Editing Concurrency with oracles (section)

== The problem ==
While there has been research into ''deterministic'' concurrency (e.g. Eleni Spiliopoulou's work with the Bristol Haskell system as described in his thesis [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.60.4364&rep=rep1&type=pdf Concurrent and Distributed Functional Systems]), models of concurrency are generally ''nondeterministic''. For programming languages like Haskell which have the goal of being [[Referential transparency|referentially transparent]], the nondeterminism associated with concurrency presents a challenge - the usual techniques for supporting it used by most other (imperative) languages cannot not be simply transferred verbatim. But neither can nondeterminism be just ignored:

<blockquote>
Nondeterminism is useful in real-time applications where the behaviour of a program should depend on the order in which external events occur. For example, it might be desirable to merge the elements of two lists in the order in which the elements become available. This type of nondeterminism appears to be necessary if functional programs are to be used for real-time computing.

:<small>[https://academic.oup.com/comjnl/article-pdf/31/3/243/1157325/310243.pdf Nondeterminism with Referential Transparency in Functional Programming Languages], F. Warren Burton.</small>
</blockquote>

Other example include:
* which out of two computations will finish first;
* which input event will arrive first;
* whether a computation will finish before an input event arrives.

For concurrent programs, nondeterminism is inescapable - events like storage, network or other external failures can occur without warning. The languages used for writing such programs must have some way to allow these programs to cope with such degradation gracefully while continuing to carry out the majority of their designated tasks, albeit at reduced capacity.