Mastering Functional Programming

Real-world side effects

In this subsection, we have already seen two instances of side effects—errors and time. The nature of the real world is such that often you don't know which side effects you will encounter.

Such side effects can include the necessity to limit the number of requests sent to a given domain because certain websites tend to block entities who send too many requests, or we might want to use proxy servers to access certain websites that are not accessible otherwise. Certain websites might have the data stored and fetched via Ajax, and ordinary techniques of scraping usually do not work.

All these scenarios can be modeled as their own side effects. The idea when working with a real-world application is always to have a look ...