Supertraits

A supertrait is a trait that comes bundled with another one: a type that implements Compare has to implement Equals too, because Compare says so at its own declaration.

Example#

trait Named {
  fn name(): String
}

trait Aged {
  fn age(): Int
}

trait Greeter with Named, Aged {
  fn greet(): String {
    "Hello, {name()}. You are {age()}."
  }
}

type Person with Greeter {
  fn name(): String {
    "Ada"
  }

  fn age(): Int {
    36
  }
}

print Person().greet()

Syntax#

trait <Name> with <Supertrait>, <Supertrait> { ... }
type <Name> with <Trait> { ... }        satisfies <Trait> and every one of its supertraits

Rules#

1. with after a trait's own name lists its supertraits. trait Compare with Equals says that every type with Compare also has Equals; trait Greeter with Named, Aged lists two.

2. A type that implements a trait has to satisfy every supertrait too, whether by a member it writes itself or one that is generated. Person above never writes with Named or with Aged - providing name and age is enough, because Greeter already requires them.

3. A default member of a trait can call the members of its supertraits directly, with no qualification. greet calls name() and age() even though Greeter itself requires neither; they come from Named and Aged.

4. A missing supertrait member is reported against the trait that asked for it, not the one the type wrote with. The message names both traits, so the fix is clear even when the supertrait was never written out.

5. A trait's own with list is a promise about every instantiation of it, so an implementation of a supertrait for the trait type may not be narrower than the promise. Member lookup reads the with list, so trait Mound<Item> with From<Iterate<Item>> makes Mound.from resolve for every Item at all - and an extend<Item: Hash> Mound<Item> with From<Iterate<Item>> beside it would leave the rest of them with a supertrait and nothing behind it. Where the capability really is conditional, it belongs on the member instead (static fn of(...items: Item): Self where Self: From<Iterate<Item>>), which is where a bound can be checked at the call.

   Does not compile

   trait Mound<Item> with From<Iterate<Item>> {
     fn size(): Int
   }
   
   type Heap<Item: Hash> with Mound<Item>, From<Iterate<Item>> {
     stored: Int
   
     static fn from(values: Iterate<Item>): Heap<Item> {
       Self(0)
     }
   
     fn size(): Int {
       stored
     }
   }
   
   extend<Item: Hash> Mound<Item> with From<Iterate<Item>> {
     static fn from(values: Iterate<Item>): Mound<Item> {
       Heap.from(values)
     }
   }
   // error: `Mound` comes with `From` for every instantiation, and this implementation holds only for some

What this is not#

A supertrait is not satisfied by writing with alone. The trait still needs every one of the supertrait's members from somewhere - the declaration itself, an extend, or generation.

type Money with Compare {
  cents: Int

  fn compare(other: Money): Ordering {
    if cents < other.cents { .Less } else if cents > other.cents { .Greater } else { .Equal }
  }
}

trait Loud {
  fn shout(): String
}

type Box with Compare {
  loud: Loud

  fn compare(other: Self): Ordering {
    .Equal
  }
}
// error: `Compare` requires `Equals`, and `Box` has no `equals`

Money's Equals comes for free because every field of Money supports it; Box holds a Loud, which has no Equals to derive from, so its Compare has nothing to stand on until equals is written by hand.

Related#

• Traits - with at the declaration, and the naming rule for a single-method trait.
• Coherence and blanket implementations - the other rule a with or extend has to satisfy.
• Declaring a type - when Equals, Hash and Show are generated.