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Re: [Haskell-cafe] Newbie: Is 'type' synonym hiding two much?
by Dmitri O.Kondratiev 23 Mar '07

23 Mar '07

> Now, in the 17.5 section of a book one may see the following declarations: > > succeed :: b -> Parse a b > > *Before looking at 'succeed' function definition* one may think that > 'succeed' is a function of *one* argument of type 'b' that returns object of > type 'Parse a b'. <Antti-Juhani Kaijanaho wrote> That's what it is. However, without looking at the definition of Parse a b, you can't tell whether that is a function or not, and therefore all you can say about succeed is that it takes *at least* one argument. </Antti-Juhani Kaijanaho wrote> At least something to know :) I'll try to remeber this idiom. > Then I do this substitution *myself as a Haskell runtime* and get in > the result the following declaration of a * real > function that Haskell runtime* works with: <Antti-Juhani Kaijanaho wrote> I'm not sure why you feel the need to talk about runtime. This all happens at compile time. </Antti-Juhani Kaijanaho wrote> I wrote "runtime" because I didn't know how in own word to term inteprepter (GHCi) and compiler (GHC). Both of them will deal with this task/ > 2. Should I search through main and imported modules for treacherous 'type' > constructs? <Antti-Juhani Kaijanaho wrote> They are not treacherous. But yes, if you want to know what a type stands for, you need to look it up. The "treacherous" thing here is that in Haskell, returning a function is the same as taking one more parameter. This may feel strange at first, but it is a very important idiom and you do need to learn to live with it if you want to use Haskell. </Antti-Juhani Kaijanaho wrote> I see now, got it! Thanks. > > succeed :: b -> Parse a b > > *Before looking at 'succeed' function definition* one may think that > 'succeed' is a function of *one* argument of type 'b' that returns object of > type 'Parse a b'. > > Yet, function definition given in the book is: > > succeed val inp = [(val, inp)] <Ian Lynagh wrote> It's common to instead write this as succeed :: b -> Parse a b succeed val = \inp -> [(val, inp)] so the definition fits the type signature better. </Ian Lynagh wrote> Yes, that make more sense to me - function that returns a "customized" function <Ian Lynagh wrote> The main advantage of the synonym is when you are /using/ the Parser library, so you can put Parser String's in sequence etc without needing to know that internally they're implemented as a function. </Ian Lynagh wrote> Do you mean that if insted of type sysnonym I would use ADT built with 'data': data Parser a b = Parser (a -> [(b, [a])]) succeed :: b -> Parse a b succeed val = Parser (\inp -> ...) then I would not be able to put "put Parser String's in sequence etc without needing to know that internally they're implemented as a function" ? Anyway, I will try to implement this version of Parser as well. <Bernie Pope wrote> I think the main issue here is that from the "user's perspective" the Parse type should be (or is) abstract. Users of Parse should not really have to know how it is internally implemented. But implementors of Parse will (of course) be concerned with those details. How you look at the Parse type depends on which camp you belong to: implementor or user. </Bernie Pope wrote> Very good observation Bernie, thanks. Yet I need to play more with Parser type to justify it for this case :) Thanks guys for all your feedback! It really helps!

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Newbie: Is‘type’ synonym hiding two much?
by Dmitri O.Kondratiev 23 Mar '07

23 Mar '07

I am learning Haskell working through Simon Thompson book "Haskell The Craft of Functional Programming" (second edition). Solving problems in the book with more or less success I arrived almost to the end of the book at the section 17.5 "Case study: parsing expressions". Most probably the question I am going to ask here is very stupid, so please bear with me. In any case, after going so far in the book I felt certain that I know what function declaration means. So I thought that declaration: F :: a -> b -> c Is the same as: F :: a -> (b -> c) And means either: - a function 'f' of one argument of type 'a' that returns a function of type (b -> c), or it can also be interpreted as: - a function 'f' of two arguments of type 'a' and 'b' returning value of type 'c' Now, in the 17.5 section of a book one may see the following declarations: succeed :: b -> Parse a b *Before looking at 'succeed' function definition* one may think that 'succeed' is a function of *one* argument of type 'b' that returns object of type 'Parse a b'. Yet, function definition given in the book is: succeed val inp = [(val, inp)] Looking at this definition, I am inclined to think that 'succeed' is a function of *two* arguments named 'val' and 'inp' in the definition ! How can such a definition be correct? Ok, lets see what is this 'Parse a b' type is: type Parse a b = [a] -> [(b, [a])] So how does this help to make definition 'succeed val inp = [(val, inp)]' sound right? Well, the only way for me to make sense out of this is as follows. In this case Haskell 'type' makes type synonym for the type: [a] -> [(b, [a])] In order not to get out of my mind I start thinking about 'type' as a macro substitution mechanism. Then I do this substitution *myself as a Haskell runtime* and get in the result the following declaration of a * real function that Haskell runtime* works with: Succeed :: b -> [a] -> [(b, [a])] Great! This last declaration matches perfectly with function definition: succeed val inp = [(val, inp)] So I start feeling better, after all, it looks like my understanding of Haskell function declarations is not flawed too much. Well, but here comes my main questions! So: 1. Should I work every time as a macro translator when I just see *!any!* function declaration? 2. Should I search through main and imported modules for treacherous 'type' constructs? 3. Where, in this case goes implementation abstraction principle? Why I must provide *all* the details about function argument type structure in order to understand how this function works? Another example of a similar function from the book is: alt :: Parse a b -> Parse a b -> Parse a b alt p1 p2 inp = p1 inp ++ p2 inp In function definition I see three parameters: p1 – matches with function declaration perfectly p2 – matches with function declaration perfectly inp – how to match this parameter with function declaration ? I can match 'inp' parameter with 'alt' function declaration *only* after working as macro processor that expands type synonym 'Parse a b' into '[a] -> [(b, [a])]' and getting *real* declaration: alt :: [a] -> [(b, [a])] -> [a] -> [(b, [a])] -> [a] -> [(b, [a])] with that matches alt p1 p2 inp = p1 inp ++ p2 inp where 'inp' matches with *last* '[a]' argument. It seems that life of "human macro processor" becomes really hard when not trivial functions with 'type' synonym arguments come into play! Where I am wrong? Please enlighten me; I am really at a loss! And thanks for reading all this! Below I give a code example of these functions. Thanks, Dima module Parser where import Data.Char type Parse a b = [a] -> [(b, [a])] none :: Parse a b none inp = [] succeed :: b -> Parse a b succeed val inp = [(val, inp)] suc:: b -> [a] -> [(b, [a])] suc val inp = [(val, inp)] spot :: (a -> Bool) -> Parse a a spot p [] = [] spot p (x:xs) | p x = [(x, xs)] | otherwise = [] alt :: Parse a b -> Parse a b -> Parse a b alt p1 p2 inp = p1 inp ++ p2 inp bracket = spot (=='(') dig = spot isDigit t1 = (bracket `alt` dig) "234"

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Re: [Hs-Generics] FW: [Haskell-cafe] SYB vs HList (again)
by oleg＠pobox.com 22 Mar '07

