February 2013 - Beginners

Building with shared libraries on OS X
by James Nelson 18 Feb '13

18 Feb '13

Hi, I've been trying to install Haskell in such a way that I can compile things with the -dynamic option on an OS X machine. It seems that with the standard Haskell Platform, the shared versions of libraries are not included, so enabling the "shared" option in ~/.cabal/config doesn't work. I've tried installing haskell-platform from scratch with homebrew (with and without --devel), and this hasn't worked either Finally, I've tried re-installing Haskell Platform with the provided .dmg from scratch, but I get errors like: - Perhaps you haven't installed the "dyn" libraries for package `text-0.11.2.3'? - Codec/Zlib.hs:47:8: Could not find module `Codec.Compression.Zlib' Perhaps you haven't installed the "dyn" libraries for package `zlib-0.5.4.0'? Is there any way to add the shared libraries to everything included in haskell platform for OS X? Regards, James

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Simple parser question
by Martin Drautzburg 18 Feb '13

18 Feb '13

Hello all, I just hit a sticking point when trying to parse something like data Exp = Lit Int -- literal integer | Plus Exp Exp where something like "1+2" should be parsed to "Plus (Lit 1) (Lit 2)". When I try to parse "1+2" my parser enters an infinite loop. I can understand why: it thinks "hmm, this expression could be a plus, but then it must start with an expression, lets check". and it tries to parse expression again and again considers Plus. When I change the rules, so it first checks for "Lit", it does parse the "1" just fine, but then gives up, because the remainder is not an expression anymore, but just a "+2". My parser is written in the style shown in Graham Hutton's book: Parser a :: String -> (a, String). I believe I am missing something obvious, but I can't see it. -- Martin

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Re: [Haskell-beginners] Stack overflow folding a map
by tcollin371 18 Feb '13

18 Feb '13

It turns out that the problem isn't the counting code in the fold operations, but instead seems to be lazy evaluation that's happening as the map is built. If I run through the count operation every few thousand inserts, then I don't get the stack overflow operation. -Truman -----Original Message----- >From: tcollin371 <tcollin371(a)earthlink.net> >Sent: Feb 16, 2013 1:13 AM >To: Hask <beginners(a)haskell.org> >Subject: [Haskell-beginners] Stack overflow folding a map > >I've been playing around with Haskell on and off for a while, and am getting a "Stack space overflow" that I can't seem to correct. I have a map containing about 2 million items. Actually, it's a map keyed off of an Int where the element is another map keyed off of a string that has three Ints as its data. The number of top level maps is pretty small (< 50), and the maps held by them hold a lot of entries. > >I want to walk through all of the entries and gather some statistics, but I keep getting the stack overflow. I worked down my test to an inner and outer fold that just counts the entries, and I still get the stack overflow. I added 'seq' calls to make sure that the count isn't building up a bunch of undone operations, and I'm using foldlWithKey, and have also tried foldlWithKey' with the same result. > >I get the count in my main function and immediately print it out. Here is the relevant code: > >outputIfNotFilteredCount1 :: Int -> KeyMap -> PuzzleMapRecord -> Int >outputIfNotFilteredCount1 inCount key entry > = seq inCount (inCount + 1) > >outputDatabaseCount1 :: Int -> Int -> InnerDB -> Int >outputDatabaseCount1 inCount smndCount innerDB > = seq inCount (inCount + outCount) > where > outCount > = Map.foldlWithKey' outputIfNotFilteredCount1 0 innerDB > >main = do >... > let > count = Map.foldlWithKey' outputDatabaseCount1 0 resultDB > putStrLn $ "Database entries:" ++ (show count) > > >Any thoughts on why this is running out of stack space? > >-Truman > > >_______________________________________________ >Beginners mailing list >Beginners(a)haskell.org >http://www.haskell.org/mailman/listinfo/beginners

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f . g or f g or f $ g?
by Martin Drautzburg 13 Feb '13

