Paths and placeholders

A placeholder in a path is typed where it appears, and reaches what the route runs by its name:

PATCH /{todo_id: Todo.id} {    input: PatchTodo    output: TodoView    run: patch_todo(todo_id, input, current_principal.user)}

Here {todo_id: Todo.id} reads one segment of the path as a todo's id and passes it to patch_todo as todo_id.

• A placeholder's type is a named type over a scalar, or an entity's id (X.id). It is never optional.
• The segment is decoded once and checked. A bad value is a 422, like any bad input.
• The methods are GET, POST, PUT, PATCH and DELETE. The runtime answers HEAD and OPTIONS itself.
• Two routes that could match the same request, such as /{todo_id: Todo.id} and /{slug: Slug}, are an error. A fixed segment wins over a placeholder, so GET /archived can sit beside GET /{todo_id: Todo.id}.