[Haskell-cafe] Making new Unboxed Algebraic Types? (A Repa Problem)

Carter Schonwald carter.schonwald at gmail.com
Mon Jun 30 19:53:55 UTC 2014

boxity has nothing to do with parallelism. none :)

On Mon, Jun 30, 2014 at 2:55 PM, Hans Georg Schaathun <
georg+haskell at schaathun.net> wrote:

> 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.]
> ¹ Yes, I know that StdGen is statistically unsound.
> ²
> http://stackoverflow.com/questions/16097418/parallel-repa-code-doesnt-create-sparks
> --
> :-- Hans Georg
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