- Beginners - Haskell.org

Re: Applicability of FP unchanging things to real world changing things
by Stephen Blackheath [to Haskell-Beginners] 25 Dec '09

25 Dec '09

Glurk, You wrote: > I think a lot of people, looking at the real world, would answer, yes, it > clearly IS the same book, now in a different state. And yes, I CAN step into > the same river twice. This is how people see and think about the real world. I think this is a really interesting question, and I may be too new at functional programming myself to do it justice. I've been doing a lot of Haskell in a short time, so my experience is fairly intuitive and this email is part of the process of making it more conscious. Here's one way to structure the library: lend :: BookID -> Library -> Library actOn :: Request -> Library -> (Library, [Result]) actOn (Lend bookID) lib = (lend bookID lib, []) ... -- | A library is a transformation from Request events (e.g. 'lend') to Result events (e.g. overdue book email), -- implemented using lazy lists. library :: Library -> [Request] -> [Result] library lib0 reqs = concat $ snd $ mapAccumL (\lib req -> actOn req lib) lib0 reqs Now, are we mutating a library, or are we not? In spite of what the more Zen-oriented Haskellers might say, it seems to me we are. Call me uneducated but I can't really see what is actually wrong with the idea of mutation of state over time, where the abstraction is appropriate. Perhaps the point is that in Haskell, this is one of many ways of looking at it. Perhaps the essential difference here is that the library goes round in an "eddy" with an inflow of events influencing it as it goes round (and producing an outflow of Result events). This difference accounts for more than it seems to. (Trying to put my finger on it...) it's a much more restricted activity than mutating state outright, that means that if you change your code locally (e.g. re-write the I/O parts), your chance of breaking things is minimal to zero. > I guess that brings up another point - that eventually we are drawn into > monads, which to me seems a bit unfortunate, because it seems that it leads us > back to imperative programming with mutable state... In many situations where I could use a state monad I don't (though I went through a stage of using them). My advice would be just write the code straight, and if the state management gets complex, a state monad may be warranted, or some better way may present itself. The great beauty of Haskell is that you have a lot of choices when a situation like this arises, and going for a state monad before you've examined the alternatives potentially restrict them. In the library example, if Library is a complicated data structure, you might want to have some lift functions that allow functions that affect only parts of the library to be lifted into a 'Library -> Library' type, e.g. liftDueDates :: (DueDates -> DueDates) -> Library -> Library liftDueDates f lib = lib { libDueDates = f (libDueDates lib) } Or to take the concept further ... There may be situations where you want to do some process where there are a number of parts of the library you want to work with, but conceptually some other way of viewing it makes sense. The "Haskell Way" would be to take the right parts of Library and create a new data structure that reflects the conception better. (In FP you often abstract things using data where you would use code in imperative languages.) If some output from that process has to contribute to a modified Library, then implement that as yet another transformation. Putting things back together like this is the extra work that Haskell makes you do - but the benefits outweigh the costs. So I'd say this: If you write your code in really plain Haskell, you'll tend towards a functional approach (e.g. why pass Library when it's better to transform library in some way first?). In that process, a state monad might suggest itself as a way of making the code more readable, but the thinking is still "plain Haskell". Then you're using a state monad with "right mindfulness" so to speak. Way of looking at it #2: If you are using a state monad because it's easier for you to understand the code that way, that's probably the wrong reason to use one. If you're using it because your straightforward Haskell is looking a bit unreadable, and the state monad tidies it up, then that's probably the right reason to use one. Way of looking at it #3: Your data needs to go through transformations from one form to another. In functional programming, you turn that process into a flow of data (or a sequence of transformations) from one form into another. If the flow loops, it starts to look a bit like mutating state, but in a way that's really only a surface appearance. One difference between state monads and mutating state in place (as done in OO languages) is that (in my experience) even with a state monad, the state mutation never really gets a chance to dominate the program. Haskellers have a saying "don't fear the monad - only IO is impure" that's applicable here. However, it's true that monads can lead to bad code, so they need to be used only where they're actually appropriate. I had an example of an excellent use for a monad recently: A parser where looking up an XML tag by its id string is abstracted as a continuation (so it can use IO if it wants) and it needs the ability to bail out if there's a parse error. A monad allowed me to write this code purely and cleanly. Another difference (state monad vs. mutating in place) is that a state monad can never lead to race conditions. Steve

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Einstein's Problem
by Patrick LeBoutillier 23 Dec '09

23 Dec '09

Hi all, I've been working hard at trying to solve Einstein's problem (http://www.davar.net/MATH/PROBLEMS/EINSTEIN.HTM) if an efficient manner using Haskell. I have posted my solution here: http://haskell.pastebin.com/m3ff1973a This exercise has been a real eye opener for me, especially for learning to work with the Maybe and List monads. I'm looking for suggestions for making the "test" function nicer. In order to make my solution fast, I generate my test cases incrementally, thereby elimating most of the cases early on. However the function looks funny (deeply nested). I'm also open to any comments about making better use of the different Haskell idioms. Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Pure Regex.PCRE Possible? (PLEAC, Fix Style)
by Volkan YAZICI 23 Dec '09

