> foo() can be an expensive operation like an HTTP call. Or it might depend on a database which can change state underneath it. It can't, in Haskell, because

> foo() can be an expensive operation like an HTTP call. Or it might depend on a database which can change state underneath it. It can't, in Haskell, because it's a pure language. That's basically the definition of pure! But you still need effects, even in a "pure" language, and the whole point of the article is about how to support effects in a pure language.