使用Repa“在二维数组上循环”的最佳方式

I find the array library Repa for Haskell very interesting, and wanted to make a simple program, to try to understand how to use it. I also made a simple implementation using lists, which proved to be much faster. My main question is how I could improve the Repa code below to make it the most efficient (and hopefully also very readable). I am quite new using Haskell, and I couldn't find any easily understandable tutorial on Repa [I find the array library Repa for Haskell very