You can however obligate that the function that invokes the outside IO be required to return valid input as a constraint in the type-system. So you wouldn't be

You can however obligate that the function that invokes the outside IO be required to return valid input as a constraint in the type-system. So you wouldn't be able to compose an unrestricted ``readLine`` function with a function that couldn't handle the invalid input. It's shifting the checks to ensure that the invalid program is completely inexpressible by construction instead of ensuring runtime safety like a Maybe would.