Just fyi, a library openstack (and myself and others) has created (in python) basically lets u program your workflow (really dataflow) as a DAG, and then the library will run it reliably (and in parallel and so-on). I'm more than willing to answer questions about how it does that if people are interested...

http://docs.openstack.org/developer/taskflow/

https://pypi.python.org/pypi/taskflow

And yes u can do things like built equation solvers to:

http://docs.openstack.org/developer/taskflow/examples.html#l...

And other neat things:

http://docs.openstack.org/developer/taskflow/#examples

That's exactly how Haskell's IO monad works (albeit a little simplified)