Effects in Rust (and Koka)
What is an effect system? According to Wikipedia, it is a formal system that describes the computational effects of computer programs, such as side effects. It is also an extension of a type system, and allows you to statically verify that your program is sound with regard to effects.
If you want to fully understand this concept, I recommend you to learn Koka. It is a beautiful language with a powerful, yet easy to understand effect system. This blog post includes Koka snippets, but they should be easy to understand without prior knowledge.