Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
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
Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
Nice tool. I'll be using it a lot from now on, I'm sure. Thanks, Neil. Michael --- On Mon, 5/30/11, Neil Mitchell ndmitch...@gmail.com wrote: From: Neil Mitchell ndmitch...@gmail.com Subject: Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings To: michael rice nowg...@yahoo.com Cc: Haskell Cafe haskell-cafe@haskell.org, Alexander Solla alex.so...@gmail.com Date: Monday, May 30, 2011, 1:55 PM 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.Randomimport Control.Applicativeimport Control.Monad.Reader import Control.Monad.Stateimport Data.Maybeimport 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 - IntrInt = 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
Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
Alexander Solla schrieb: 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 I'm not a fan of explicit if-then-else's, but my preferred case words of [] - {- no more words -} ... w:ws - ... would work perfectly. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
Yes, very nice. Thanks. Michael --- On Wed, 5/25/11, Henning Thielemann schlepp...@henning-thielemann.de wrote: From: Henning Thielemann schlepp...@henning-thielemann.de Subject: Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings To: michael rice nowg...@yahoo.com Cc: Haskell Cafe haskell-cafe@haskell.org Date: Wednesday, May 25, 2011, 3:41 AM Alexander Solla schrieb: 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 I'm not a fan of explicit if-then-else's, but my preferred case words of [] - {- no more words -} ... w:ws - ... would work perfectly. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
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.Randomimport Control.Applicativeimport Control.Monad.Readerimport Control.Monad.Stateimport Data.Maybeimport 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 - IntrInt = 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 100Blessed 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 100Blessed 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
Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
My comments are in-line, marked off with On Tue, May 24, 2011 at 4:09 PM, 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 I don't see what f is for, since it doesn't do anything with the key. 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 I'm not a fan of explicit if-then-else's, but my preferred alternative won't win much either. (see http://osdir.com/ml/haskell-cafe@haskell.org/2011-05/msg00612.html for an example of what I'm talking about) generate :: GeneratorState2 (Maybe String) generate = do ((pfx1,pfx2),gen) - get mp - ask let suffixList = mp ! (pfx1,pfx2) I'm not sure how you're guaranteed that mp ! (pfx1, pfx2) exists, at first glance. lookup uses Maybe semantics, in the case there is no result. let (index,newGen) = randomR (0, (length suffixList)-1) gen I might use a function like: listRange :: [a] - (Int, Int) listRange list = (0, (length $ l) - 1) This is a common enough pattern to abstract away. 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 rInt is fair enough, but you can also have the same effect with an explicit type signature ((read n) :: Int) I tend to prefer the latter, personally. 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)) Nice use of functor application. {- [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
Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
Thanks, Alexander. Not as much red ink as I was expecting. My monitor is acting up, so I may be be dropping off the radar unexpectedly while acquiring a replacement. Each prefix is guaranteed to have at least one following word in its suffix list (possibly more) because the map is built from the actual Psalms text. The only prefix for which this does not occur is the final two words of the text, thus the TRUE case in buildMap puts the sole non_word in the suffix list. The generate function then _always_ builds a prefix from the first two words of the text, Blessed is, looks up its suffix list, chooses the next word randomly, and creates a new prefix by pairing the second word of the old prefix and the new word. Repeat. I've attached the AWK and C++ sources for the same Markov exercise if you wish to examine them. Both just write a single word on each line so I cobbled a Haskell program (lines.hs) to turn them into lines. Michael --- On Tue, 5/24/11, Alexander Solla alex.so...@gmail.com wrote: From: Alexander Solla alex.so...@gmail.com Subject: Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings To: michael rice nowg...@yahoo.com Cc: Haskell Cafe haskell-cafe@haskell.org Date: Tuesday, May 24, 2011, 10:42 PM My comments are in-line, marked off with On Tue, May 24, 2011 at 4:09 PM, 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.Randomimport Control.Applicativeimport Control.Monad.Reader import Control.Monad.Stateimport Data.Maybeimport 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 I don't see what f is for, since it doesn't do anything with the key. 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 I'm not a fan of explicit if-then-else's, but my preferred alternative won't win much either. (see http://osdir.com/ml/haskell-cafe@haskell.org/2011-05/msg00612.html for an example of what I'm talking about) generate :: GeneratorState2 (Maybe String)generate = do ((pfx1,pfx2),gen) - get mp - ask let suffixList = mp ! (pfx1,pfx2) I'm not sure how you're guaranteed that mp ! (pfx1, pfx2) exists, at first glance. lookup uses Maybe semantics, in the case there is no result. let (index,newGen) = randomR (0, (length suffixList)-1) gen I might use a function like: listRange :: [a] - (Int, Int) listRange list = (0, (length $ l) - 1) This is a common enough pattern to abstract away. let word = suffixList !! index if (word == non_word) then return Nothing else do put ((pfx2,word),newGen) return (Just word) rInt :: String - IntrInt = read rInt is fair enough, but you can also have the same effect with an explicit type signature ((read n) :: Int) I tend to prefer the latter, personally. 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)) Nice use of functor application. {-[michael@hostname ~]$ ghc --make markov.hs[1 of 1] Compiling Main ( markov.hs, markov.o ) Linking markov ...[michael@hostname ~]$ cat psalms | ./markov 111 100Blessed 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