16 · Reflection & Metaprogramming
Built-In Derives
Use @derive(Debug, Eq, Clone) to auto-generate standard trait implementations for a struct.
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@derive(Debug, Eq, Clone) instructs the compiler to generate implementations
of the named traits for the annotated type. The built-in derives are themselves
written in Zolo via the same @derive_for mechanism — no privileged compiler
path — so they serve as executable documentation for writing custom derives.
Apply @derive(Debug, Eq, Clone) to a User struct, then verify tostring, equality comparison, and clone all work without any hand-written impl.
11-derive-builtin.zolo
// Feature: `@derive(Trait)` generates trait impls at compile time. The built-in
// derives (Debug, Eq, Clone for structs; Eq/Clone/Hash/Ord for newtypes) are
// themselves written in Zolo via the same public `@derive_for` mechanism
// (dogfooded — no privileged compiler path).
@derive(Debug, Eq, Clone)
struct User { id: int, name: str }
let u = User.new(7, "ada")
print(tostring(u))
// expected: User { id: 7, name: ada }
print(u == User.new(7, "ada"))
// expected: true
let c = u.clone()
print(c == u)
// expected: true
See also