For everybody's reference, here is summary information of the exercise (each quoted paragraph starts with a double-quote, and the entire section ends with a single double-quote): The 10th ICFP Programming Contest: July 20 - 23, 2007, organized by Utrecht http://save-endo.cs.uu.nl/ Problem ("Task") Description: http://save-endo.cs.uu.nl/Endo.pdf Background of Problem (from "[Chapter] 1 Background" of the above-mentioned "Problem ('Task') Description": "Department of Information and Computing Sciences Utrecht University "Tenth Interstellar Contest on Fuun Programming "Morph Endo! "Task Description "Endo is an alien life form, belonging to the species of the Fuun. Endo needs your help! Earth’s environmental conditions can be harsh for a life form not properly adapted. Endo had the bad luck of being dropped on Earth by an Interstellar Garbage Collector. Both the life form and its faithful space ship Arrow were severely hurt in the crash, and even worse, after leaving the damaged space craft Endo was hit by a cargo container that was also dropped by the Garbage Collector. "Endo is now in serious trouble. It cannot survive on planet Earth in its present form, and Arrow is running low on power. According to Arrow, who was the one to have contacted us, the only hope for Endo is to change its DNA and thereby adapt it to the conditions of our planet. To this end, Arrow has been able to come up with a form in which Endo will survive. Unfortunately, given its current condition, Arrow lacks the resources for coming up with proper DNA modifications itself. "Your task is therefore to help us find such a DNA sequence within 72 hours. Shortly thereafter, Arrow’s power will run out for good, and since only Arrow can perform the genetic modification on Endo, this would mean Endo’s definite end. "Admittedly, we are partially responsible for the current state of urgency. It took us a long time before realizing the nature of Arrow’s emergency message and decoding the information that was provided to us. We are sorry but nevertheless hope that you, the ICFP programming contest audience, have the proper expertise to solve this problem within the harsh time constraints. "Fortunately, there’s also good news. When we weren’t yet aware of the complete story of Endo and its struggle for life, we have been working on decoding the specification of Fuun DNA. We now know that Fuun DNA works significantly different from human DNA, and that the process of DNA resequencing can be properly described as an algorithm. The main purpose of this document is to describe this algorithm." To solve the above task, you are given the following tools: * Endo's DNA string (MD5 hash c496125ef1d22a61cb86aeb1a02c4092) http://save-endo.cs.uu.nl/endo.zip * the source picture http://save-endo.cs.uu.nl/source.png * the target picture http://save-endo.cs.uu.nl/target.png The actual description of the problem is 20 pages long, which is too long to quote here. Correct me if I'm wrong, but because the prefix should not be too long, and chemical process of modifying Endo with the prefix should not consume too much energy, this appears to be an example of constraint programming. Since constraint programming can often be carried out by constraint logic programming, I might suggest a logic programming approach for this problem. According to "Applications and libraries/Compilers and interpreters - HaskellWiki" (http://haskell.org/haskellwiki/Applications_and_libraries/Compilers_and_inte...), CHR (Constraint Handling Rules) is "A concurrent committed-choice constraint logic programming language, implemented using GHC's software transactional memory." The URL listed for the associated PS file is http://www.comp.nus.edu.sg/~sulzmann/manuscript/chr-stm.ps, but when I click on the link, I get a "403 Forbidden" error. However, I was able to find the following associated Web site with Google: Constraint Handling Rules (CHR) http://www.cs.kuleuven.ac.be/~dtai/projects/CHR/ -- Benjamin L. Russell On Wed, 23 Jul 2008 22:11:10 -0300, "Rafael Gustavo da Cunha Pereira Pinto" <rafaelgcpp@gmail.com> wrote:

Did any of you tried to do a Haskell implementation of the ICFPC 2007 problem?

I was thinking of using it as a learning exercise, but I am afraid of the stack. My approach is:

1) use Data.ByteString.Lazy.Char8 to read the contents of the DNA file 2) create a recursive function process::ByteString -> a that will call itself.

I have a few problems:

a) the DNA is 8MB long. How can I ensure the stack will hold a recursive call? b) there is an "abnormal ending" function called finish that is called anywhere in the code. Is it a good approach to return Empty to end processing? c) should I go "monadic", keeping the dna on a state monad?

Thanks

On Wed, 23 Jul 2008 22:11:10 -0300, "Rafael Gustavo da Cunha Pereira Pinto" <rafaelgcpp@gmail.com> wrote:

[...]

I have a few problems:

a) the DNA is 8MB long. How can I ensure the stack will hold a recursive call?

Forgive me if I'm wrong, but on a hunch, but this sounds like an issue that could possibly be solved using tail recursion. For example, according to "Recursion in a monad - HaskellWiki" (http://haskell.org/haskellwiki/Recursion_in_a_monad), here is a monadic do-block reads 'n' lines from stdin, using a linear recursive process: main = f 3 f 0 = return [] f n = do v <- getLine vs <- f (n-1) return $! v : vs This runs as follows: $ runhaskell A.hs 1 2 3 ["1","2","3"] Rewriting the code to make it use a linear iterative process (using tail recursion) results in the following code: f 0 acc = return (reverse acc) f n acc = do v <- getLine f (n-1) (v : acc) Here, "acc" stands for an accumulator, which is a register (here, a variable simulating a register) to store temporary values. This rewrite enables the process to use constant stack space. Using this idea, it may be possible to rewrite your recursive function process::ByteString -> a to use constant stack space. -- Benjamin L. Russell