13 Feb '13

Hello all I frequently get confused over f . g vs f g. I do understand the following With: g :: a->b f :: b ->c f.g :: a->c However f g is a type error, because then f would have to accept a function (a->b) as its first parameter, but it only accepts a b. Is there a way to easily remember when to use f.g and when to use f g without having to do this type algebra. -- Martin

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Re: [Haskell-beginners] Beginners Digest, Vol 56, Issue 22
by Patrick Lynch 13 Feb '13

13 Feb '13

Good morning Brent. Thank you for your recomendation. I went to Amazon and checked the Table of Contents on the book you mentioned... It costs $90 and its not really what I'm looking for. I'm going to approach this from a different direction. It is: 1. I'm looking for a the Haskell definition of the category Hask...If you can help me out here, I'd appreciate it. 2. Functor, Monoid and Monad are all classes in Haskell...I'm going to try to determine how they are related to Hask and to each other. To do this, I'll review my two favorite Haskell books: "Real World Haskell" by O'Sullivan etal and "Learn You a Haskell for Great Good!..." by Lipovaca. I'll review Functor, Monoid and Monad in these two books and do the associated exercises [though Lipovaca book does not contain exercises]. Unfortunately, neither book contains an explanation of Hask [and all of the other Haskell books in my library also do not define Hask]. 3. If you know of an online course pertaining to Haskell and CT, I'd appreciate hearing about it. 4. I may, "shudder", even consider going back to university and pick up two or three courses in regard to this. I'll let you know how I make out - I'm heading off on a long, long journey Good day, Patrick ----- Original Message ----- From: <beginners-request(a)haskell.org> To: <beginners(a)haskell.org> Sent: Tuesday, February 12, 2013 4:43 PM Subject: Beginners Digest, Vol 56, Issue 22 > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Beginners Digest, Vol 56, Issue 20 (Brent Yorgey) > 2. Re: f . g or f g or f $ g? (Martin Drautzburg) > 3. Re: f . g or f g or f $ g? (Emanuel Koczwara) > 4. Parsec simple question (Sean Cormican) > 5. Re: f . g or f g or f $ g? (Patrick Mylund Nielsen) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 12 Feb 2013 14:14:15 -0500 > From: Brent Yorgey <byorgey(a)seas.upenn.edu> > Subject: Re: [Haskell-beginners] Beginners Digest, Vol 56, Issue 20 > To: beginners(a)haskell.org > Message-ID: <20130212191415.GA5182(a)seas.upenn.edu> > Content-Type: text/plain; charset=utf-8 > > Hi Patrick, > > I would highly recommend > > Jeremy Gibbons, "Calculating Functional Programs". In Roland > Backhouse? Roy Crole and Jeremy Gibbons, editors, Algebraic and > Coalgebraic Methods in the Mathematics of Program > Construction. Vol. 2297 of Lecture Notes in Computer Science. Pages > 148?203. Springer?Verlag. 2002. > > http://www.cs.ox.ac.uk/publications/publication2360-abstract.html > >>From what you've said it sounds like this is exactly what you're > looking for. > > -Brent > > On Tue, Feb 12, 2013 at 10:02:14AM -0500, Patrick Lynch wrote: >> Good morning, >> >> I've actually have 6 books in my library. They are: >> 1. "Algebra Third Edition" by MacLane and Birkhoff >> [I couldn't get anything out of this book] >> 2. "Categories for the Working Mathemetician - Second Edition" by Mac >> Lane >> [I could get through the first chapter and then gave up] >> 3. "Conceptual Mathematics - A first introduction to categories" by >> Lawvere and Schanuel >> [The first chapter was understandable but the following chapters >> were completely undecipherable] >> 4. " Basic Category Theory for Computer Scientists" by Pierce >> [I could get through all three chapters but couldn't do the >> exercies - and could not see how CT applies to Haskell {especially, >> Monoids, Monads and Functors}] >> 5. "Categories and Computer Science" by Walters >> [This one made me cry] >> 6. "Category Theory" by