Implementing Std Traits

Implement core::cmp::Ord on your type so it works with comparison operators and generic Ord bounds.

Implementing a standard trait lets the language treat your type exactly like a built-in. Implement core::cmp::Ord — one method, cmp, returning -1 / 0 / 1 — and all four comparison operators (<, >, <=, >=) are derived from it automatically. Your type also satisfies any T: Ord bound in generic functions:

Money implements core::cmp::Ord via cmp; smaller<T: core::cmp::Ord> accepts it as a generic argument.

18-impl-std-trait.zolo

Playground

// Feature: implementing Ord so YOUR type works with comparison + Ord bounds
// Syntax: `impl Ord for MyType { fn cmp(self, other: MyType) -> int { ... } }`
// When to use: you want `<`/`>`/`<=`/`>=` on your struct, and to pass it to a
// generic `fn f<T: Ord>`. Implement `cmp` (returns -1 / 0 / 1) — all four
// comparison operators are derived from it, and the `T: Ord` bound is satisfied.
// (`==`/`!=` already work structurally on every struct, no impl needed.)

struct Money (
  cents: int,
) {
  impl core::cmp::Ord {
    fn cmp(self, other: Money) -> int {
      match self.cents - other.cents {
        d if d < 0 => -1,
        d if d > 0 => 1,
        _ => 0,
      }
    }
  }
}

fn smaller<T: core::cmp::Ord>(a: T, b: T) -> T { a > b && b || a }

print(smaller(Money { cents: 500 }, Money { cents: 300 }).cents)  // 300
print(Money { cents: 5 } < Money { cents: 9 })                     // true
print(Money { cents: 5 } == Money { cents: 5 })                    // true (structural)
// expected:
// 300
// true
// true

A few things to note:

== and != are structural by default — every struct gets them for free without an explicit Eq impl.
cmp drives all four operators — you only write one method and the language derives the rest from it.
Qualified trait path — impl core::cmp::Ord (the full path) is the same as impl Ord; using the qualified form makes the origin clear.

Challenge

Add a second struct Weight with a grams: float field and the same core::cmp::Ord impl. Pass both Money and Weight to smaller and confirm that the same generic function works for both types.

Find your way through Zolo

Start here