Circular dependencies can be removed in almost every case by splitting out a large module into smaller ones and adding an interface or two.
In your bot example, you have a circular dependency where (for example) the bot needs to read messages, then run a command from a module, which then needs to send messages back.
v-----------\
bot command_foo
\-----------^
This can be solved by making a command conform to an interface, and shifting the responsibility of registering commands to the code that creates the bot instance.
main <---
^ \
| \
bot ---> command_foo
The bot
module would expose the Bot
class and a Command
instance. The command_foo
module would import Bot
and export a class implementing Command
.
The main
function would import Bot
and CommandFoo
, and create an instance of the bot with CommandFoo
registered:
// bot module
export interface Command {
onRegister(bot: Bot, command: string);
onCommand(user: User, message: string);
}
// command_foo module
import {Bot, Command} from "bot";
export class CommandFoo implements Command {
private bot: Bot;
onRegister(bot: Bot, command: string) {
this.bot = bot;
}
onCommand(user: User, message: string) {
this.bot.replyTo(user, "Bar.");
}
}
// main
import {Bot} from "bot";
import {CommandFoo} from "command_foo";
let bot = new Bot();
bot.registerCommand("/foo", new CommandFoo());
bot.start();
It’s a few more lines of code, but it has no circular dependencies, reduced coupling, and more flexibility. It’s easier to write unit tests for, and users are free to extend it with whatever commands they want, without needing to modify the bot
module to add them.
But the real questions is, can we change them?
Imagine this: