I think the best way to approach this question is the see CAR Tony Hoare's "Billion Dollar Mistake" talk. A fellow stands up at the end and says, in detail, so

I think the best way to approach this question is the see CAR Tony Hoare's "Billion Dollar Mistake" talk. A fellow stands up at the end and says, in detail, something that can be summarized as "it's all golden until you invoke the I/O monad."