An Operational Semantics for the Parallel Language Eden

The functional parallel language Eden — suitable for the description of parallel and concurrent algorithms in a distributed setting — is an extension of Haskell with a set of coordination features. In this paper we present a formal operational semantics for the kernel of Eden, or more precisely, for a λ-calculus widened with explicit parallelism and potentially infinite communication channels. Eden overrides the lazy nature of Haskell on behalf of parallelism. This interplay between laziness and eagerness is accurately described by the semantics proposed here, which is based on Launchbury's natural semantics for lazy evaluation, and is expressed through a two-level transition system: a lower level for the local and independent evaluation of each process, and an upper one for the coordination between all the parallel processes in the system. As processes are created either under demand or in a speculative way, different scheduling strategies are possible — ranging from a minimal one that only allows the main thread to evolve, to a maximal one that evolves in parallel every active binding.