Considered, not built: routes that carry a function

A design note from the engineering record, moved out of ROADMAP.md on 2026-09-03 and kept as written. The roadmap itself now holds only the project's state; the reasoning lives here.

Router.match returns an Int and the caller dispatches on it, which is why three of five Mojo apps skip the router and hand-write if path == …. Route-to-function is reachable on Mojo 1.0 — verified by spike, not assumed. The spelling is thin: a closure trait (def (X) raises -> Y) is an AnyTrait, refused as a List element and refused outright as a struct field, but def (X) thin raises -> Y is a concrete type that lists and struct fields both take, including generically — List[def (mut Self.T, ...) thin raises -> HTTPResponse] inside a struct RouteTable[T] dispatches and mutates T correctly.

What stopped it is a type cycle, not the language. Every Mojo app in this tree keeps its state in the handler and the handler owns the router, so RouteTable[NotesHandler] as a field of NotesHandler is infinitely recursive. The fix is to split app state into a type the handler owns alongside the table — a real design, and a real rewrite of the showcase app. Building the table before an app wants it would add an API with zero call sites, which is the condition auth.mojo and response_cache.mojo are already in. Build it with the first app that asks; the spelling above is the part that was unknown.