[Haskell-cafe] Asking for advice on programming style and also performance

Dear Haskell Cafe, I am working on a solver for Slitherlink puzzles in Haskell. Today I am completely puzzled that a small change I made had a huge performance impact. I thought the change to be merely cosmetic to make a function a little more elegant and the code more readable. But this change slowed down my program by a factor of 100! I would greatly appreciate advice from experienced Haskellers why this has happened. The program is hosted on github; this is the direct link to the change with unexpected outcome: https://github.com/ctbo/slitherlink/commit/b6f58699258ef68ddee21a1346bd18446... The program itself isn't commented at all, but I have written a sketch of how it works in the wiki: https://github.com/ctbo/slitherlink/wiki/How-it-works While I am at it: I would also appreciate any advice you might have on the programming style in general. I am relatively new to Haskell and this is my first nontrivial program. So any suggestions for improvements are welcome! Thanks for your time, -- Harald Bögeholz (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */