Is the monadic IO construct in Haskell just a convention?

Question

Is the monadic IO construct in Haskell just a convention or is there is a implementation reason for it?

Could you not just FFI into libc.so instead to do your IO, and

Answer 1

That depends on what the meaning of "is" is—or at least what the meaning of "convention" is.

If a "convention" means "the way things are usually done" or "an agreement among parties covering a particular matter" then it is easy to give a boring answer: yes, the IO monad is a convention. It is the way the designers of the language agreed to handle IO operations and the way that users of the language usually perform IO operations.

If we are allowed to choose a more interesting definition of "convention" then we can get a more interesting answer. If a "convention" is a discipline imposed on a language by its users in order to achieve a particular goal without assistance from the language itself, then the answer is no: the IO monad is the opposite of a convention. It is a discipline enforced by the language that assists its users in constructing and reasoning about programs.

The purpose of the IO type is to create a clear distinction between the types of "pure" values and the types of values which require execution by the runtime system to generate a meaningful result. The Haskell type system enforces this strict separation, preventing a user from (say) creating a value of type Int which launches the proverbial missiles. This is not a convention in the second sense: its entire goal is to move the discipline required to perform side effects in a safe and consistent way from the user and onto the language and its compiler.

Could you just FFI into libc.so instead to do IO and skip the IO Monad thing?

It is, of course, possible to do IO without an IO monad: see almost every other extant programming language.

Would it work anyway or is the outcome undeterministic because of Haskell evaluating lazy or something else, like that the GHC is pattern matching for IO Monad and then handling it in a special way or something else.

There is no such thing as a free lunch. If Haskell allowed any value to require execution involving IO then it would have to lose other things that we value. The most important of these is probably referential transparency: if myInt could sometimes be 1 and sometimes be 5 depending on external factors then we would lose most of our ability to reason about our programs in a rigorous way (known as equational reasoning).

Laziness was mentioned in other answers, but the issue with laziness would specifically be that sharing would no longer be safe. If x in let x = someExpensiveComputationOf y in x * x was not referentially transparent, GHC would not be able to share the work and would have to compute it twice.

What is the real reason?

Without the strict separation of effectful values from non-effectful values provided by IO and enforced by the compiler, Haskell would effectively cease to be Haskell. There are plenty of languages that don't enforce this discipline. It would be nice to have at least one around that does.

In the end you end you endup in a sideeffect. So why not do it the simple way?

Yes, in the end your program is represented by a value called main with an IO type. But the question isn't where you end up, it's where you start: If you start by being able to differentiate between effectful and non-effectful values in a rigorous way then you gain a lot of advantages when constructing that program.