Hi Andy, This is a good question, and something we hit in GHC quite often too. Our "solution" is to use a mixture of strictness annotations, deepSeq, smart constructors, and hand-waving optimism that things will be evaluated soon enough anyway. Having to occasionally deepSeq the structore to force the thunks has quite a few problems, as you say. A better approach might be to establish a guarantee that the data type isn't leaky; that is, every field is either strict, or guaranteed to be deepSeq'd at construction by a smart constructor. To enforce the smart constructor, you might want ReadOnlyConstructors (see the Haskell' proposal). So for things like this:

regs :: !Array Int RegVal

You either use a strict Array type, or deepSeq the Array when constructing the record. To support record update without having to re-deepSeq everything in the record you would want to provide record updaters as part of the abstract datatype. Hope this helps... Cheers, Simon