[jolmg]

Reader is similar, but I wouldn't say equivalent, because only those things working in the Reader can use that environment. For example, when you have project A using project B using project C, you can't define a putStrLn such that project C can use it and be able to override its output stream from project A, while keeping B completely ignorant of that ability of putStrLn or that C even uses putStrLn.

Reader is like a middle-ground between dynamic-scoping and explicit passing of a context argument around as you would in a statically-scoped language.

[catern]

Reader is like implicit parameters: dynamic scoping with static types. https://dl.acm.org/doi/10.1145/325694.325708

[jolmg]

I know what Reader is, but my point still stands.

[catern]

Okay... I wasn't suggesting you don't know what Reader is... you said:

>Reader is like a middle-ground between dynamic-scoping and explicit passing of a context argument around

I just wanted to point out that the-thing-which-Reader-is, is sometimes called "implicit parameters", and there are papers written about it.