[Haskell-cafe] How to translate Repa 2 program to efficient Repa 3 code?

[Michael Serra]

Hi Haskellers,
I've posted this question (and my code) to stack overflow as well (
http://stackoverflow.com/questions/10747079/what-are-the-key-differences-between-the-repa-2-and-3-apis),
so if anyone here has the answer, I'll post it to that site for the world's
reference.  Using the Repa 2 API, I have written some simple image
convolution tests which run more than fast enough.  The trick to getting
good performance was to call 'force' after every array transformation.  I
can't quite figure out the analogous thing to do with Repa 3 - at
stackoverflow you can see my Repa 3 code, which runs correctly but very
slowly.  It is not clear to me how exactly the monadic "computeP" functions
in Repa 3 are intended to be used - I have several calls to 'force' in my
Repa 2 code, but only 1 call to computeP in the Repa 3 version.  I've read
the excellent "Numeric Haskell" Repa tutorial by Don S, but it doesn't
cover Repa 3.  My thanks in advance!
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