&gt; To my knowledge, there is no known system in mainstream use that reliably enforces that check-at-every-stage policy.<p>I got the impression the author is talking about Haskell. In pure code, Haskell can behave exactly that way, and the checks don&#x27;t even bother that much, they are easy to deal with.<p>Problem is, not even Haskell is purely that way. IO operations just throw exceptions just like in any other language.