On 11/29/12 2:17 PM, Ivan Salazar wrote:
The bad side is that direct translation of algorithms are almost always very slow and the work needed to make them perform is very mind bending.
Indeed. The thing is, all algorithms make (implicit) assumptions about the cost model of the underlying language. The problem comes about because the assumptions made in most algorithms are not valid in Haskell. This isn't just an issue of laziness; the entire computational model of Haskell (e.g., STG) is radically different from imperative languages.
For example, in many cases just passing arguments around (a la the State monad) is much faster than using ST and explicit mutation. GHC does a lot of clever code reorganization, but mutation breaks countless purity laws and so it inhibits the application of these optimizations. Unfortunately, the vast majority of algorithms out there assume you're working in a language where mutation is essentially free. I'm not talking about any significant theoretical issues about using mutation or not; I'm just talking about the implicit assumptions that algorithm implementers make. If you believe mutation is essentially free then it makes sense to create an initial object and then incrementally mutate it this way and that until you get to where you want. But, a great deal of the time there's nothing actually stopping us from gathering all the information and allocating the final result directly. However, if you're not used to thinking in those terms, then the conceptual reorganization required to see how to gather all the data at once is indeed mind-bending.
-- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