22 Mar '07

[Please follow-up to generics(a)haskell.org] S. Alexander Jacobson wrote: > HLists require you to define Labels and basically only use label > values that are themselves either scalar or HLists. > ... > With SYB you create field labels using newtype (or data) declarations > e.g. > > data Salary = S {salary::Float} > > With HList, label declarations are really verbose e.g. > > data SalaryLabel deriving(Typeable) > type Salary = Field (Proxy SalaryLabel) Int > salary = proxy :: Proxy FooLabel Actually there is no requirement that HList record names must be scalar `labels', must be Proxies and require such a complex declaration. From HList's high point of view, any collection can be a record provided the type of each item is unique and there is some way to extract the value associated with that type. The HList library provides two implementations of Records (and there was one more, obsolete now). There could be more. For example, I have just committed a yet another implementation, http://darcs.haskell.org/HList/src/RecordD.hs Here a record is a list of things that have a type and a value and provide a way to extract that value. The example from the end of this file seems worth quoting: > data Name = Name String String deriving Show > newtype Salary = S Float deriving Show > data Dept = D String Int deriving Show > > person = (Name "Joe" "Doe") .*. (S 1000) .*. (D "CIO" 123) .*. emptyRecord > > -- could be derived automatically, like Typeable... > instance Fieldish Name (String,String) where > fromField (Name s1 s2) = (s1,s2) > instance Fieldish Salary Float where > fromField (S n) = n > instance Fieldish Dept (String,Int) where > fromField (D s n) = (s,n) > > test1 = show person > -- When a field acts as a label, only its type matters, not the contents > test2 = person .!. (Name undefined undefined) > test3 = person .!. (undefined::Salary) > test5 = person .!. (D "xxx" 111) > I don't know exactly how HList handles default values but I assume you > can restrict use of those values to explicit deserialization contexts. > Is that correct? I'm not sure what you mean about the restriction of default values to deserialization contexts. Anyway, HList provides a left-biased union of two records: hLeftUnion r1 r2 is the record r1 augmented with all the fields from r2 that didn't occur in r2. One may consider r2 to be the record with default fields and the corresponding values. > It would be really nice if there was some way to tell Haskell that > HLists have no more fields than the ones you happen to be getting and > setting in your code. Effectively that would mean you get data > structure inference not just function type inference which would be > really cool! I'm not sure I follow. Could you outline an example of the code you wish work? Incidentally, a lot of the library depends on the record types being members of some specific classes. One can define > newtype ClosedRecord = ClosedRecord r To make a ClosedRecord to be a record from which we can extract the values of some fields, we merely need to say > instance HasField l r v => HasField l (ClosedRecord r) v > where hLookupByLabel l (ClosedRecord r) v = hLookupByLabel l r v Since we did not make this record the member of HExtend or HAppend, it is not extensible.

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How do I do conditional tail recursion in a monad?
by DavidA 22 Mar '07

22 Mar '07

Hi, I would like to write a function untilM, which would be to until as mapM is to map. An example use would be if I had a function dice :: State Int Int which returns random dice throws within a state monad. Then I'd like to be able to do something like untilM (\(s,p)-> s>=100) f (0,1) where f (s,p) = do d <- dice return (s+d, p*d) This should throw dice until the sum exceeds 100, and keeping track of the product as we go along. The problem is that I get stuck trying to manage the interaction of the conditional and the recursion in untilM. Let's start with until: until p f x = if p x then x else until p f (f x) So I figure untilM should look something like: untilM :: Monad m => (a -> Bool) -> (a -> m a) -> a -> m a untilM p f x = return (if p x then x else untilM p f (f x)) The problem is that the two branches of the conditional have different types. If I try to remedy that by changing "then x" to "then return x", it still isn't happy. (My real question is, how do I do conditional tail recursion in a monad?) Thanks in advance.

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Newbie vs. laziness
by Alex Queiroz 22 Mar '07

22 Mar '07

Hallo list, I don't quite get how ($!) works. I have this function: dbCreateIndices con dtn dof = do let dummy = newDbItem let query = "SELECT id FROM " ++ (dtn dummy) ++ " ORDER BY " ++ (dof dummy) ids <- liftM (map fromSql . concat ) $! quickQuery con query [] return $ IntMap.fromList $ zip ids [0..] quickQuery returns a lazy list of results, and I expected ($!) to make it strict. But my program crashes if I use it this way. But, if I add a print to the function: dbCreateIndices con dtn dof = do let dummy = newDbItem let query = "SELECT id FROM " ++ (dtn dummy) ++ " ORDER BY " ++ (dof dummy) ids <- liftM (map fromSql . concat ) $! quickQuery con query [] print ids return $ IntMap.fromList $ zip ids [0..] print makes the ids list strict and all is well. How is the correct way to use ($!) ? Cheers, -- -alex http://www.ventonegro.org/

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Re: [Haskell-cafe] Lazy IO and closing of file handles
by Bertram Felgenhauer 22 Mar '07

22 Mar '07

Ketil Malde wrote: > Bertram Felgenhauer wrote: > >>> type Subject = String > >>> data Email = Email {from :: From, subject :: Subject} deriving Show > >data Email = Email {from :: !From, subject :: !Subject} deriving Show > ...except that From and Subject are Strings, and thus the strictness > annotation only forces WHNF. I.e., you also need to modify parseEmail > to force these. > > -k You're right. Actually, the program will be strict enough if the From header always precedes the Subject header (which was the case in my tests), but that's not immediately obvious. Modifying getHeader to force its result is the clean solution, say: getHeader = forceString . fromMaybe "N/A" . flip lookup headers forceString s = length s `seq` s Having to rely on GC to close the fds quickly enough is another problem; can this be solved on the library side, maybe by performing GCs when running out of FDs? Bertram

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Is ([] -> []) an arrow?
by Chad Scherrer 22 Mar '07

22 Mar '07

In John Hughes's "Programming With Arrows" (http://www.cs.chalmers.se/~rjmh/afp-arrows.pdf) he discusses a "stream function" type newtype SF a b = SF {runSF :: [a] -> [b]} and gives instance Arrow SF where .... He gives some examples using this, and everything seems to go just fine. But in Ross Patterson's "Arrows and Computation" (http://www.soi.city.ac.uk/~ross/papers/fop.html) he says that newtype ListMap i o = LM ([i] -> [o]) is ALMOST an arrow. Now, I've heard (but never verified) that IO fails to satisfy some monad laws, yet here we are, using it as a monad. Is a similar kind of thing going on here? Has anyone hit any snags in using this instance? Thanks, Chad

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Partial Evaluation
by jim burton 22 Mar '07

22 Mar '07

I am reading Hudak's paper Modular Domain Specific Languages and Tools [1] and am confused by his use of the term `Partial Evaluation'. I understand it to mean supplying some but not all arguments to a function, e.g. (+3) but it seems to mean something else too. This is in the context of optimising performance: "We have used existing partial evaluation techniques to do this...Unfortunately, there does not currently exist a suitable, easy-to-use partial evaluator for Haskell. Our approach was to convert the Haskell program to Scheme, partially evaluate the Scheme program, and then translate back into Haskell." What does P.E, mean here? Thanks, [1] Available http://wiki.ittc.ku.edu/lambda/Image:Hudak-Modular_Domain_Specific_Language…

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There can be only one fix? Pondering Bekic's lemma
by Nicolas Frisby 21 Mar '07

21 Mar '07

Bekic's lemma [1], allows us to transform nested fixed points into a single fixed point, such as: fix (\x -> fix (\y -> f (x, y))) = fix f where f :: (a, a) -> a This depends on having "true products," though I'm not exactly sure what that means. Mutual recursion can also be described with recursion and products f = \a -> ... g a ... g = \b -> ... f b ... can be defined as (f, g) = fix (\knot -> \a -> ... (snd knot) a ..., \b -> ... (fst knot) b ...) My question is: Given products and a fixed point combinator, can any pure expression be transformed into a corresponding expression that has just a single use of fix? If yes, has there been any usage of such a transformation, or is it just crazy? If no, could you provide a counter-example? Thanks for playing along, Nick [1] - Bekic has an entire book, but the most available reference for this I found is Levent Erkök's thesis regarding MonadFix/mfix (pgs 16 and 141). [Also, I cannot figure out how to get the proper symbol above that c in Bekic... sorry about that. I'd appreciate if someone told me how; does Unicode even have it?]

