[omaranto]

Haskell does not force you to indicate that a function has possible side effects in the program source code, the type of the function will however have such an indication. Here main has type IO (), and the IO indicates it can do arbitrary I/O and mutation. Haskell will infer the type for you so you don't need to declare it in the source code.

So I disagree with the claim "without anything indicated by the function main", and would amend to "without anything indicated explicitly by the source code, leaving the only indication in the inferred type".