Result of foldAllP isn't forced.

[Ben Lippmeier]

Report from Chris Reade:

That is why I expected this code to work without the explicit seq , but these are needed to avoid the "Performing nested parallel computation sequentially" problem.

writeHeatMapBMP :: String 
                -> Array U DIM2 Double
                -> IO()
writeHeatMapBMP filename arr
  = do maxVal <- foldAllP max 0.0 arr
       minVal <- foldAllP min 0.0 arr
       arrImageOut <- minVal `seq` maxVal `seq`
                      computeP
                      $  R.map rgb8OfDouble
                      $  R.map (rampColorHotToCold minVal maxVal) arr
       writeImageToBMP filename arrImageOut

The implementation of foldAllP should ensure that the result is evaluated before it returns.