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Design and state...
by Magnus Therning 21 Mar '07

21 Mar '07

Recently I've been hacking together a small Haskell module on top of ptrace(2). It's comming along nicely, but it's very low level. The signals arrive in a list: wait :: Int -> IO [Signal] and processing of the signals can be done using e.g. mapM_: wait childPid >>= mapM_ processSignal In a few cases this low level requires quite a bit of extra code. Code needs to be repeated in every program using the module. Dealing with breakpoints is a good example: - Setting a breakpoint requires storing the old instruction at the address - When hitting a breakpoint the old instruction should be put back, EIP backed up, perform one single-step, and last put the breakpoint back again. I believe adding another layer on top of what I have now seems like a good idea. AFAICS that means having a state (I must keep track of the original instruction). That can easily be done by wrapping IO in a StateT: forEachSignal:: Int -> (Signal -> IO ()) -> StateT DbgEnv IO () forEachSignal childPid procSignal = ... (Or would it be better using IORefs and "OO" in the way described in "IO Inside"[1]?) What I am struggling with is how to allow the user of the module to have state as well. I can see how "OO"[1] can be used to solve it, but is there a more elegant, maybe more idiomatic (Haskellic??), way? Or am I missing a more elegant way of dealing with this? /M [1]: http://haskell.org/haskellwiki/IO_inside#Example:_emulating_OOP_with_record… -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus

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Lazy IO and closing of file handles
by Pete Kazmier 21 Mar '07

21 Mar '07

When using readFile to process a large number of files, I am exceeding the resource limits for the maximum number of open file descriptors on my system. How can I enhance my program to deal with this situation without making significant changes? My program takes one or more directories that contain email messages, stored one per file, and prints a list of all the email threads. Here is a snippet of output: New addition to the Kazmier family Casey Kazmier Memoization in Erlang? Thomas Johnsson Ulf Wiger $AL/EAB$ As a newcomer to Haskell, I am intrigued by lazy evaluation and how it can influence one's designs. With that said, I wrote the program as a sequence of list manipulations which seemed quite natural to do in Haskell starting with reading the contents of the each file. Here is the algorithm at the high level: 1. Read contents of all files returning a list of Strings 2. Map over the list and parse each String as an Email 3. Sort the list of Emails 4. Group the list of Emails by Subject 5. Map over the grouped list to create a list of Threads 6. Finally, print the list of Threads It is my understanding that, as a result of lazy IO, the entire file does not need to be read into memory because parseEmail only inspects the topmost portion of the email (its headers), which is a key part of my design as some of the files can be quite large. Unfortunately, as soon as I run this program on a directory with more than 1024 files, GHC craps out on me due to resource limits. It seems that the handles opened by readFile remain open. Would this be common across all Haskell implementations? How do I go about fixing this without making a significant number of changes to my program? Did I make a mistake in steps 1 and 2 above? Should I have read and parsed a single file at a time, and then move on to the next? I'd appreciate any other comments on the program as well. I feel this is the best example of Haskell code that I have written. Compared to the first version of this program I wrote a few months ago, this is a hundred times better. Here is the program: > module Main where > > import Control.Monad (filterM, liftM) > import Data.List > import Data.Maybe > import System.Directory > import System.Environment > > type From = String > type Subject = String > data Email = Email {from :: From, subject :: Subject} deriving Show > data Thread = Thread [Email] > > instance Show Thread where > show (Thread emails@(e:es)) = title ++ senders > where > title = newline . bolder . subject $ e > sender = newline . indent . from > senders = concatMap sender emails > newline = (++ "\n") > indent = (" " ++) > bolder = ("\27[0;32;40m" ++) . (++ "\27[0m") > > main = > getArgs >>= > mapM fileContentsOfDirectory >>= > mapM_ print . threadEmails . map parseEmail . concat > > fileContentsOfDirectory :: FilePath -> IO [String] > fileContentsOfDirectory dir = > setCurrentDirectory dir >> > getDirectoryContents dir >>= > filterM doesFileExist >>= -- ignore directories > mapM readFile > > parseEmail :: String -> Email > parseEmail text = > Email (getHeader "From") (getHeader "Subject") > where > getHeader = fromMaybe "N/A" . flip lookup headers > headers = concatMap mkassoc . takeWhile (/="") $ lines text > mkassoc s = case findIndex (==':') s of > Just n -> [(take n s, drop (n+2) s)] > Nothing -> [] > > threadEmails :: [Email] -> [Thread] > threadEmails = > map Thread . groupBy (fuzzy (==)) . sortBy (fuzzy compare) > where > fuzzy fn e e' = stripReFwd (subject e) `fn` stripReFwd (subject e') > stripReFwd = stripSpaces . reverse . stripToColon . reverse > stripSpaces = dropWhile (==' ') > stripToColon = takeWhile (/=':')

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toClockTime
by Jeff Polakow 21 Mar '07

21 Mar '07

Hello, On my system, GHC 6.6 running on windows xp, System.Time.toClockTime fails on calendar times later than (by days, I didn't check hours, minutes, etc.) January 18, 2038. Is this a bug? thanks, Jeff --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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what I learnt from my first serious haskell programm
by Fawzi Mohamed 21 Mar '07

21 Mar '07

Hi everybody, I came to haskell recently (one month) and I have now written my first serious program. I am still busy improving it, but here is a small report of what I learned and and my impressions of the language. I found no show stoppers, but a couple of things that I didn't like much. * namespaces * First off something that disturbed me but does not seem to be discussed much are namespaces, or rather the lack of them. Yes I know that haskell does have namespaces, but with object oriented languages you basically get a namespace for each class, in haskell you need modules, and I often like to have groups of related types in the same module. This might seem like a small issue, but the fact when a program or library grows it gets worse, and for me having a consistent way of calling functions is very important, it lets me spend less time thinking about naming. This is something makes the difference between a set of libraries and well thought out framework. Records then also put everything in the module namespace, and it seems that this misfeature has already been discussed. Anyway the namespaces would not be a problem if one would use always the most specialized function (and throw an error is no such function exist) as for example is done by Aldor ( http://www.aldor.org ) a language with a very nice type system, that unfortunately has been kind of dying for a long time because of its closed source. So I am wondering how people cope with them, share your opinion, for me the best thing seem to be to try to use one module per "big" type, and then "import qualified x as y", what are good coding practices? * space leaks * Sure enough in my program I had a space leak, actually an exponential space leak. The short story: it did bite me, and I think that many people that have problems with the performance in haskell, is because of them. The whole story is that I wanted to find the optimal node at depth n of an infinite tree. I even wrote down the list of the optimal nodes at each depth from which I would take some, but even before compiling it I realized that this would never be efficient. So I passed the maximum depth as argument to the function that generates and immediately reduces the tree. It had a huge space leak, and I knew what was happening, the function wad doing a breath first search of the tree instead of a depth first search, so I put a seq to make it go deeper first. It helped, but a space leak was still there. So I used the profiler, and basically the default profiler is useless for space leaks (it just says the total amount allocated which was constant). After some thinking I finally found out (a posteriori obvious) that I had to fully evaluate the actual node the the depth, and finally return the list. In the meantime I had introduced a couple of seq too much, and also switched to a strict data structure (which helped a little but was not the biggest issue. I really looked into memory profiling after having fixed the problem (but having seen how much work it could be even when you know where the problem is). I tried to use it and I think that it would have been useful, I will see how much with the next bug. With respect to this bibliographical heap profile (+RTS -hb) is broken on my AMD 64 Linux machine with GHC 6.6.20070129, I submitted a bug. * vector & matrices * A thing that bothered me a little was the absence of good standardized vectors and matrices, for now I rolled my own 3x3, I have seen numerical prelude, maybe in the future thre will be a standard interface for matrixes... Rolling my own mattrixes/vectors I have hit against some limitations of classes, nothing terrible, but not so nice, and something that I gather is known for example the fact that you cannot overload + or (in haskell 98) you cannot have overlapping instances. Again (see namespaces problem) changing names for the operations is the way out but it is not so nice. * parallel * For the future I want to make my algorithm parallel using threads (should be easy) . Then I was thinking about being parallel between computers, so to have really make this exercice a test if I really can switch to haskell for my programming, I was thinking about MPI, suggestions? thanks for having read till here :) Fawzi

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Re: [Haskell] Call-by-name: moved to haskell-cafe
by Dan Weston 21 Mar '07

