Reader fulfills a bunch of OO patterns. I like to think of it as "Environment", "Capability", "Configuration", etc., and is related in a way to Dependency Inje

Reader fulfills a bunch of OO patterns. I like to think of it as "Environment", "Capability", "Configuration", etc., and is related in a way to Dependency Injection. Basically, you have some computation/object that isn't complete without some "setup info" which, from the point of view of the code being written, is "just there". Like if you had a program that needed access to a particular interface, but you didn't want to manually pass it to every function that cares about it; instead you'd make an object with that interface as its state and write your program as methods on that object. That's just Reader in disguise. In fact, C++ methods are basically in the Reader monad; they all have an implicit "this" argument which the compiler plumbs around for you.