Hi, could someone offer som advice on handling composite types with the Repa library? I can see a /long/ way around my problem, but I wonder what is the /best/ way ... The problem is that I am trying to parallellise a probabilistic algorithm using Repa. To handle pseudo-randomness in parallel, I couple each data element with a generator state, in a tuple. The generator state is of type TFGen (tf-random package) or StdGen¹ (random package), neither of which is an Unbox instance. Hence, I have been using boxed repa arrays. The code does not execute in parallel, and if I understand correctly this is because of the Boxed type. There is a workaround², but that requires an NFData instance. To make a NFData instance and work through the code to support it should be as simple as it is tedious, but it would still leave the other drawbacks of a boxed type. (In particular, I do sequential fold because the Repa fold function did not support boxed types.) I could of course rewrite the published libraries to use tuples instead of algebraic types, and benefit from the pre-made Unbox instances, but sounds messy and error-prone (as well as boring). If I have to I have to. Is there a quick and robust way to make Unbox instances from composite datatypes? Or any other advice on how to approach the matter? Both random ideas, references, and concrete solutions are welcome. [The other element of the tuple is an vector of unboxed constituent type. I am not sure how that's going to work, but I'll cross that bridge when I get there. For the time being it is the generator that stops type checking.] TIA ¹ Yes, I know that StdGen is statistically unsound. ² http://stackoverflow.com/questions/16097418/parallel-repa-code-doesnt-create... -- :-- Hans Georg