Procrastinating variables ( PVar s) are meant to be used in cases where you want to give someone a value that you do not have available yet, but will definitely have ready by the time that they need to use it.

PVar s have the advantage that you do not make the user of your value execute some kind of unwrapping routine in order to get access to the value within. For example, this is useful when you are constructing closures that you want to go ahead and construct now even though some of the values that they contain are not available yet.

PVar s are implemented with a lazy thunk that reads from an IORef ; before the IORef is written to, it contains _|_ (an exception with a descriptive error message) so that an error is raised in the user code if the variable is accidently accessed before the value is ready.