Awodey >> [Again, I could only get through the first chapter] >> >> I tried viewing the videos by Eugenia Cheng - but I just couldn't >> follow her presentation. >> I also viewed another video entitled Hasket and CT - but it was given >> by a mathematican and I couldn't follow this fellow - I'll have to >> get the link for this. >> I finally viewed a video by Brian Beckman in regard to FP [eg: F# and >> Haskell - and mentions C# as a comparison non-FP language], >> specifically, in regard to Monoids and it was great - however, he >> didn't go into CT - but he did recommend that the viewer pursue it. >> >> I'm really not interested in all of the categories in the world...I >> specifically am interested in how CT applies to Hask, Monoids, Monads >> and Functors. >> All of the Haskell authors that I've read have 'ducked' the CT issue >> - mentioning that it is not necessary to understand Haskell. I often >> was left with the thought: are they correct or do they simply not >> understand CT. >> If you know of a Computer Scientist [and, please, not a >> Mathematician] who has written in regard to Haskell and CT, please >> let me know. >> >> Hopefully, there is a reference that is available that will help me - >> if not, I plan on revisting my Haskell books ["Learn You a Haskell >> for Great Good!..." by Lipovaca and "Real World Haskell" by >> O'Sullivan, etal] and focus in on Functors, Monoids, Monads and Hask. >> I'll also view Brian Beckman's video again and see if I can really >> follow his example. >> >> Thank you >> >> >> ----- Original Message ----- From: <beginners-request(a)haskell.org> >> To: <beginners(a)haskell.org> >> Sent: Tuesday, February 12, 2013 6:00 AM >> Subject: Beginners Digest, Vol 56, Issue 20 >> >> >> >Send Beginners mailing list submissions to >> >beginners(a)haskell.org >> > >> >To subscribe or unsubscribe via the World Wide Web, visit >> >http://www.haskell.org/mailman/listinfo/beginners >> >or, via email, send a message with subject or body 'help' to >> >beginners-request(a)haskell.org >> > >> >You can reach the person managing the list at >> >beginners-owner(a)haskell.org >> > >> >When replying, please edit your Subject line so it is more specific >> >than "Re: Contents of Beginners digest..." >> > >> > >> >Today's Topics: >> > >> > 1. Re: Haskell and Category Theory (Rustom Mody) >> > 2. Re: Haskell and Category Theory (Joe Fredette) >> > 3. Re: Effective use of nested Monads (David Hinkes) >> > >> > >> >---------------------------------------------------------------------- >> > >> >Message: 1 >> >Date: Mon, 11 Feb 2013 19:01:51 +0530 >> >From: Rustom Mody <rustompmody(a)gmail.com> >> >Subject: Re: [Haskell-beginners] Haskell and Category Theory >> >To: The Haskell-Beginners Mailing List - Discussion of primarily >> >beginner-level topics related to Haskell <beginners(a)haskell.org> >> >Message-ID: >> ><CAJ+Teoc=Yn9aWi-7OnOcLPPiywZ+rv__wiEjhTGdVD_0bP_qDg(a)mail.gmail.com> >> >Content-Type: text/plain; charset="iso-8859-1" >> > >> >On Mon, Feb 11, 2013 at 10:37 AM, Brent Yorgey >> ><byorgey(a)seas.upenn.edu>wrote: >> > >> >>On Sun, Feb 10, 2013 at 12:44:11PM -0500, Patrick Lynch wrote: >> >>> Good morning, >> >>> I've tried to read 5 books on Category Theory and finally have >> >>> admitted >> >>defeat. >> >>> What I'm looking for is simply a book that is geared to Haskell and >> >>Category that can be understood by mere mortals. >> >>> I was trained as an Electrical Engineer, so my math is quite >> >>good, but > I >> >>just don't get Category Theory from these books. >> >>> If anyone can recomment a book on Category Theory and Haskell, >> >>> written >> >>by a Computer Scientest [no more Mathematicians for me], I welcome it. >> >>> Thanks, >> >>> Patrick >> >> >> >>Can you tell us which books you've tried to read? >> >> >> >>-Brent >> >> >> > >> >Well Brent is not going to say it I guess so someone needs to: >> >Typeclassopedia http://www.haskell.org/haskellwiki/Typeclassopedia >> >is required reading >> >

