Speaking as a Haskell programmer, this is not a problem. You put “IO” in the function signature and “do” in the body, and that’s workable. Or some other Monad. You get so many choices, which is its own problem, but “having to use monads” is not a problem in practice.
There are other problems that make Haskell hard to work with. This just happens to not be it.
:)
Unfortunately, most programmers trying to learn Haskell have not taken any abstract algebra courses, so these definitions and laws are deemed insufficient. Regular people want to know the exact nature of a thing — not just a list of mathematical properties that hold — and so they insist that there must be something more. But there’s nothing there!
EDIT: Apologies, I missed the subtle point this post was making!
I think chongli is actually in agreement with you
What are the biggest or most important ones in your opinion?
The other common problem I see is not actually a Haskell problem but merely a problem Haskell exacerbates: many students fail to think through their types prior to starting an implementation. I frequently had students in office hours trying to force their way through a problem and when questioned about the types were unable to reason at the type level intuitively. I suspect this reflects larger problems in my university's undergraduate program, but I figured I'd comment on it anyway.