Your absolutely right: it's not true. Well, not entirely true. There are two ways functional languages get around this: Most functional language don't force f

Your absolutely right: it's not true. Well, not entirely true. There are two ways functional languages get around this: Most functional language don't force functions to be pure. So, while they tend to be composed of functions that produce the same output for a given input, they aren't entirely. That is, they're just like any other language in this respect. Nonetheless, functional programs tend to separate the impure parts using patterns such as https://www.destroyallsoftware.com/screencasts/catalog/functional-core-imperative-shell https://www.destroyallsoftware.com/screencasts/catalog/funct... Then there are the languages where functions actually are necessarily pure - mostly Haskell. But this just means that the imperative actions aren't functions, not that they don't exist. The big impact is that, since a pure function can't invoke an impure action - or it wouldn't be pure - you have to use function-code-imperative-shell or your program won't compile.