21 Mar '07

Dan Weston wrote: > Douglas Philips wrote: >> On 2007 Mar 20, at 3:30 PM, Dan Weston indited: >> >>> I looked up John Backus on wikipedia and followed a link to ALGOL: >>> http://en.wikipedia.org/wiki/ALGOL_60 >>> where the following "undesirable" property of call-by-name is mentioned. >>> >>> "ALGOL 60 allowed for two evaluation strategies for parameter >>> passing: the common call-by-value, and call-by-name. Call-by-name had >>> certain limitations in contrast to call-by-reference, making it an >>> undesirable feature in language design. For example, it is impossible >>> in ALGOL 60 to develop a procedure that will swap the values of two >>> parameters if the actual parameters that are passed in are an integer >>> variable and an array that is indexed by that same integer variable. >>> However, call-by-name is still beloved of ALGOL implementors for the >>> interesting thunks that are used to implement it." >>> >>> I suppose that call-by-name is still beloved of Haskell implementors >>> as well? >> >> Notice that the "problem" with call-by-name is when side-effects are >> involved. In a pure-functional-environment those "problems" don't >> arise... >> >> --Doug > > It was the phrase "making it an undesirable feature in language design" > that jumped out at me. Here "language" is an implicitly universally > quantified variable, and the phrase beta-reduces to "call-by-name is an > undesirable feature in Haskell design". >

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parsec error message
by david karapetyan 21 Mar '07

21 Mar '07

i get the following error message new.o:fake:(.text+0x85): undefined reference to `parseczm2zi0_TextziParserCombin atorsziParsecziChar_oneOf_closure' new.o:fake:(.rodata+0x4): undefined reference to `parseczm2zi0_TextziParserCombi natorsziParsecziChar_oneOf_closure' collect2: ld returned 1 exit status when compiling the following code module Main where import System.Environment import Text.ParserCombinators.Parsec hiding (spaces) symbol = oneOf "!$%&|*+-/:<=>?@^_~" main = do return 1 any help is greatly appreciated. i am running ghc-6.6 on a windows xp machine.

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Two Other AI Sources
by Adam Wyner 21 Mar '07

21 Mar '07

For the Haskell and AI work, we ought to consider AI programming books in addition to Russell and Norvig: /Paradigms of AI Programming: Case Studies in Common Lisp/, P. Norvig, 1991. /Prolog Programming for Artificial Intelligence/, I. Bratko, 1990./ Artificial Intelligence Techniques in Prolog/, Y. Shoham, 1994. Perhaps there are newer additions (e.g. Bratko), but the problems and solutions from these languages are presented. We can build on them rather than starting from scratch or even just the theoretical outline of Russell and Norvig. Adam

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Search monad
by Edsko de Vries 20 Mar '07

20 Mar '07

Hey, I have a structure containing Xs in various places, like so data X data Structure = Structure .. [X] .. [X] .. And I defined mapStructure mapStructure :: (X -> X) -> (Structure -> Structure) I then wanted to use mapStructure to define queries as well as transformations on structures. I generalized mapStructure to mapStructureM: mapStructure :: Monad m => (X -> m X) -> (Structure -> m Structure) and then defined the following search monad: > data Search f a b = Found (f a) b > > class Monad (m a) => SearchMonad m a where > found :: a -> m a a > > fromSearch :: Search f a b -> f a > fromSearch (Found a _) = a > > search :: (SearchMonad m a) => (a -> Bool) -> a -> m a a > search f a > | f a = found a > | otherwise = return a Instances of the monad for finding one and for finding all elements: > instance SearchMonad (Search Maybe) a where > found a = Found (Just a) a > > instance SearchMonad (Search []) a where > found a = Found [a] a > > instance Monad (Search Maybe a) where > return b = Found Nothing b > Found (Just a) a' >>= f = case f a' of > Found _ b -> Found (Just a) b > Found Nothing a' >>= f = f a' > > instance Monad (Search [] a) where > return b = Found [] b > Found as a' >>= f = case f a' of > Found as' b -> Found (as ++ as') b Here is a simple sample session with ghci *Util> fromSearch $ mapM (search even) [1,3,5] :: Maybe Int Nothing *Util> fromSearch $ mapM (search even) [1,2,3,4,5] :: Maybe Int Just 2 *Util> fromSearch $ mapM (search even) [1,2,3,4,5] :: [Int] [2,4] What I'm wondering about is if this monad is an instance of a more general monad(if so, which one?), and generally if people have any comments about these definitions. Thanks! Edsko

4 3

wanted: haskell one-liners (in the perl sense of one-liners)
by Thomas Hartman 20 Mar '07

20 Mar '07

Okay, I am aware of http://haskell.org/haskellwiki/Simple_unix_tools which gives some implementation of simple unix utilities in haskell. But I couldn't figure out how to use them directly from the shell, and of course that's what most readers will probably wnat. Or let me put it another way. Is there a way to do find -maxdepth 1 -type f | xargs du | perl -ane 'print "\$F[0]\n"' | perl -e '$sum += $_ while <>; print "$sum\n"' as a shell command that idiomatically uses haskell? For non-perlers, that sums up the disk usage of all files in the current directory, skipping subdirs. print "\$F[0]\n looks at the first (space delimited) collumn of output. perl -e '$sum += $_ while <>; print "$sum\n"' , which is I guess the meat of the program, sums up all the numbers spewed out of the first column, so in the end you get a total. So, anyone out there want to establish a haskell one liner tradition? :) thomas.

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Re: [Haskell-cafe] Re: wanted: haskell one-liners (in the perl sense of one-liners)
by Thomas Hartman 20 Mar '07

20 Mar '07

You contributed #12:14 < Thunder> @pl \s -> drop (length s - n) s #12:14 < lambdabot> drop =<< subtract n . length But, on second thought, 1) I can't use this as a drop-in replacement for the non points free (right term?) version, and 2) I don't really understand it. Still, I would be curious to see if there is a way to make this useful somehow. 2007/3/20, Lutz Donnerhacke <lutz(a)iks-jena.de>: > In iks.lists.haskell, you wrote: > > This takes a long file containing mostly numerical data, filters out > > the numerical data, and sorts it numerically. (Not the same thing as > > sorting alphabetically, which is what you get by default, or using the > > unix sort utility). Maybe there's some flag to make unix sort util act > > like this? Enh, who cares, now we have haskell. :) > > sort -g > > > Thanks to Thunder > > I was not that helpful. >

2 1

Haskell-cafe] Haskell and a new web developement mailinglist?
by Dimitry Golubovsky 20 Mar '07

20 Mar '07

Marco Weber wrote: >I've been talking to Chris Eidhof the last days and we'd like to suggest adding >another specialized haskell mailinglist: Haskell and web developement. I support the idea to have such a mailing list. Thanks. -- Dimitry Golubovsky Anywhere on the Web

1 0

NYTimes.com: John W. Backus, 82, Fortran Developer, Dies
by paul.hudak＠yale.edu 20 Mar '07

20 Mar '07

This page was sent to you by: paul.hudak(a)yale.edu. John Backus, inventor of Fortran, Turing Award winner, and also an early pioneer in functional programming, died Saturday at his home in Oregon. Many of us have fond memories of him in the earlier days of our careers, and we all owe a lot to him for giving credibility to functional programming through his Turing Award lecture, "Can Programming Be Libarated From the von Neumann style?" Here is an article from the New York Times. BUSINESS | March 20, 2007 John W. Backus, 82, Fortran Developer, Dies By STEVE LOHR Mr. Backus assembled and led the I.B.M. team that created Fortran, the first widely used programming language. http://www.nytimes.com/2007/03/20/business/20backus.html?ex=1175054400&en=d… ---------------------------------------------------------- ABOUT THIS E-MAIL This e-mail was sent to you by a friend through NYTimes.com's E-mail This Article service. For general information about NYTimes.com, write to help(a)nytimes.com. NYTimes.com 500 Seventh Avenue New York, NY 10018 Copyright 2007 The New York Times Company

