Immutability as absense of over-writes or mutations of existing values (in Erlang one just introduces a new binding for a new value) is obviously enough. Stat

Immutability as absense of over-writes or mutations of existing values (in Erlang one just introduces a new binding for a new value) is obviously enough. State and I/O is easily incapsulable inside closures with CPS or with explicit message passing. For a lazy language one needs some ADT, like Monad, for explicitly "enforcing" a strict order of evaluation for a specific set of expressions, because I/O implies a strict order. Think of it as a transformation from a set to sequence (by defining an order as an ADT). Lifting State or IO into Monadic world is mere a trick to satisfy a typechecker and enforce a strict order by using an ADT. A strict language with optional lazynes could use ordinary type-tags. Nothing to see here.