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 functi

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.