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Re: [Haskell-beginners] Beginners Digest, Vol 56, Issue 20
by Patrick Lynch 12 Feb '13

12 Feb '13

Good morning, I've actually have 6 books in my library. They are: 1. "Algebra Third Edition" by MacLane and Birkhoff [I couldn't get anything out of this book] 2. "Categories for the Working Mathemetician - Second Edition" by Mac Lane [I could get through the first chapter and then gave up] 3. "Conceptual Mathematics - A first introduction to categories" by Lawvere and Schanuel [The first chapter was understandable but the following chapters were completely undecipherable] 4. " Basic Category Theory for Computer Scientists" by Pierce [I could get through all three chapters but couldn't do the exercies - and could not see how CT applies to Haskell {especially, Monoids, Monads and Functors}] 5. "Categories and Computer Science" by Walters [This one made me cry] 6. "Category Theory" by Awodey [Again, I could only get through the first chapter] I tried viewing the videos by Eugenia Cheng - but I just couldn't follow her presentation. I also viewed another video entitled Hasket and CT - but it was given by a mathematican and I couldn't follow this fellow - I'll have to get the link for this. I finally viewed a video by Brian Beckman in regard to FP [eg: F# and Haskell - and mentions C# as a comparison non-FP language], specifically, in regard to Monoids and it was great - however, he didn't go into CT - but he did recommend that the viewer pursue it. I'm really not interested in all of the categories in the world...I specifically am interested in how CT applies to Hask, Monoids, Monads and Functors. All of the Haskell authors that I've read have 'ducked' the CT issue - mentioning that it is not necessary to understand Haskell. I often was left with the thought: are they correct or do they simply not understand CT. If you know of a Computer Scientist [and, please, not a Mathematician] who has written in regard to Haskell and CT, please let me know. Hopefully, there is a reference that is available that will help me - if not, I plan on revisting my Haskell books ["Learn You a Haskell for Great Good!..." by Lipovaca and "Real World Haskell" by O'Sullivan, etal] and focus in on Functors, Monoids, Monads and Hask. I'll also view Brian Beckman's video again and see if I can really follow his example. Thank you ----- Original Message ----- From: <beginners-request(a)haskell.org> To: <beginners(a)haskell.org> Sent: Tuesday, February 12, 2013 6:00 AM Subject: Beginners Digest, Vol 56, Issue 20 > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Haskell and Category Theory (Rustom Mody) > 2. Re: Haskell and Category Theory (Joe Fredette) > 3. Re: Effective use of nested Monads (David Hinkes) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 11 Feb 2013 19:01:51 +0530 > From: Rustom Mody <rustompmody(a)gmail.com> > Subject: Re: [Haskell-beginners] Haskell and Category Theory > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Message-ID: > <CAJ+Teoc=Yn9aWi-7OnOcLPPiywZ+rv__wiEjhTGdVD_0bP_qDg(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > On Mon, Feb 11, 2013 at 10:37 AM, Brent Yorgey > <byorgey(a)seas.upenn.edu>wrote: > >> On Sun, Feb 10, 2013 at 12:44:11PM -0500, Patrick Lynch wrote: >> > Good morning, >> > I've tried to read 5 books on Category Theory and finally have admitted >> defeat. >> > What I'm looking for is simply a book that is geared to Haskell and >> Category that can be understood by mere mortals. >> > I was trained as an Electrical Engineer, so my math is quite good, but >> > I >> just don't get Category Theory from these books. >> > If anyone can recomment a book on Category Theory and Haskell, written >> by a Computer Scientest [no more Mathematicians for me], I welcome it. >> > Thanks, >> > Patrick >> >> Can you tell us which books you've tried to read? >> >> -Brent >> > > Well Brent is not going to say it I guess so someone needs to: > Typeclassopedia http://www.haskell.org/haskellwiki/Typeclassopedia > is required reading >

