Am 07.02.13 03:37, schrieb Jacob Thomas: > Hello > > I'm new to Haskell, and need help with figuring out the Either type... > Any example to show how this works?
Maybe a program I wrote a while ago can help you get it. https://github.com/ctbo/slitherlink In Slitherlink.hs there is a function readProblem that can fail. It returns either a String with an error message (Left) or the problem read (Right). [...] readProblem :: String -> Either String Problem readProblem s = do pl <- readProblemList s when (null pl) $ Left "Problem is empty." let columns = length $ head pl when (columns == 0) $ Left "Problem starts with an empty line." unless (all ((== columns) . length) pl) $ Left "Problem not rectangular." let rows = length pl return $ listArray ((0, 0), (rows-1, columns-1)) $ concat pl [...] Or if you don't feel comfortable with using the Either monad, look at the even simpler function readConstraint a few lines earlier in the same file. In the main solve.hs program this is used to output a meaningful error message to the user if the input file is invalid: [...] where work s n = case readProblem s of Left e -> putStrLn e Right p -> do [...do something with the problem p...] Hope this helps. Harald -- Harald Bögeholz <b...@ct.de> (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 */ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe