Haskell is a bit tricky to talk about here because it has other big differences on laziness and suchlike, and this means there are pervasive monads. If you instead consider a language more like JavaScript where async functions return values wrapped in promises and therefore require special versions of lots of things like Array.prototype.forEach for async-returning functions (ok, language feature of generators helps here). The point I’m trying to get at is that putting io-ness into an argument works better than putting it into the return type because it is easier to pass an extra argument when using other language features an harder to do an extra thing with returned values.