Hi Michael, You've used quite a few entirely redundant brackets. The tool HLint ( http://community.haskell.org/~ndm/hlint) will tell you which ones.
Thanks, Neil On Wed, May 25, 2011 at 12:09 AM, michael rice <nowg...@yahoo.com> wrote: > The input file: http://dl.dropbox.com/u/27842656/psalms > > The Markov chain exercise from "The Practice of Programming", > Kermighan/Pike. Sample runs at the end. > > Michael > > ============================ > > import System.Environment(getArgs) > import System.Random > import Control.Applicative > import Control.Monad.Reader > import Control.Monad.State > import Data.Maybe > import Data.Map > > type Prefix = (String,String) > type GeneratorState1 = State ((Map Prefix [String]),Prefix,[String]) > type GeneratorState2 = StateT (Prefix,StdGen) (Reader (Map Prefix > [String])) > > non_word = "\n" > > f key new old = new ++ old > > buildMap :: GeneratorState1 (Map Prefix [String]) > buildMap = do (mp,(pfx1,pfx2),words) <- get > if (Prelude.null words) > then {- No more words. Return final map (adding non_word > for final prefix). -} > return (insertWithKey' f (pfx1,pfx2) [non_word] mp) > else do {- Add word to map at prefix & continue. -} > put (insertWithKey' f (pfx1,pfx2) [head words] mp, > (pfx2,(head words)), tail words) > buildMap > > generate :: GeneratorState2 (Maybe String) > generate = do ((pfx1,pfx2),gen) <- get > mp <- ask > let suffixList = mp ! (pfx1,pfx2) > let (index,newGen) = randomR (0, (length suffixList)-1) gen > let word = suffixList !! index > if (word == non_word) > then > return Nothing > else do > put ((pfx2,word),newGen) > return (Just word) > > rInt :: String -> Int > rInt = read > > main = do (seed:nwords:_) <- (Prelude.map rInt) <$> getArgs > contents <- getContents > putStrLn $ unwords $ catMaybes $ runReader (evalStateT (sequence > $ replicate nwords generate) ((non_word,non_word),mkStdGen seed)) > (evalState buildMap > (singleton (non_word,non_word) [], (non_word,non_word), words contents)) > > {- > [michael@hostname ~]$ ghc --make markov.hs > [1 of 1] Compiling Main ( markov.hs, markov.o ) > Linking markov ... > [michael@hostname ~]$ cat psalms | ./markov 111 100 > Blessed is the LORD, in thine own cause: remember how the foolish people > have blasphemed thy name. In the courts of the righteous: The LORD taketh > pleasure in the desert. And he led them with the wicked, and with the whole > earth, is mount Zion, on the sides of thine only. O God, and was troubled: I > complained, and my God. My times are in thy praise. Blessed be God, which is > full of the LORD is good: for his wondrous works. Now also when I am small > and despised: yet do I put my trust: how say ye to > [michael@hostname ~]$ cat psalms | ./markov 666 100 > Blessed is the LORD, and cried unto thee, Thy face, LORD, will I remember > thee from the beginning: and every one that is weaned of his heart to any > wicked transgressors. Selah. They return at evening: they make ready their > arrow upon the people; and thou hast destroyed all them that fight against > them that trust in thee: and let my tongue cleave to the heavens by his > power for ever; and thy lovingkindnesses; for they have laid a snare before > them: and that my ways were directed to keep thy word. Mine eyes fail while > I have said that > [michael@hostname ~]$ > > --- On *Tue, 5/24/11, Alexander Solla <alex.so...@gmail.com>* wrote: > > > From: Alexander Solla <alex.so...@gmail.com> > Subject: Re: [Haskell-cafe] [Maybe Int] sans Nothings > To: "Haskell Cafe" <haskell-cafe@haskell.org> > Date: Tuesday, May 24, 2011, 5:01 PM > > > > Personally, I find non-functional values without Eq instances to be > > degenerate. So I really do not mind superfluous Eq constraints. I > > would not hesitate to use filter ((/=) Nothing) in a function whose type > > has no free type variables. It's just a bit of plumbing inside of a > > more complex function. > > Sometimes it seems to be better to not allow Eq on Float and Double. > Since most algebraic laws do not hold for those types, it is more often > an error than an intention to compare two Float values. And how to > compare (IO a) values? > > > Floats, Doubles, and IO are all "degenerate" types, for the reasons you > outline. (Admittedly, Float and Double have Eq instances, but invalid Eq > semantics) Notice how their value semantics each depend on the machine your > runtime runs on, as opposed to merely the runtime. Bottom is another one of > these degenerate types, since comparisons on arbitrary values are > undecidable. > > Also, by thinking about function types, you often > get interesting use cases. Thus I would not assume too quickly that a > type will always be instantiated by types other than a function type. > Thus I would stick to (filter isJust) and use this consistently for > monomorphic and polymorphic types. > > > I am not suggesting (filter ((/=) Nothing)) /over/ (filter isJust). > Obviously, once one is aware of a better tool, one should use it. But I > am suggesting that for simple cases which are unlikely to change in any > substantive way, we should probably just use the tools we already know of, > as opposed to searching for the "right" one. Both might involve costs. > There is a cost involved in going to Google, thinking up a search term, > finding that Data.Maybe has relevant functions, picking the right one. It > takes less time to write "filter ..." than to type "haskell removing nothing > from list", for example. When dealing with known unknowns, there is a > balance to be made, and it is not easy. > > Michael's choice to ask the list imposed costs. (Not that we mind, we're > all volunteers, after all). But it probably took 10 minutes to get the > first reply. He could have written a bit of code that worked correctly, > given the context of his problem, in 20 seconds. Then again, he probably > worked on a different bit of code until somebody sent a solution, so we > probably only have to account for the time spent in context switching, for > everyone involved. > > > -----Inline Attachment Follows----- > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org <http://mc/compose?to=Haskell-Cafe@haskell.org> > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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