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A record says which fields are required, and `...` copies one with changes

A struct type lists its fields. A field with a default may be left out, and a field without one must be given, so a config can't be built half-filled. A struct is a value, so to make a variant, ... copies every field of an existing value and you name only what changes; the original stays as it was. No builder class, no setter that edits a config other code is already using.

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struct HttpConfig:    host: string  # ← no default: required    timeout_ms: i32 = 5000    retries: i32 = 3pub fn main() -> void $ Console:    production := HttpConfig { host: "api.shop.example" }    payments := HttpConfig { ...production, retries: 0 }  # ← copy, change one field    for config in [production, payments]:        println("${config.host}: ${config.timeout_ms} ms, ${config.retries} retries")# Delete `host: "api.shop.example"` so the first literal# reads `HttpConfig {}`, and Run:#     missing-required-field: `host` is required in `HttpConfig`

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A record says which fields are required, and ... copies one with changes

A record says which fields are required, and `...` copies one with changes