[Haskell-cafe] monads, memoization etc

Ilya Razenshteyn ilyaraz at gmail.com
Sat Aug 8 01:17:04 UTC 2015

quickfix: 10M'th, not 1M'th of course.


On Fri, Aug 7, 2015 at 6:11 PM, Ilya Razenshteyn <ilyaraz at gmail.com> wrote:

> Hi everyone,
> I'm new to Haskell and decided to learn a bit about monads and their
> utility. Actually, what worked nicely for me was: to read first several
> pages from "Computational lambda-calculus and monads", then do exercises
> from
> https://tonymorris.github.io/blog/posts/20-intermediate-haskell-exercises/,
> and then map these exercises to actual functions in the standard library.
> Then, I decided to implement memoization to practice a bit with ST,
> HashTable's and such. Here is what I managed to get:
> http://pastebin.com/79pwjLPL . This code computes Fibonacci numbers while
> caching the intermediate values in a hash table. I tried to make the code
> as fast as possible (it runs in under 15 seconds, when computing 1M'th
> Fibonacci number modulo 999983). It uses the package hashtables.
> I have several question about all this.
> First, is there a cheap way to speed this code up? Note that I'm
> interested in a universal memoization strategy, that is I don't want to use
> arrays instead of hash tables etc.
> Second, is this code the right way of doing what it does (assuming that I
> really care about the performance)?
> Third question. Currently, the code passes "cache" everywhere. It would be
> natural to combine ST with something like ReaderT to store it there. What
> is a way to do it? I tried something like "ReaderT (HashTable s) (ST s)
> Int", but I have not managed to get it work.
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