> The type system could just "look" for those mutations That's a very big "just". Typescript and mypy don't look through function call boundaries, doing so wou

> The type system could just "look" for those mutations That's a very big "just". Typescript and mypy don't look through function call boundaries, doing so would open a lot of problems, I believe. > or the language could add mutability to the type system. I agree there, the core issue is that the call to `foo` leaks a an object where the member is rebindable through assignment, but the "flow typing" assumes that doesn't happen. Ideally such a call should be a barrier for flow typing, unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). A similar problem but with a global instead: https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuBbARiAThAPopwwEAvBACwBMA3AFA0BmSCAxmFAPYIT3vsAUASngA3dlAAmEAN40Ic6CXyFaAXzqMWbThDQBDTIPjJ0WabPlR6EPjGJ2lwAWfkuImcEkxcADLRdqXHn4BP3l3ME8uKFU6UEgADyNUDGxSPQMQumZOAGd2UAA6YHYAcz54kKA https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuB... This should be handled differently. Every function should be assumed to modify globals, unless they can be marked to be pure.