Then the program dies with an exception. What if a memory allocation in a Haskell program fails? It can happen just as well because allocating memory is an impure operation which you still have to perform from pure functions. Haskell just happens to pretend it has infinite memory available because forcing the programmer to attempt handle out of memory conditions would be impractical and annoying. Haskells "purity" is just a chimera, albeit a useful one. And it's just as useful being able to pretend network operations are pure too.