Proposal for unifying traits and interfaces

Unifying traits and interfaces

A high-level description of the changes proposed here already appears on the Note development roadmap page under the headings "Extend interfaces to full traits" and "Enforce implementation coherence". What follows is a more detailed explanation of the proposal. There are three parts:

• Adding default impls to ifaces
• Allowing iface composability
• Instance coherence: only one impl per iface/type pair

Then, rename iface to trait and that's it!

Adding default impls to ifaces

Motivating example

In the middle::typeck::infer module (henceforth infer), there's a combine interface, and implementations of that interface for the three "type combiners" lub, sub, and glb. All three impls are required to implement all of the methods in the combine interface, even though some of the implementations are identical in two (or in all three!) of the type combiners. Right now, infer deals with this by defining an out-of-line method for each method for which there are multiple identical implementations, and having all the different implementations call the out-of-line-method.

For example, here's what it looks like for the modes method. In fact, there are nine methods in infer that are this way -- modes is just a representative example. (The infcx method is also identical in all three, but since all it does is return *self, it's just identically implemented directly in all three instead of calling off to an out-of-line method.)

iface combine {
    fn infcx() -> infer_ctxt;
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode>;
    ...
}

impl of combine for sub {
    fn infcx() -> infer_ctxt { *self }
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        super_modes(self, a, b)
    }
    ...
}

impl of combine for sub {
    fn infcx() -> infer_ctxt { *self }
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        super_modes(self, a, b)
    }
    ...
}

impl of combine for glb {
    fn infcx() -> infer_ctxt { *self }
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        super_modes(self, a, b)
    }
    ...
}

// Out-of-line method
fn super_modes<C:combine>(
    self: C, a: ast::mode, b: ast::mode)
    -> cres<ast::mode> {

    let tcx = self.infcx().tcx;
    ty::unify_mode(tcx, a, b)
}

Under this proposal, we could put the default implementation in the interface, and the above code becomes the following:

trait Combine {
    fn infcx() -> infer_ctxt { *self }
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        let tcx = self.infcx().tcx;
    	ty::unify_mode(tcx, a, b)
    }
    ...
}

impl sub: Combine {
    ... // only methods for which the default impl isn't enough
}

impl sub: Combine {
    ... // only methods for which the default impl isn't enough
}

impl glb: Combine {
    ... // only methods for which the default impl isn't enough
}

This code also makes three superficial syntax changes: first, changing the iface keyword to trait; second, changing impl of X for Y to impl Y: X; and third, capitalizing the trait name (with this last change intended just as a programming convention, not as something that the compiler will enforce). We'll stick to this syntax for the rest of the proposal.

Required and provided methods

Traits, as they appear in the literature, have a set of provided methods, implementing the behavior that a trait provides, and a (possibly empty) set of required methods that the provided methods can be written in terms of. For the required methods, only the names and types are specified, not the implementation.

In Rust, methods with no implementation will be considered required, and methods with an implementation will be considered provided. This removes the need for something like a req keyword, and required and provided methods can be intermingled and appear in any order in the trait definition.

// eq is required; neq is provided
trait Eq {
    fn eq(a: self) -> bool;

    fn neq(a: self) -> bool {
        !self.eq(a)
    }
}

Allowing iface composability

A defining characteristic of traits is that they are composable: a trait C can extend other traits: In this example, the Ord trait extends Eq. (The < is pronounced "extends". This syntax isn't set in stone yet; also under consideration is <:.)

trait Ord < Eq {

    fn lt(a: self) -> bool;

    fn lte(a: self) -> bool {
        self.lt(a) || self.eq(a)
    }

    fn gt(a: self) -> bool {
        !self.lt(a) && !self.eq(a)
    }

    fn gte(a: self) -> bool {
        !self.lt(a)
    }
}

impl int: Ord {
    fn lt(a: self) -> bool {
        self < a
    }

    fn eq(a: self) -> bool {
        self == a
    }
}

Because Ord extends Eq, the impl of Ord for the int type has to implement the required methods of both Ord and Eq -- in this case, lt and eq.

One place in the Rust compiler that could benefit from this so-called "interface inheritance" is called out by a FIXME for issue 2616 in core::num. We might be able to clean up duplicated code between core/int_template.rs and core/uint_template.rs with this kind of strategy.