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Re: [Haskell-cafe] what I learnt from my first serious haskell programm
by Fawzi Mohamed 20 Mar '07

20 Mar '07

oops, I realized now that my answer did not go to the list. For the record here is it: Thanks to everybody for the answers and to Henning Thielemann for the useful links: > http://haskell.org/haskellwiki/Qualified_names > > http://haskell.org/haskellwiki/Category:Style > I agree with apfelmus that recursive module should be supported (as the standard requires) this would make it easier to use one module per main type (the distributed approach), this approach was also what I had in mind, but it comes very close to introducing by hand one namespace per type from which came my rant about missing namespaces... Fawzi

1 0

ANN: MIME Strike Force
by Jeremy Shaw 20 Mar '07

20 Mar '07

Hello, If you have tried to do any MIME processing using Haskell, you are likely to have found two things: 1) There are a lot of MIME libraries for Haskell 2) None of them do everything you want So, I propose that we form a MIME Strike Force and create the one, true MIME library. The plan is something like this: 1) Document all the things the MIME library needs to support. 2) Pick the technology, and design the infrastructure for supporting these features. For example, I don't think we will be able to use Parsec because: i) We want to support ByteString ii) We want to be able to lazily parse the message 3) Try to reuse as much existing code as possible to implement the design. I have started step 1 by creating this page: http://www.haskell.org/haskellwiki/Libraries_and_tools/MIMEStrikeForce Please add your comments, requirements, etc. If you are interesting in helping contrib ideas, code, or flames, please let me know. If there is enough interest, we should probably set up a mailing list for discussion. j. ps. This *might* make a decent SoC project, but only if it results in the one, true MIME library. We definitely do not need another incomplete MIME library floating around.

4 3

Matrices in Haskell
by Ivan Miljenovic 20 Mar '07

20 Mar '07

Some of you might recall me annoying people on #haskell over the New Year about my Latin Squares project. Well, I'm looking at re-doing it from scratch, but the first thing I need to do is find a new way of representing my square. I have been using a list of lists ([[a]]) to represent a matrix. The problem with this data structure is that I need to be able to manipulate matrices as both row-oriented and column-oriented data structures, as I need to examine the values in the columns as well as in the rows. As it stands, I was doing this by transposing the matrix, manipulating it, then transposing it back again. This is a pain, as it takes up about 15% to 20% of the run time. The other problem with using a list of lists is that the only reason I'm sure that the matrix is valid (i.e. all the rows are the same length, etc.) is because I created it that way, not because the data structure requires it. As such, I'd like to know if there's any way of storing a an n-by-n matrix such that the algorithm/function to get either the rows or the columns is less than O(n^2) like transposition is. I did try using an Array, but my (admittedly hurried and naive) usage of them took longer than a list of lists, was more difficult to manipulate, and wasn't required separate inverse functions to row and cols. Also, since I need to look all throughout the list of values, it's still O(n^2), but this time for both rows and columns. I know that when doing something similar to this in Java a few years ago (a primitive Sudoku solver, to be precise), I could represent the rows and columns as to separate 2D arrays, with rows(i,j) pointing to the same object as cols(j,i). Is something like this possible in Haskell? in particular, I will be storing lists of possible values in the cells of the matrix, so the capability to backtrack would be very nice, as I'd be trying each value at a time to see if it is valid. I'd also want to use such a matrix implementation next semester for a project, which I plan on being a quick comparison of various programming languages as to ease of use and efficiency for matrix-based computing problems. -- Ivan Lazar Miljenovic

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Re: There can be only one fix? Pondering Bekic's lemma
by oleg＠pobox.com 20 Mar '07

20 Mar '07

Nicolas Frisby wrote: > My question is: Given products and a fixed point combinator, can any > pure expression be transformed into a corresponding expression that > has just a single use of fix? Albert Y. C. Lai pointed out model-theoretical and CPU-practical answers. There is also a Haskell-automatic answer. That is, polyvariadic fixpoint, a combinator to obtain a mutually least fixpoint of several terms, is expressible in Haskell itself, via the regular fix: http://okmij.org/ftp/Computation/fixed-point-combinators.html#Poly-variadic I like its inferred type fix':: [[a->b]->a->b] -> [a->b] which is truly the type of the (applicative, or eta-expanded) regular fix with some round parentheses replaced with square ones.

1 0

Haskell and AI
by Adam Wyner 20 Mar '07

20 Mar '07

I agree with Andrew Wagner re: Haskell and AI. There are some relevant resources on Haskell, Maths, and Logic: *The Haskell Road To Logic, Maths And Programming (Paperback) * by Kees Doets <http://www.amazon.com/exec/obidos/search-handle-url/103-5196905-1677457?%5F…> (Author), Jan van Eijck <http://www.amazon.com/exec/obidos/search-handle-url/103-5196905-1677457?%5F…> (Author) http://www.amazon.com/Haskell-Road-Logic-Maths-Programming/dp/0954300696/re… van Eijck has another book on Computational Semantics. Elsewhere, there is work on learning algorithms in Haskell. Broadly, we might start by gathering known resources, projects, and people. We could consider one of the standard AI books as a guide for chapters, sections, and problems to fill in with Haskell approaches. I'd be very interested to contribute to this. Cheers, Adam Wyner Message: 7 Date: Mon, 19 Mar 2007 12:51:19 -0400 From: "Andrew Wagner" <wagner.andrew(a)gmail.com> Subject: Re: [Haskell-cafe] Haskell Chess To: haskell-cafe(a)haskell.org Message-ID: <b8a8636e0703190951y21c7cb01qcb4f5b58089b169d(a)mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Here's my take on this: I've thought for a while now that Haskell needs a general toolkit for AI. After all, functional programming has long been recognized for being good at AI, yet you rarely hear about it being done in Haskell. Anyway, my suggestion would be to concentrate on methods of AI. For example, if we implement alpha-beta polymorphically enough, it should be trivial to use it for any game that it makes sense to use it on. This kind of thing is what I was thinking of when I talked about some "fundamental design" ideas I had. Things like writing a Board type-class, so that you can implement any board representation you want to for chess. Or writing alpha-beta in terms of positions and moves, regardless of what kind of game you're talking about (I also think you simply must use unfoldTree, as this is a beautiful instance of it). Things like learning, and other general strategies, should be developed independently of any particular game, IMHO.

3 3

Re: [Haskell] Haskell Chess
by dons＠cse.unsw.edu.au 20 Mar '07

20 Mar '07

smazanek: > Hello again, > > I got a lot of interesting and useful comments on my posting > about Haskell Chess. Somebody suggested using the program > for benchmarks. Several people asked me to open the program > for contributions. And others were just interested in the exercises. > > It is probably the best to branch the development, so that we > have a teachers' and a hackers' version (ease of understanding > and learning paradigms vs. efficiency and implementation of > full ruleset). I will do the necessary steps next week (svn, etc). > Somebody already offered help for cabalizing hsChess. hsChess has been cabalised, and is available now via darcs; darcs get http://www.cse.unsw.edu.au/~dons/code/contrib/hsChess/ You can also browse the source directly: http://www.cse.unsw.edu.au/~dons/code/contrib/hsChess/ > Several people asked me for the exercises so I started the wiki > page > http://haskell.org/haskellwiki/Learning_Haskell_With_Chess > on this topic. I will add more content when I am back to office. > > Every contribution and discussion is welcome. I am happy to host the code at the above url, unless Steffen, you have a better place to host the repository? -- Don

7 11

Re: [Haskell-cafe] N and R are categories, no?
by Dominic Steinitz 20 Mar '07

20 Mar '07

> I haven't formally checked it, but I would bet that this endofunctor > over N, called Sign, is a monad: Just to be picky a functor isn't a monad. A monad is a triple consisting of a functor and 2 natural transformations which make certain diagrams commute. If you are looking for examples, I always think that a partially ordered set is a good because the objects don't have any elements. A functor is then an order preserving map between 2 ordered sets and monad is then a closure (http://en.wikipedia.org/wiki/Closure_operator) - I didn't know this latter fact until I just looked it up. Dominic.

