You can, but: A) Legitimate libraries don't (unless the IO action is in fact pure) B) Rogue libraries that do this will not generally work: laziness, optimiza

You can, but: A) Legitimate libraries don't (unless the IO action is in fact pure) B) Rogue libraries that do this will not generally work: laziness, optimizations, RTS races can all make the IO action run 0..N times, arbitrarily. C) It doesn't change the fact that in Haskell, the XML library exposes the weird XML behavior of looking up external entities by being in IO (my original point) -- because of A.