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 Zolo source, spliced next

// to the type. The source can be `impl Trait for Self` OR — when you just want

// to add a method without declaring a trait — an INHERENT `impl Self { ... }`.


@derive_for(FieldList)
fn derive_field_list(info) -> str {
  var names = "\"\""
  var first = true
  for f in info.fields {
    if first == false { names = names + " + \",\"" }
    names = names + " + \"" + f.name + "\""
    first = false
  }
  // Inherent impl: adds `field_list` directly onto the type — no `trait`

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

  return "impl " + info.name + " \{ 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