5 5

N and R are categories, no?
by Steve Downey 20 Mar '07

20 Mar '07

EOk, i'm trying to write down, not another monad tutorial, because I don't know that much yet, but an explication of my current understanding of monads. But before I write down something that is just flat worng, I thought I'd get a cross check. (and I can't get to #haskell) Monads are Functors. Functors are projections from one category to another such that structure is preserved. One example I have in mind is the embedding of the natural numbers into the real numbers. The mapping is so good, that we don't flinch at saying 1 == 1.0. The functor that takes us from N to R is probably a Monad, that is, if N and R are categories. The real hard part is tying together how unit, join and bind produce a spacesuit that can protect apples from nuclear waste. I'm still getting that clear in my head, although my recent blinding flash of obviousness that M a is a type, and that of course types can do interesting things, I think gets me further along.

3 4

Algorithms
by P. R. Stanley 20 Mar '07

20 Mar '07

Hi "When you write a general solution for a class of problems, as opposed to a specific solution to a single problem, you have written an algorithm." Discuss! Regards, Paul

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Haskell and AI
by Andrew Wagner 20 Mar '07

20 Mar '07

> I agree with Andrew Wagner re: Haskell and AI. > > There are some relevant resources on Haskell, Maths, and Logic: > > The Haskell Road To Logic, Maths And Programming (Paperback) > by Kees Doets (Author), Jan van Eijck (Author) > > http://www.amazon.com/Haskell-Road-Logic-Maths-Programming/dp/0954300696/re… > I liked this book. What parts did you have in mind specifically as being relevant to AI? > van Eijck has another book on Computational Semantics. Elsewhere, there is > work on learning algorithms in Haskell. > > Broadly, we might start by gathering known resources, projects, and people. > We could consider one of the standard AI books as a guide for chapters, > sections, and problems to fill in with Haskell approaches. I like this idea. On the other thread [1] Ricardo Herrmann suggested following the structure of "AI: A Modern Approach", the classical text [2] by Russell and Norvig. Do others agree? I think we should carve out a section of the wiki, in the Applications and Libraries section [3] for AI, which should then be split into (at least) a section each for relevant papers, code, interesting AI problems with their solutions, and a table-of-contents type page, structured according to a text like this. Comments? > > I'd be very interested to contribute to this. > Maybe we should assemble an AI strike force? :) > Cheers, > Adam Wyner > [1] http://www.haskell.org/pipermail/haskell-cafe/2007-March/023646.html [2] http://aima.cs.berkeley.edu/ [3] http://www.haskell.org/haskellwiki/Libraries_and_tools

1 0

System.Random StdGen read fails on some strings?
by Fritz Ruehr 20 Mar '07

20 Mar '07

According to the documentation for System.Random (see <http:// haskell.org/ghc/docs/latest/html/libraries/base/System-Random.html>): > In addition, read may be used to map an arbitrary string (not > necessarily one produced by show) onto a value of type StdGen. In > general, the read instance of StdGen has the following properties: > > * It guarantees to succeed on any string. > * ... On the other hand, I get the following on the (admittedly stupid, spur-of-the-moment) String argument "whateva": > Hugs> :l System.Random > System.Random> read "whateva" :: StdGen > > Program error: Prelude.read: no parse > > System.Random> map read $ words "this is a test of the System dot > Random StdGen read, which seems to fail only on ... whateva" :: > [StdGen] > [4580 1,440 1,101 1,4584 1,445 1,1485 1,35261 1,1377 1,32825 > 1,34047 1,13422 1,14037 1,13637 1,469 1,4132 1,4514 1,453 1,626 1, > Program error: Prelude.read: no parse Am I missing something here? Or am I just being punished for the stupidity of my particular choice of String? :) -- Fritz PS: this happens with both Hugs Version: September 2006 and GHC Interactive, version 6.6, running on Mac/Tiger, for what that's worth.

4 4

Type synonym application
by C Rodrigues 20 Mar '07

20 Mar '07

Type synonyms aren't applied as I would expect during kind checking. What's going on here? type WithList a b = b [a] type FooPair a b = (b, a -> b) -- error: `WithList' is applied to too many type arguments ints1 :: WithList Int FooPair [Int] ints1 = ([1], id) -- error: `FooPair' is not applied to enough type arguments ints2 :: (WithList Int FooPair) [Int] ints2 = ([1], id) -- after manually applying the first type synonym ints3 :: FooPair [Int] [Int] ints3 = ([1], id) _________________________________________________________________ Live Search Maps find all the local information you need, right when you need it. http://maps.live.com/?icid=hmtag2&FORM=MGAC01

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Re: [Haskell] Haskell Chess
by Andrew Wagner 20 Mar '07

20 Mar '07

Andrzej, I'd love to hear some of your thoughts on these things. I agree with you about brute-force not being the best approach in haskell (or maybe at all). I think we should switch to haskell-cafe, since that's where much of this discussion has gone, and that's more for extended discussions anyway. On 3/18/07, Andrzej Jaworski <himself(a)poczta.nom.pl> wrote: > Didactic it may be but brute force on search trees means in Haskell problem > with garbage collection. So students may build a punch but loose a street > fight. > > Instead of using Haskell as a hammer I would rather explore what monadic > programming can offer in terms of encapsulating constraints, heuristics, > modal logic or agents. Chess is a human game so why not use computing along > this line and employ Haskell on humane terms? The force is with us! > > Cheers, > -Andrzej > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

2 1

Re: [Haskell] Summer of Code project proposal
by Neil Mitchell 20 Mar '07

20 Mar '07

Hi Nikhil, haskell-case is probably a better choice of list for this - haskell is more for announcements. > > I am Nikhil N, a third year Computer Science student from India. I am > looking at building a good > IDE for people learning Haskell(as part of SoC).This IDE will be modeled > on Dr Scheme or blueJ. Have you looked at something based on HIDE or GuiHaskell? The latter is certainly in the SoC database (I added it), the former was an active project a while ago. How much work do you think is involved in writing an IDE? I suspect its a massive amount - much more than one summers worth.... Thanks Neil

1 0

Re: Performance Help
by Dominic Steinitz 19 Mar '07

19 Mar '07

Thanks. > Fusing the ws is trickier. Directly appealing to the fibonacci-number > example is not recommended because this would mean to keep the last 16 > ws in memory and shifting them right to left by hand. But as the Are you saying this because we don't want a 16-tuple? > "Alternate method of computation" on the website suggests, you can > delegate the shifting to an index that shifts around mod 16. Having a > mutable array is helpful, then. Are you saying that because haskell doesn't really have mutable arrays, this is ruled out? > Of course, you can also fill a large static (boxed!) array of 80 Word8s via > > ws :: Data.Array.IArray.Array Int Word8 > ws = accumArray 0 (0,80) (curry snd) $ > zip [0..15] xs ++ [(i, xxor i) | i<-[16..80]] > where > xxor i = ws ! (i-16) `xor` > ws ! (i-3) `xor` ws ! (i-8) `xor` ws ! (i-14) > > or something like that (I didn't care about correct indices and bounds). > GHC can fuse such array accumulations. Why is an array better than a list? Is it because (!) is O(1) and (!!) is O(n)? > > In general, keeping stuff in lists is not wrong, but ByteStrings are > more adapted to current CPU and RAM architecture. I'm not clear how ByteStrings help here. We need to put bits into 32 words and operate on these. Dominic.

