Regular, shape-polymorphic, parallel arrays in Haskell
This recent paper describes the state-of-the-art in Data Parallel Haskell (DPH). Some of the code referred to in the paper is available here . This post is in response to a question from one of the coauthors, Roman Leshchinskiy, asking what is wrong with the paper. The problem with this paper is, in a word, cache . Specifically, the widespread adoption of CPU caches two decades ago forced a change in the way programmers traverse data structures when performance is of interest. The old style was to iterate directly using a for loop. The new style is either a cache aware set of loops that exploit knowledge of the sizes of caches in the machine, or a cache oblivious style that uses divide and conquer to subdivide the problem until it fits in-cache regardless of the cache's size. This paper uses neither of the modern approaches, opting instead to use direct loops that have not been seen in real numerical code for several decades. The paper incorrectly asserts that these are "wi...