Pure functional programming doesn't preclude side-effects like IO; it makes side-effects explicit rather than implicit!
At my previous job, we used pure functional programming to ensure that custom programs only had access to certain side-effects (most importantly, not IO). This meant that it was trivial to run these custom programs in multiple environments, including various testing environments, and production.
At my previous job, we used pure functional programming to ensure that custom programs only had access to certain side-effects (most importantly, not IO). This meant that it was trivial to run these custom programs in multiple environments, including various testing environments, and production.