23 Dec '09

Hi, I'm trying to implement FixStyle[1] program of PLEAC[2] for Haskell. (It's missing in the Haskell version of PLEAC.) For this purpose, I needed regular expressions. (Now, I have two problems!) Despite, my code is, IMHO, quite purely functional, Regex.PCRE.String functions totally mess up the design with impure functions they introduce even for really simple string matching stuff. If you'd check out the sources[3], you'll see below impure functions. transDictRegex :: IO PCRE.Regex matchRegex :: String -> IO (String, String, String) translate :: String -> IO String However, if I'm not mistaken, above functions _should_ be absolutely pure. Is it possible to encapsulate impure PCRE functions into their pure equivalents? BTW, why are PCRE functions impure? Regards. [1] http://pleac.sourceforge.net/pleac_perl/strings.html [2] http://pleac.sourceforge.net/ [3] http://hpaste.org/fastcgi/hpaste.fcgi/view?id=14422

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expression parser
by John Moore 22 Dec '09

22 Dec '09

Hi, Could some one point out what I'm doing wrong below. This is a parser which takes an arithmetic expression built up from single digits. Turns 3*4+2 into (3*4)+2 etc.It giving me the last expression in a do statement or parse error on -> (+++) :: Parser a -> Parser a -> Parser a expr :: term( '+' expr |"") term -> factor('*' term |"") expr :: Parser Int expr = do t <- term do char '+' e <- expr return (t + e) +++ return t term :: Parser Int term = do f <- factor do char '*' t <- term return (f * t) +++ return f factor :: Parser Int factor = do d <- digit return (digitToInt d) +++ do char '(' e <- expr char ')' return e eval :: String -> Int eval xs = fst(head(parse expr xs)) John

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inserting tupels inside list as tupels in list
by kane96＠gmx.de 22 Dec '09

22 Dec '09

maybe the subject is a little bit confusing :p I have a organiser list like that and have to add events (like panic, xmas, ...) which is also a type of tupel for a specific date: aliceOrg = [ ( (21,Dec,2009), [panic,aliceBDay] ) ( (25,Dec,2009), [xmas] ) ( (10,Jan,2010), [bobBDay] ) What function(s) can I use to add these nested lists and tupels here? -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/chbrowser

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print all days of calendar
by kane96＠gmx.de 21 Dec '09

21 Dec '09

I have to create a calendar like this: type Calendar = [Date] Date is: type Day = Int data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show) type Year = Int type Date = (Day,Month,Year) I shall create a function: calendar :: Year -> Calendar that return a calendar for the given year like: [(1, Jan, 2009), (2, Jan, 2009), ..., (31, Dec, 2009)] I also have a function from a previous exercise which checks if a given date is valid. Is there a function for a loop that iterates from 1 to n and checks if the date is valid. If it's valid it should return the date otherwise it should jump to the next month or end at the end of the year? Or is it better to do it on another way with this data I have? -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/chbrowser

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Enum for natural numbers
by kane96＠gmx.de 21 Dec '09

21 Dec '09

Hi, I have data Nat = Z | S Nat deriving (Eq,Ord,Show) toEnum should return Z for negative numbers. I did something like this but the problem is I don't know how to set "less than". I tried > and lt: instance Enum Nat where toEnum (gt 0) = S Z toEnum (lt 1) = Z -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01

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Type constraints question
by David Gordon 21 Dec '09

21 Dec '09

Hi Folks, Total newbie here. I don't know if I am having a syntactic problem or a conceptual problem. This code: data Test = Test data Test2 = Test2 class MyClass a where getChild :: (MyClass b) => a -> b instance MyClass Test where getChild = Test2 instance MyClass Test2 where getChild = Test results in: [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:10:15: Couldn't match expected type `Test -> b' against inferred type `Test2' In the expression: Test2 In the definition of `getChild': getChild = Test2 In the instance declaration for `MyClass Test' test.hs:13:15: Couldn't match expected type `Test2 -> b' against inferred type `Test' In the expression: Test In the definition of `getChild': getChild = Test In the instance declaration for `MyClass Test2' Failed, modules loaded: none. Is this a reasonable thing to try and do in Haskell? If not, I have a lot more questions... ;) many thanks, David

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`coarbitrary' is not a (visible) method of class `Arbitrary'
by Amy de Buitléir 19 Dec '09

19 Dec '09

I'm trying to follow the instructions in http://haskell.org/haskellwiki/How_to_write_a_Haskell_program. I'm at the step where we've just written a QuickCheck test: amy@localhost ~/forbairt/haq $ cat Tests.hs import Char import List import Test.QuickCheck import Text.Printf main = mapM_ (\(s,a) -> printf "%-25s: " s >> a) tests instance Arbitrary Char where arbitrary = choose ('\0', '\128') coarbitrary c = variant (ord c `rem` 4) -- reversing twice a finite list, is the same as identity prop_reversereverse s = (reverse . reverse) s == id s where _ = s :: [Int] -- and add this to the tests list tests = [("reverse.reverse/id", test prop_reversereverse)] But when I try to run it, I get the following errors: amy@localhost ~/forbairt/haq $ runhaskell Tests.hs Tests.hs:10:4: `coarbitrary' is not a (visible) method of class `Arbitrary' Tests.hs:17:33: Not in scope: `test' I think this has something to do with the changes between QuickCheck v1 and v2. Can someone tell me what I need to do to make this example work? Thank you in advance! Amy

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Parallel mutation
by Jon Harrop 18 Dec '09

18 Dec '09

Can you mutate different parts of the same array from different threads in parallel using GHC? Someone once told me this was possible but I cannot see how... -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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