[eyegor]

What they mean to say is: when I was working with a language that enforced pure functions, I had to actually consider purity. It's rare to see a way to enforce purity in procedural languages, whereas most fp langs support it.

[nixpulvis]

Are we talking about even roughly the same concept of functional purity [1]? Nothing is stopping a pure function from representing EOF as -1.

Implementing IO in a "pure" way, is however another discussion.

[1]: https://en.wikipedia.org/wiki/Pure_function

[eyegor]

Mostly, do you know of a single procedural language with a concept of IO monads in its stdlibs?