> A value of type `IO a` is a computation which, when performed, does some I/O before returning a value of type a.
So your function foo does no IO in itself. It returns a "computation" for main to perform. And only main can do this, since the runtime calls main. You can call foo as much as you like, but nothing will be printed until you bind any of the returned IO values.
Comparing it to other languages is a bit misleading since Haskell is lazy. putStrLn isn't even evaluated until the IO value is needed. So even "before returning" is wrong no matter how you choose to define "inside".
It is not. The documentation and the type very clearly shows this:
https://hackage.haskell.org/package/base-4.21.0.0/docs/Prelu...
> A value of type `IO a` is a computation which, when performed, does some I/O before returning a value of type a.
So your function foo does no IO in itself. It returns a "computation" for main to perform. And only main can do this, since the runtime calls main. You can call foo as much as you like, but nothing will be printed until you bind any of the returned IO values.
Comparing it to other languages is a bit misleading since Haskell is lazy. putStrLn isn't even evaluated until the IO value is needed. So even "before returning" is wrong no matter how you choose to define "inside".