ADR 0019: Named and default arguments for Lyth functions

Status: Accepted

Context

Backend functions accumulate optional settings: timeouts, page sizes, retry counts, flags. Go answers with option structs, functional options, or several functions per variant, and boolean arguments stay unreadable at call sites (render(t, false, true)). Kotlin answers with default values and named arguments. Lyth constructs structs with named arguments already, so calls looked inconsistent without them.

Decision

• A parameter may declare a default value. Calls to functions and methods declared in Lyth may name arguments: positional ones first, named ones in any order. A trailing lambda fills the last parameter; a variadic parameter is positional only.
• Defaults are filled in at the call site. The Go signature stays exactly what the parameter list says, and a call lowers to an ordinary positional call.
• A default is resolved where the function is declared, so a caller’s locals cannot change it. It must be a plain expression (no control flow and no error or null operators) and cannot refer to other parameters. In a pub function it may use only pub names, because calls in other packages contain it.
• Arguments are evaluated in the order written, then the defaults. Explicit arguments move into temporaries only when the positional Go call would otherwise run side effects in a different order.
• Go functions take positional arguments only.

Alternatives

• Defaults implemented in the callee (pointer or variadic parameters, or a generated wrapper per arity): changes the API that Go callers see, or adds functions. Call-site expansion keeps Lyth and Go signatures identical.
• Defaults that refer to earlier parameters (end: Int = start): useful occasionally, but it needs temporaries for every such call. It can be added later without breaking code.
• Named arguments for Go functions: Go parameter names are not part of an API. They change between versions, and can be unnamed, without Go considering it a breaking change. Lyth code would break when a dependency renamed one.
• Option structs only: still the right tool for large configurations, and struct construction already takes named fields (http.Server(Addr = ...)).

Consequences

Go callers of a Lyth package pass every argument. Since Go builds every package from source, a changed default reaches all callers at their next build. Interface methods cannot declare defaults, because a call through an interface does not know which implementation’s defaults would apply. Adding a parameter with a default is compatible for Lyth callers but not for Go callers.