2 1

Re: Re: HS-Plugins 1.0 chokes on simple test, WinXP GHC-6.6 (Conal Elliott)
by Vivian McPhail 19 Mar '07

19 Mar '07

Hi, I just setup and installed hs-plugins from darcs on WinXP using ghc-6.6 and MSYS. The hs-plugin test suite all passes. Can you send me something that generates your error and I'll have a look at it. Vivian > Message: 2 > Date: Sat, 17 Mar 2007 18:06:12 -0700 > From: "Conal Elliott" <conal(a)conal.net> > Subject: Re: [Haskell-cafe] Re: HS-Plugins 1.0 chokes on simple test, > WinXP GHC-6.6 > To: haskell-cafe(a)haskell.org, "Donald Stewart" <dons(a)cse.unsw.edu.au> > Message-ID: > <ea8ae9fb0703171806h641da4a8q28fa19b86bd2ad85(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > Glad to know, Ian. Thanks. > > I'd sure love to have a working hs-plugins again, especially > in time to help with my ICFP submission. I wouldn't begin to > know how to crack something like this bug. Does anyone have > any ideas? > > Cheers, - Conal > > > On 3/17/07, Ian Lynagh <igloo(a)earth.li> wrote: > > > >c:/ghc/ghc-6.6/HSbase.o: unknown symbol `_free' > > > > > > > > >Don mentioned this is a known problem. Is it on > anyone's todo list? > > > > Not mine. > > > > > > > On 3/17/07, Ian Lynagh <igloo(a)earth.li> wrote: > > > > On Fri, Mar 16, 2007 at 09:52:10PM -0700, Conal Elliott wrote: > > > BTW, to get hs-plugins to build, I changed two lines in > > > hs-plugins/configure. > > > > As it happens, I sent Don a similar patch last night, so hopefully > > it'll be fixed in darcs soon. > > > > > First I tried tr -d '\n', but it didn't work, and I don't > know why. > > > > Because it's a \r you're trying to delete. > > > > > On 3/16/07, Conal Elliott <conal(a)conal.net> wrote: > > > > > > > >I got hs-plugins to compile fine on winxp, but now when > I run it I > > > >get > > a > > > >crash with this message: > > > > > > > >c:/ghc/ghc-6.6/HSbase.o: unknown symbol `_free' > > > > > > > >Don mentioned this is a known problem. Is it on > anyone's todo list? > > > > Not mine. > > > > > > Thanks > > Ian > > > > >

2 1

Visibility of definitions in Hugs Prelude
by Fritz Ruehr 19 Mar '07

19 Mar '07

I am wondering about the visibility of definitions in the Hugs Prelude. More specifically, when I ask for info about the Eq class, I see a lot of instances, including: > instance Eq Key On the other hand, Key is not in scope: > Hugs> :info Key > Unknown reference `Key' I imagine this has to do with importing/exporting, or maybe some modern Hugs empty-Prelude funkiness. Or perhaps it is because Key is a "non-standard export" (but that is just a comment in the Prelude). In any case, is it intended that ":info" will tell me about things that I (and it) can't otherwise see? Please forgive my naivete re module rules in advance ... . -- Fritz

1 0

Using external programs with Haskell in windows.
by Christian Lean 19 Mar '07

19 Mar '07

I'm looking for a way to run an external program and get the results in haskell. Something similar to HSH but that will work in windows. I don't need anything too complex, just to provide the command to be run as a string and get the result as a string. Is this possible? One of the HSH examples is all I need: runS $ "echo /etc/pass*" -> "/etc/passwd /etc/passwd-"

2 4

Stack Space Overflow.
by John T Davis 19 Mar '07

19 Mar '07

oHello, I'm writing a simulation of a game of Nim with genetic programming. I've run into a stack space overflow problem with one of my functions (part of a class instance), which I've included below. crossover_mutate enp = enp { progPop = ansPop, numProgs = (numProgs enp) + (crossNum enp)} where crossResult = crossNimPlayerPop (progPop enp) (seed enp) (theBoard enp) (numProgs enp) (crossNum enp) mutateResult = mutateNimPlayerPop crossResult (mutateRatio enp) popParts = crossResult ++ mutateResult newPop = Set.toAscList (Set.fromList popParts) ansPop = tournament (newPop, newPop) (theBoard enp) Now, here's where it gets weird. I've got test data that I've run all the constituent functions (crossNimPlayerPop, mutateNimPlayerPop, tournament) on, and they all work as they should. Now, when I run the same data on crossover_mutate, I get the aforementioned stack space overflow. I'm totally stumped on this... I've even run the equivalent of ansPop on the command line, and despite how truly unreadable the code is in that format, it runs fine. Any suggestions or thoughts would be greatly appreciated. Have a nice day. :-) Thank you, John T Davis

3 2

Does inlinePerformIO = unsafePerformIO . unsafeInterleaveIO semantically?
by Isaac Dupree 18 Mar '07

18 Mar '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Compare the guarantees (in practice and/or theoretically) of inlinePerformIO and unsafePerformIO . unsafeInterleaveIO (i.e. function composition) (aside from inlinePerformIO being better optimized). Are they the same, or if not, what do they mean - what are the differences? Not yet understanding any of these operations fully, Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFF/QsBHgcxvIWYTTURAtC6AJ4rKqhfmVi5cLC2MhXF9J4Pxk30KQCdGnQ7 Wd0BrIreZhIc0Gzm/tQLa78= =poy9 -----END PGP SIGNATURE-----

3 2

Haskell and SSL
by Thomas David Baker 18 Mar '07

18 Mar '07

I'm writing an application that scrapes some data from some websites. I was using Network.Browser from http://haskell.org/http/. So far, so good. However, now I need to scrape some websites that use SSL. Network.Browserdoesn't seem to support this (although for some reason it does work on one of the sites so maybe I'm getting this wrong). Someone on #haskell pointed me at hsgnutls which is an wrapper for parts of GNU TLS. But I actually had more success writing a wrapper around wget. I know that darcs uses curl in a similar way for some stuff but it still feels like I'm doing the Wrong Thing. Does anyone know of a Haskell library that does support scraping SSL enabled web pages? Or would adding it to Network.Browser be terribly difficult? Looking at GNU TLS it looks more complicated than I imagined. Thanks in advance for your help, Tom

2 1

HS-Plugins 1.0 chokes on simple test, WinXP GHC-6.6
by Alistair Bayley 18 Mar '07

18 Mar '07

Does anyone have hs-plugins working on WinXP with ghc-6.6? When I run the simple test below I get this error: Main: c:/ghc/ghc-6.6/HSbase.o: unknown symbol `_free' Main: user error (Dynamic loader returned: user error (resolvedObjs failed.)) Am I doing something obviously dumb? Alistair module Test1 where test1 = putStrLn "test1" module Main where import Prelude hiding (catch) import Control.Exception import Data.List import System.Environment import System.Plugins instance Show (LoadStatus a) where show (LoadFailure errors) = "LoadFailure - " ++ (concat (intersperse "\n" errors)) show (LoadSuccess m p) = "LoadSuccess" main = do a <- getArgs let modName = case a of (n:_) -> n _ -> "Test1" let modPath = "./" ++ modName ++ ".o" let method = "test1" fc <- catch (load modPath [""] [] method) (\e -> return (LoadFailure ["Dynamic loader returned: " ++ show e])) case fc of LoadFailure errors -> do fail (concat (intersperse "\n" errors)) LoadSuccess modul proc -> do let p :: IO (); p = proc proc

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How do I avoid stack overflows?
by oleg＠pobox.com 18 Mar '07

18 Mar '07

DavidA wrote: > I'm trying to write some code which involves lots of matrix multiplications, > but whenever I do too many, I get stack overflows (in both GHCi 6.4.2, and > Hugs May 2006). By placing a couple of strictness annotations, your test' gives the expected answer (given some time) on Hugs. GHCi unfortunately runs into some kind of bug (it says so itself), an unimplemented opcode. The test'' below gives that bug instantly... import List (transpose) -- not needed here -- foldl' f z [] = z -- foldl' f z (h:t) = (foldl' f $! f h z) t -- sum' l = foldl' (+) 0 l map' f [] = [] map' f (h:t) = scons (f h) (map f t) -- strict cons. Could be associated with an infix op, e.g., :$ scons :: a -> [a] -> [a] scons x l | x `seq` l `seq` False = undefined scons x l = x:l u <.> v = sum $ zipWith (*) u v a <<*>> b = multMx a (transpose b) where multMx [] _ = [] multMx (u:us) bT = scons (map' (u <.>) bT) (multMx us bT) id3 = [[1,0,0],[0,1,0],[0,0,1]] -- test = iterate (<<*>> id3) id3 !! 1000000 iterate' f x = x : seq x' (iterate' f x') where x' = f x test' = iterate' (<<*>> id3) id3 !! 1000000 test'' = head $ drop 1000000 $ iterate' (<<*>> id3) id3

