To be or not to be …lazy (In a Parallel Context)

Laziness restricts the exploitation of parallelism because expressions are evaluated only on demand. Thus, parallel extensions of lazy functional languages, like Haskell, usually override laziness to some extent. The purpose of the present work is to analyze how and to which extent strictness should be introduced in a lazy language to design a parallel extension of it. Towards this end, we have considered different evaluation strategies mixing laziness and eagerness for the language Eden –a parallel extension of Haskell–, we have given formal definitions for each, and we have implemented them in an interpreter to be able to run examples with alternative evaluation models, so that we can observe the intermediate and final states of the processes in the system, in terms of heaps of closures. Although the study is based on Eden, the concepts involved and the conclusions that we have obtained can be transferred to other parallel and functional languages.