example.nim

type
  CommandKind = enum eval, replace
  CodeCmd = object # not required, but makes my point clearer
    kind: eval
    code: string
  ReplaceCmd = object
    kind:       replace
    element_id: string
    content:    string
  
  Command = CodeCmd | ReplaceCmd

proc exec_eval(command: CodeCmd) =
  discard

proc exec_replace(command: ReplaceCmd) =
  discard

proc dispatch(command: Command) =
  case command.kind
  of eval:
    exec_eval(command)
  of replace:
    exec_replace(command)

let c = Command(kind: eval, c: CodeCmd(code: "alert('hi')"))
dispatch(c)