The opposite would be better: an "impure" keyword for specifying impure functions.
Or maybe, you could pass some kind of "scope" interface that provides functions for executing side effects. So "impure(IO) function printStuff()" brings all the functions in IO into scope for the body of printStuff.
Or maybe, you could pass some kind of "scope" interface that provides functions for executing side effects. So "impure(IO) function printStuff()" brings all the functions in IO into scope for the body of printStuff.