Builder chain

struct Server:
    host: String
    port: i32
    workers: i32

struct ServerBuilder:
    host: String
    port: i32
    workers: i32

impl Server:
    def builder() -> ServerBuilder:
        return ServerBuilder(host=String.from_str("localhost"), port=80, workers=1)

impl ServerBuilder:
    def host(self: Self, value: view str) -> Self:
        var next = self
        next.host = String.from_str("127.0.0.1")
        return next

    def port(self: Self, value: i32) -> Self:
        var next = self
        next.port = value
        return next

    def workers(self: Self, value: i32) -> Self:
        var next = self
        next.workers = value
        return next

    def build(self: Self) -> Server:
        return Server(host=self.host, port=self.port, workers=self.workers)

def main() -> i32:
    let server = Server.builder()
        .host("127.0.0.1")
        .port(3000)
        .workers(4)
        .build()
    if server.host.len == 9:
        return 0
    return 1

What’s worth noticing:

• Each builder method takes self: Self by value and returns the next owned value: the call chain consumes the previous instance and produces the next.
• Self resolves to the surrounding impl’s receiver type. Same as writing ServerBuilder here, but reads cleaner.
• The chain uses indented continuation: Server.builder() followed by indented .host(...), .port(...), etc. This is the supported method-chain form.
• The consuming chain moves String storage out of self between calls: this is permitted because ServerBuilder does not define drop.

For the rules on which receiver shapes work in chains, see Methods and impl.