement<FromClause<users::table>, diesel::query_builder::select_clause::DefaultSelectClause<FromClause<users::table>>, diesel::query_builder::distinct_clause::NoDistinctClause, diesel::query_builder::where_clause::WhereClause<diesel::expression::grouped::Grouped<diesel::expression::operators::Eq<users::columns::id, posts::columns::id>>>>`

Incremental information

requirements on the impl of
  LoadQuery●
'_, _, (●
i32, String
)
for
  SelectStatement●
FromClause●
users::table
, DefaultSelectClause●
FromClause●
users::table
, NoDistinctClause, WhereClause●
Grouped●
Eq●
users::columns::id, posts::columns::id

Summary

 App::new()
  .insert_resource(Timer(0))
  .add_system(run_timer)
  .run();

LoadQuery●
'_, _, (●
i32, String
)
: SelectStatement●
FromClause●
users::table
, DefaultSelectClause●
FromClause●
users::table
, NoDistinctClause, WhereClause●
Grouped●
Eq●
users::columns::id, posts::columns::id

What approach would you take if you were to build a trait debugger?
Are there related problems where this approach could be useful? (E.g., I think debugging Coq tactics is closely relatead.)
What features would you want in such an interface?

Gavin Gray (@gavinleroy)	ETH Zürich
Will Crichton	Brown University

	trait IntoString {
	fn to_string(&self) -> String;
	}

	impl IntoString for (i32, i32) {
	fn to_string(&self) -> String {
	format!(
	"({}, {})",
	self.0, self.1
	)
	}
	}

	impl<T: IntoString>
	IntoString for Vec<T> {
	fn to_string(&self) -> String
	{ /* ... */ }
	}

	fn main() {
	let v: Vec<(i32, i32)> = vec![
	(0, 1), (2, 3)
	];
	println!("{}", v.to_string());
	}

	fn main() {
	let v: Vec<String> = vec![
	String::from("We're"),
	String::from("Strings!")
	];
	println!("{}", v.to_string());
	}

	error[E0599]: the method `to_string` exists for struct `Vec<String>`,
	but its trait bounds were not satisfied
	--> src/main.rs:24:20
	\|
	24 \| println!("{}", v.to_string());
	\| ^^^^^^^^^
	\| method cannot be called on `Vec<String>` due to unsatisfied trait bounds
	\|
	= note: doesn't satisfy `String: IntoString`
	= note: doesn't satisfy `Vec<String>: IntoString`
	\|
	note: trait bound `String: IntoString` was not satisfied
	--> src/main.rs:14:9
	\|
	14 \| impl<T: IntoString>
	\| ^^^^^^^^^^ unsatisfied trait bound introduced here
	15 \| IntoString for Vec<T> {
	\| ---------- ------

	Timer: Resource.
	Res<T>: SystemParam :-
	T: Resource.

	fn(P0): SystemParamFunc :-
	P0: SystemParam.

	fn(TheWorld): ExclSystemParamFunc.

	F: IntoSystem :-
	F: SystemParamFunc.

	F: IntoSystem :-
	F: ExclSystemParamFunc.

	F: IntoSystemConfigs :-
	F: IntoSystem.

	App::new()
	.insert_resource(Timer(0))
	.add_system(run_timer)
	.run();