[tome]

> at the edges where we need to interact with run time exceptions, the state of resources external to the process, etc. Even in Haskell you can write your code procedurally if you want. You won’t get the benefits of denotational design

I'd go further and say that effect systems, MTL etc. implement denotational design for procedural code. You can say exactly what your operations denote: a procedure that can throw exceptions only, update state only, a combination of both but not I/O, etc. etc.