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Creating static libraries using GHC
by Tom Hawkins 17 Mar '07

17 Mar '07

I have a chunk of Haskell code I would like wrap up and distribute as a library. Is there a way to build a static library (*.a) that includes my code plus the Haskell runtime, which C programs can easily link against? Here is what I have tried so far... ghc --make -fffi MyLib # Builds MyLib.o and MyLib_stub.o. gcc -c -I/usr/local/lib/ghc-6.4.1/include MyLibWrapper.c ar -r libMyLib.a MyLib.o MyLib_stub.o MyLibWrapper.o This works fine when I use ghc to compile and link a C program... ghc main.c libMyLib.a But if I use gcc, it throws a lot of unresolved references. I have no problem compiling with ghc, but the folks using my library probably won't have it installed. I added -v to ghc to see how it calls gcc; it seems to link in different libraries based on what Haskell libraries are being used, and it undefines a bunch is symbol references. Are there any switches to have ghc return a single archive with everything included? Please excuse my ignorance of the linking process. Thanks for any help! -Tom

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Newbie question: ghc search path for .c files
by Seth J. Fogarty 17 Mar '07

17 Mar '07

Hi, I am having difficulty compiling a program that uses a foreign library. The readme says to first compile a C file: :> ghc mipFFI.c -c mipFFI.c:1:0: lpkit.h: No such file or directory Alright, that's not uncommon. gcc confirms :> gcc mipFFI.c mipFFI.c:1: lpkit.h: No such file or directory mipFFI.c:2: HsFFI.h: No such file or directory So I then include the directory and gcc finds the lpkit.h file. :> gcc -I /home/sfogarty/lib/lp_solve_5.5 mipFFI.c mipFFI.c:2: HsFFI.h: No such file or directory However ghc does not seem to accept this :> ghc -i/home/sfogarty/lib/lp_solve_5.5 mipFFI.c mipFFI.c:1:0: lpkit.h: No such file or directory I've tried every variation of spacing, capitalization, copying the files to the local directory, simply listing the files ahead. Nothing seems to work! I am horribly confused! And another question while I am here: -package posix and -package lang don't seem to be included on my distribution of ghc. I am having minimal success, as yet, finding where to download them. If I could get a pointer, I'd be very appreciative. If not, I'm sure I'll find it eventually. -- Seth Fogarty sfogarty@[gmail.com|rice.edu|livejournal] Neep-neep at large AIM: Sorrath "Let us not seek to satisfy our thirst for freedom by drinking from the cup of bitterness and hatred." - Martin Luther King, Jr.

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Lack of expressiveness in kinds?
by Andrew Wagner 17 Mar '07

17 Mar '07

Ok, so I'm inching closer to understanding monads, and this question popped up today. Consider the following 2 declarations: data Foo a = Bar a data (Ord a) => Baz a = Bah a Note that both of these have kind * -> *. However, Baz could never be an instance of monad, because there is a restriction on the types it can operate on. Foo, however, is completely polymorphic, without limitation. It seems to me that there ought to be a way to express the difference between the two in the type/kind system. For example, you can almost, but not quite, say that in the declaration "class Monad m where..", m must be of kind *->*, but that's not quite enough to say, because of this example. Am I just missing something, or is there a reason the kind of Baz shouldn't be something other than *->*?

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ANNOUNCE: Google Summer of Code - open for business
by Malcolm Wallace 16 Mar '07

16 Mar '07

We are pleased to announce that haskell.org has been officially accepted as a mentoring organisation for this year's Summer of Code. Student applications are now open! http://code.google.com/summerofcode.html http://hackage.haskell.org/trac/summer-of-code/ Students: want to earn $4500 for 3 months work hacking in Haskell? Go to our ideas page, browse around, and write up a proposal for something you would like to work on. You must be available to work basically full-time for the months of June to August (inclusive). Proposals are competitive against other students. Last year, nine were funded. Your deadline for application is 24th March. Anyone else: Got any good ideas for coding projects, but no time to work on them yourself? Go to our wiki and add it in to the database! Or do you feel you could be either a primary or backup mentor for a student project in an area of your interest, even if it isn't a project you defined yourself? We need willing members of the community to foster the new student talent, so sign up on the wiki's 'people' page if you would like to be involved. Obviously you will need Haskell coding experience, but also be able to advise on time management, strategies for gaining a userbase and keeping them happy, ways of interacting with the wider Haskell community, and so on. Regards, Malcolm

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Typical?
by Hans van Thiel 16 Mar '07

16 Mar '07

Hello All, Us who're learning Haskell have a lot in common... that is, if Amazon has it right... ------------ We recommend: STAR TREK-USS ENTERPRISE D by Corgi Classics Recommended because you purchased or rated: * The Haskell School of Expression: Learning Functional Programming through Multimedia . . . We recommend: Cooking Entrees With the Micheff Sisters: A Vegan Vegetarian Cookbook by Brenda Micheff Walsh Recommended because you purchased or rated: * Purely Functional Data Structures ------------ To be fair, other recommendations were more traditional ...:) Hans van Thiel

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Generalised merge
by Paul Johnson 16 Mar '07

16 Mar '07

The most common kind of primitive recursive function I find myself writing these days is a variation on the theme of merging two sorted lists. You can see some examples in my Ranged Sets library at http://ranged-sets.sourceforge.net/. For instance: -- | Set union for ranged sets. Infix precedence is left 6. rSetUnion, (-\/-) :: DiscreteOrdered v => RSet v -> RSet v -> RSet v -- Implementation note: rSetUnion merges the two lists into a single -- sorted list and then calls normalise to combine overlapping ranges. rSetUnion (RSet ls1) (RSet ls2) = RSet $ normalise $ merge ls1 ls2 where merge ls1 [] = ls1 merge [] ls2 = ls2 merge ls1@(h1:t1) ls2@(h2:t2) = if h1 < h2 then h1 : merge t1 ls2 else h2 : merge ls1 t2 -- | Set intersection for ranged sets. Infix precedence is left 7. rSetIntersection, (-/\-) :: DiscreteOrdered v => RSet v -> RSet v -> RSet v rSetIntersection (RSet ls1) (RSet ls2) = RSet $ filter (not . rangeIsEmpty) $ merge ls1 ls2 where merge ls1@(h1:t1) ls2@(h2:t2) = rangeIntersection h1 h2 : if rangeUpper h1 < rangeUpper h2 then merge t1 ls2 else merge ls1 t2 merge _ _ = [] Union also has its own merge function. The worst case I've come across was at work. I can't talk about the details, but it involved manipulating two functions of time represented by lists of samples. So I had a type TimeFunc = [(Value, Time)], and the job was to compare two TimeFuncs with samples at different times. Step 1 was to interpolate each TimeFunc with the values for the times in the other TimeFunc, giving a result of type [(Value, Value, Time)] for the union of all the times in both original TimeFuncs. I wrote a truly hairy zipTimeFunc function with guards to match each possible case. It worked, but it must have been 100 lines if you include the comments to explain each case and demonstrate totality. So I'm wondering if anyone has a more general pattern. Unfold? Some variation on a theme of zip? I once tried writing a generalised merge, but it needed half a dozen functions as arguments to handle all the various cases. It was kludgier than just rolling a new merge routine every time. And I don't think it would have handled the TimeFunc case. Paul.

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