Trait composition is also order-independent: the trait Ord could extend multiple traits at once, with the order being irrelevant. For instance, the trait

trait Ord < Foo, Bar {
    ...
}

is the same as

trait Ord < Bar, Foo {
    ...
}

Conflict resolution

In the literature, traits do some cool conflict resolution stuff when traits being combined have methods with the same name, and we might want to do that eventually, but we can punt for now and just do what Rust already does if a type implements multiple interfaces that define a method with the same name -- that is, raise a compile-time "multiple applicable methods in scope" error. In the case of the Ord < Foo, Bar example above, for instance, we could (for now) raise an compile-time error if Foo and Bar have methods with conflicting names.

Instance coherence

An iface that presents a group of functions without mandating any particular implementation -- as is the case with all ifaces in Rust as it stands -- leaves open the possibility of different conflicting implementations for a particular type. This is known as the "instance coherence" problem (although in Rust we could call it "implementation coherence"), or just the "coherence" problem for short.

Consider the following program (due to gwillen), which compiles and runs in Rust today:

use std;

mod ht {
    iface hash {
        fn hash() -> uint;
        fn tostr() -> str;  // putting this into the interface is just
                            // a hack to give us a way to print
                            // keys. This doesn't go here at all.
    }

    type t<K,V> = [(K, V)];  // doesn't matter, we don't use it

    fn create<K:hash,V>() -> @t<K,V> {
        @[]/~
    }

    fn put<K:hash,V>(ht: @t<K,V>, k: K, v: V) {
        io::println(#fmt("ht put: %s hash to %ud", k.tostr(), k.hash()));
    }
}

mod Module1 {
    impl of ht::hash for uint {
        fn hash() -> uint { ret self; }
        fn tostr() -> str { ret #fmt("%ud", self); }  
    }
    fn foo() {
        let h = ht::create::<uint, str>();
        ht::put(h, 3u, "hi"); // 3u.hash() == 3u here
        Module2::bar(h);
    }
}

mod Module2 {
    impl of ht::hash for uint {
        fn hash() -> uint { ret self / 2; }
        fn tostr() -> str { ret #fmt("%ud", self); }
    }
    fn bar(h: @ht::t<uint, str>) {
        ht::put(h, 3u, "ho"); // 3u.hash() == 1u here
    }
}

fn main() {
    Module1::foo();
}

The output of this program is:

ht put: 3d hash to 3d
ht put: 3d hash to 1d

If put had really been inserting into a hash table instead of just printing, the table would end up with both "hi" and "ho" in it, even though we thought we were storing them under the same key, and "ho" should have overwritten "hi". The problem arises because there are conflicting implementations of the hash iface in Module1 and Module2. Neither one is wrong by itself, but when compiled together we get unexpected behavior.

Under instance coherence, only one implementation of a trait per type is allowed. Haskell enforces this property at link time, because conflicting implementations might only be discovered when separate compilation units are linked. However, in Rust we're interested in forbidding conflicting implementations sooner.

Consider a program consisting of four interlinked crates C1, C2, C3, and C4, where C1 defines the trait C1Trait and C2 defines the type C2Type. Under instance coherence, crate C3 would not be allowed to directly define an impl of C2Type for C1Trait, because this leaves us open to the possibility of crate C4 doing the same thing, resulting in conflicting implementations. However, this is not a problem for expressivity, because it is always possible for C3 to create a new nominal alias for C2Type and then define an impl for that type. Alternatively, both the trait and the impl could be defined in C1. Here's a sketch of how it would work:

// Instance coherence enforcement should forbid this situation
// (assuming each module is a separate crate)

mod C1 {
    trait C1Trait { ... }
}

mod C2 {
    type C2Type = int;
}

mod C3 {
    import C1;
    import C2;
    
    // conflicts with potential other impls
    impl C2Type: C1Trait { ... } 
}

// But this should be OK:

mod C3v2 {
    import C1;
    import C2;
    
    enum local_myType = myType;
    impl local_myType: C1Trait { ... }
}

// As should this:

mod C1v2 {
    import C2;

    trait C1Trait { ... }
    impl C2Type: C1Trait { ... }
}

mod C3v2 {
    import C1v2; // imports the trait as well as its impl; can't
                 // accidentally use a conflicting impl
}