Learning a programming language that is immature and substantially different from what’s commonly used in the industry makes you faster at programming, specifically debugging and unblocking. These skills generalize to any programming and make you faster at ‘figuring things out’ next time something unexpected happens.
For example, rust or clojure are significantly different from common languages like Java or Ruby that you can’t rely on previous experience to fumble your way through writing it (like JavaScript). Errors and tooling tend to be worse and often obscure (e.g. clojure’s error messages). This forces you to learn the language deeper to understand what’s going on and read much more code (e.g. reading source code of the language itself or a particular library).
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Faster is more productive because it is a multiplier on tasks you spend a lot of time doing (thinking, writing, typing, debugging).