GP is correct. There are zero side-effects inside a pure program. All the side-effects happen outside it, in a "runtime" (a better name IMO is "imperative shell

GP is correct. There are zero side-effects inside a pure program. All the side-effects happen outside it, in a "runtime" (a better name IMO is "imperative shell") that exists totally separate from the program. The main() function of a pure program executes completely and terminates before any side effect has a chance to happen. I put quotes around runtime because it's not a runtime like an interpreter in dynamic languages, or a library that you can call from inside your pure program. This runtime just calls your main() function, which return a complex value (it's a chain of lambdas, to spoil it) that is interpreted by this runtime or "imperative shell". I understand this sounds unintuitive and might seem very confusing, but this is what makes useful pure programs possible. These couple presentations show how this type of boundary between imperative and functional code works with simpler type of program [1]. The only difference is that Haskell's "imperative shell" is lower-level than the presenter's. It only deals with IO, etc, whereas the presenter's imperative shell also has some domain code. [1] https://www.destroyallsoftware.com/talks/boundaries https://www.destroyallsoftware.com/talks/boundaries / https://www.destroyallsoftware.com/screencasts/catalog/functional-core-imperative-shell https://www.destroyallsoftware.com/screencasts/catalog/funct...