Reified laziness
Short post, longer ones in progress.
One of the really neat things about the Par monad is how it explicitly reifies laziness, using a little structure called an IVar (also known in the literature as I-structures). An IVar is a little bit like an MVar, except that once you’ve put a value in one, you can never take it out again (and you’re not allowed to put another value in.) In fact, this precisely corresponds to lazy evaluation.
The key difference is that an IVar splits up the naming of a lazy variable (the creation of the IVar), and specification of whatever code will produce the result of the variable (the put operation on an IVar). Any get to an empty IVar will block, much the same way a second attempt to evaluate a thunk that is being evaluated will block (a process called blackholing), and will be fulfilled once the “lazy computation” completes (when the put occurs.)
It is interesting to note that this construction was adopted precisely because laziness was making it really, really hard to reason about parallelism. It also provides some guidance for languages who might want to provide laziness as a built-in construct (hint: implementing it as a memoized thunk might not be the best idea!)