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Parsec simple question
by Sean Cormican 12 Feb '13

12 Feb '13

I'm having the following issue but I have no idea what to do about it, clearly I'm missing something. integer = P.integer lexer identifier = P.identifier lexer data Expression = ID String | Num Integer deriving (Show) name :: Expression name = ID "string" number :: Expression number = Num 123 whileParser :: Parser Expression whileParser = whiteSpace >> expr8 expr8 :: Parser Expression expr8 = name <|> number The error I'm getting is the following: Couldn't match expected type `Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity Expression' with actual type `Expression' In the first argument of `(<|>)', namely `name' In the expression: name <|> number In an equation for `expr8': expr8 = name <|> number Failed, modules loaded: none. Also there is an issue where i have the following code changed: name = ID "string" to name = ID identifier number = Num 123 to number = Num integer which gives me two further errors regarding: Couldn't match expected type `String' with actual type `Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity String' and Couldn't match expected type `Integer' with actual type `Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity Integer' Now I am sure this is something silly but thanks for any help!

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Haskell and Category Theory
by Patrick Lynch 12 Feb '13

12 Feb '13

Good morning, I've tried to read 5 books on Category Theory and finally have admitted defeat. What I'm looking for is simply a book that is geared to Haskell and Category that can be understood by mere mortals. I was trained as an Electrical Engineer, so my math is quite good, but I just don't get Category Theory from these books. If anyone can recomment a book on Category Theory and Haskell, written by a Computer Scientest [no more Mathematicians for me], I welcome it. Thanks, Patrick

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Effective use of nested Monads
by David Hinkes 11 Feb '13

11 Feb '13

Hi haskell-beginners, I'm starting to come to the idea of exposing a Monad as a means of controlling an API. So, I've started creating my own Monad data types based on classical monads. However, I'm running into a problem regarding creating monad definitions when using nested Monads. For example: newtype Example m o = Example { runExample :: State Int (m o) } Is there a clean way to make Example a monad? instance Monad m => Monad (Example m) where -- return is easy return = Example . return . return -- bind is hard. -- f :: o -> Example m p -- a :: Example m o a >>= f = ... My intuition tells me that this should be simple, I should just use the State's bind operation, but I can't seem to make it work. Any advise would be great. Thanks, Dave -- David Hinkes

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Type of a Group.
by Robert Goss 09 Feb '13

09 Feb '13

Dear all, Looking to learn a little haskell I tried to program up a little group theory but feel I am stuck on what the types should be. At first it seemed obvious (and the algebra package does this) that the type of a group should be given by: class Group g where mul :: g -> g -> g inv :: g -> g unit :: g My problem is this seems to assume that the type of group you are in is encoded by the type system. I first ran into problems with this when I wanted to define a cyclic group. The only way I could define the type was either to define each cyclic group separately so have C2, C3, C4, ... or parametrise it over the class Nat. So a cyclic group would have the type Cyclic (Succ(Succ( ... Succ(Zero)) ... )) which would consistently define all cyclic groups but is hardly any better. For example a computation mod a large prime p is not going to be pleasant. I came up with a partial solution by realising that a group is defined as a set X and some operations on it to get class Group g x where mul :: g -> x -> x -> x inv :: g-> x -> x unit :: g -> x Making it easy to define cyclic groups and all my old groups carry over. But now to evaluate an expression I need to hang onto the group i am in and pass it around. As i am newish to haskell I wanted to know if I have missed a simpler more obvious way of doing things? All the best, Robert Goss

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