Haskell values that cannot be evaluated immediately.
Procrastinating variables (PVars) 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.
PVars 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.
PVars 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.
NOTE: PVars are modeled closely on the IVar implementation in the ivar-simple package. The major difference is that if you try to read an IVar before it has been given a value, it blocks until the value is available, whereas reading from a PVar before it is ready raises an exception. The reason behind the different symantics for PVar is because if the user accidently accesses the value too early, you want there to be a lot of noise to let him or her know about it, rather than merely blocking the thread indefinitely and causing them to wonder what went wrong.