[mbrock]

Actually I/O in general is something that almost always comes from outside the language, so to speak. How do you define a file writing function in C? You have to appeal to some already existing magic syscall which actually cannot be formally defined in the base language. It has no real semantics in the base language, it’s a magic function-like thing that happens to have a “side effect.”

Same with Haskell, only that the base language semantics absolutely forbids functions with side effects, so you have to think of a different way to model it. The IO type is that way. It models effects not as side effects of function application but with these explicitly sequenced command values that are returned from the main function.

It’s just a different formalism. Just like how math equations don’t themselves have any physical effects but can still model physical systems in different ways. Computer languages don’t do I/O, they model and represent, with different semantics and restrictions.

[Rerarom]

You are right, but this is something that is not usually emphasized. I think I could have been persuaded back then that with enough cleverness you could write printf out of if's and while's :)

Also, some years ago I even thought -- but not consciously -- that you could implement preemptive multitasking just from software (you can do it trivially via emulation, but it's slow and most importantly it's not what happens in actual OSes, where the code you write is the actual code that runs on the CPU). I think when I began reading about assembly and OS development, I was expecting the solution to appear at some point. But again, I was not aware of it (the moment I became, I realized how absurd it was), so I got disappointed in assembly without even knowing why.