Developer Bob Nystrom uses a colorful allegory to expose the friction between synchronous and asynchronous functions in programming languages. By imagining a language where every function must be declared either 'red' or 'blue,' he illustrates how this constraint creates cascading problems for developers trying to write reusable code.

The thought experiment reveals that red functions can only be called from other red functions, forcing developers to 'color' their entire call chains when using async APIs. This mimics real-world JavaScript challenges where Node.js core libraries often expose asynchronous interfaces that taint everything they touch.

Nystrom's strawman language demonstrates why higher-order functions like filter() become problematic when function colors must match. Making such utilities flexible enough to handle both sync and async callbacks requires careful design that most languages still struggle with today.

The piece ultimately argues that async/sync boundaries represent a fundamental language design flaw that forces developers to constantly manage invisible state, making code reuse and library design significantly harder than it should be.