The spec changing is the best part! (Or conversely, the worst part when you are 100% dynamic). You change your types to match the new spec (eg adding a case to

The spec changing is the best part! (Or conversely, the worst part when you are 100% dynamic). You change your types to match the new spec (eg adding a case to a discriminated union) and you immediately get feedback on places in your code that are definitely wrong/incomplete now. It won’t find every change you need to make but it’s a big load off and frees up some cognitive capacity for what’s left.