Custom Derive

@derive_for(Trait) registers a comptime function as the code generator for that trait. The function receives typeinfo(Self) and returns a Zolo source string — either an impl Trait for Self or an inherent impl Self { ... } — which the compiler splices next to the type definition. No compiler modification required.

Register a FieldList derive with @derive_for, generate an inherent impl that joins field names as a comma-separated string, and verify a.field_list() returns "id,owner,balance".

12-derive-custom.zolo

Playground

// Feature: define your OWN derive. `@derive_for(Trait)` registers a comptime

// function that receives typeinfo(Self) and returns structured Zolo syntax,

// spliced next to the type. The result can be `impl Trait for Self` OR — when

// you just want a method — an INHERENT `impl Self { ... }`.


@derive_for(FieldList)
fn derive_field_list(info) -> Syntax<.Items> {
  let names = info.fields.map(|field| field.name).join(",")
  // Inherent impl: adds `field_list` directly onto the type — no `trait`

  // declaration needed, and it coexists with any hand-written `impl` blocks.

  return quote items {
    impl ${info.ref} {
      fn field_list(self) -> str { return ${names} }
    }
  }
}

@derive(FieldList)
struct Account {
  id: int,
  owner: str,
  balance: int,
}

let a = Account.new(id: 1, owner: "x", balance: 0)
print(a.field_list())
// expected: id,owner,balance

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