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Re: [Fwd: Re: type class VS struct/functor]
by Ashley Yakeley 20 Jan '02

20 Jan '02

At 2002-01-18 18:59, Ahn Ki-yung wrote: >Each of these Set types takes a string, but the membership test is >different. This is an annoying case in Haskell, because you can make a >String a member of the Eq typeclass in only one way. Your example seemed to translate into Haskell quite straightforwardly... module FunctorTest where { import Char; lowercase = fmap toLower; data EQ' t = EQ' { eq :: t -> t -> Bool }; data SET elt set = SET { empty :: set, add :: elt -> set -> set, mem :: elt -> set -> Bool }; fSet :: EQ' t -> SET t [t]; fSet mEq = SET { empty = [], add = \elt set -> elt:set, mem = let { mm elt [] = False; mm elt (x:xs) = if eq mEq x elt then True else mm elt xs; } in mm }; sensitiveCase :: EQ' String; sensitiveCase = EQ' { eq = \s s' -> s == s' }; insensitiveCase :: EQ' String; insensitiveCase = EQ' { eq = \s s' -> (lowercase s) == (lowercase s') }; sensitiveSet = fSet(sensitiveCase); insensitiveSet = fSet(insensitiveCase); } -- Ashley Yakeley, Seattle WA

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schrist@gmx.net
by c-schnell＠t-online.de 19 Jan '02

19 Jan '02

Please add me to the mailinglist.

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[Fwd: Re: type class VS struct/functor]
by Ahn Ki-yung 19 Jan '02

19 Jan '02

Hmm ... How do you solve this in Haskell ? -------- Original Message -------- Subject: Re: type class VS struct/functor Date: Sat, 19 Jan 2002 01:34:32 GMT From: neelk(a)alum.mit.edu (Neelakantan Krishnaswami) Reply-To: neelk(a)alum.mit.edu Organization: ATT Broadband Newsgroups: comp.lang.functional References: <3C48C86E.9282BE00(a)bawi.org> On Sat, 19 Jan 2002 10:14:22 +0900, =?EUC-KR?B?vsix4r+1?= <kyagrd(a)bawi.org> wrote: > > What does ML struct/functor have anything better than type classes ? > For the user type classes feels like implicit functor istantiations > to specific sturucture, and struct/functor seems just bugglling the > user to do more typing which can be automated by using type classes. The advantage of functors shows up when you need to have multiple implementations of a module for a type. For instance, suppose you want to implement a set, and you write the functors (OCaml below): module type EQ = sig type t val eq : t -> t -> bool end module type SET = sig type elt type set val empty : set val add : elt -> set -> set val mem : elt -> set -> bool end module Set(Eq : EQ) : SET with type elt = Eq.t = struct type elt = Eq.t type set = elt list let empty = [] let add elt set = elt :: set let rec mem elt set = match set with | [] -> false | x :: xs -> if Eq.eq x elt then true else mem elt xs end Now, suppose you want two kinds of sets of string, one of which is case-sensitive and one of which is not. You can easily do this with functors like so module SensitiveCase = struct type t = string let eq s s' = (s = s') end module InsensitiveCase = struct type t = string let eq s s' = (String.lowercase s) = (String.lowercase s') end module SensitiveSet = Set(SensitiveCase) module InsensitiveSet = Set(InsensitiveCase) Each of these Set types takes a string, but the membership test is different. This is an annoying case in Haskell, because you can make a String a member of the Eq typeclass in only one way. However, I think it's true that typeclasses are less verbose in the common case. Personally, I want a language with both typeclasses and functors, so that I can use either as the problem requires. Neel

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Re: functions not in type classes
by Ashley Yakeley 19 Jan '02

19 Jan '02

At 2002-01-18 03:06, Cagdas Ozgenc wrote: >Same story, you shouldn't have had a Container of Char only in the first >place. Well, what about a file-handle? Not necessarily exactly a Container (though it might be), but it's always "of Char", or better yet, "of Word8". You could make a generalised file-handle to be of anything, but your read and write functions would necessarily have to be restricted. For instance, given this: class Stream1 c where read :: c a -> Integer -> IO [a] write :: c a -> [a] -> IO () data FileHandle1 a = ... fileRead :: FileHandle1 Word8 -> Integer -> IO [Word8] fileWrite :: FileHandle1 Word8 -> [Word8] -> IO () ...you cannot make FileHandle1 an instance of Stream1. However, you could do this: class Stream2 s a | s -> a where read :: s -> Integer -> IO [a] write :: s -> [a] -> IO () data FileHandle2 = ... fileRead :: FileHandle2 -> Integer -> IO [Word8] fileWrite :: FileHandle2 -> [Word8] -> IO () instance Stream2 FileHandle2 Word8 where read = fileRead write = fileWrite -- Ashley Yakeley, Seattle WA

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Re: functions not in type classes
by Ashley Yakeley 19 Jan '02

19 Jan '02

At 2002-01-18 03:53, Cagdas Ozgenc wrote: >You shouldn't be using multi paramater type classes in place of type >constructors. Yes you should, it's more general. > Then you will have the ambiguity problems that you mention on >you web site. That's what fundeps are for... > Multi parameter type classes have the benefit of using multi >dispatch based on two non related types. Or two related types, if you prefer. -- Ashley Yakeley, Seattle WA

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Re: functions not in type classes
by Ashley Yakeley 19 Jan '02

19 Jan '02

At 2002-01-18 03:06, Cagdas Ozgenc wrote: >Well can you give examples of some useful functions? I can live without id, >and (.) Well what about [] and 'c'? [] :: [a] 'c' :: Char Not functions, of course. But Haskell definitions are just values: it would be odd if a restriction applied to some values (functions) and not others. -- Ashley Yakeley, Seattle WA

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Re: functions not in type classes
by Ashley Yakeley 18 Jan '02

18 Jan '02

At 2002-01-18 00:06, Cagdas Ozgenc wrote: >Why does Haskell let you write functions that are not a part of type class? Well, what classes should such functions as const, id and (.) be members of? const :: a -> b -> a; const x y = x; id :: a -> a; id x = x; (.) :: (b -> c) -> (a -> b) -> (a -> c); (.) f g x = f (g x); >For example instead of > >elem :: Eq a => a -> [a] -> Bool >map :: (a -> b) -> [a] -> [b] > >class Container a where > elem :: Eq b => b -> a b -> Bool > map :: (b -> c) -> a b -> a c > >would define methods that can work on all containers, Actually this would only work on container type-constructors. If you had a type that was some kind of container only of Char, you could not make it an instance of your Container. >and create a discipline to write more reusable and generic functions. There's already a generalised version of map, called fmap, that works on any Functor type. Picking the correct generalisation is often not obvious... -- Ashley Yakeley, Seattle WA

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memoization hints?
by Feuer 18 Jan '02

18 Jan '02

It seems that under some circumstances it might be good for a programmer to be able to tell the implementation not to remember something. In particular, intermediate data structures may be remembered for much longer than they should be. If I am not mistaken, full laziness (which I think ghc uses with -O ?) can make this worse. Would it be beneficial to have some kind of annotations to help deal with this? I don't really know how it would be implemented. Take a standard sort of example: fib n = let f = 0:1:zipWith (+) f (tail f) in f !! n -- code that first calculates fib 1000, then fib 3, fib 4, then does some other stuff I believe that the entire list will be maintained (when compiled with ghc -O) after calculating fib 1000, despite the fact that it will never be used again. If I could say instead fib n = let forget f = 0:1:zipWith (+) f (tail f) in f !! n and somehow get it to forget about f as often as possible (somehow suppressing laziness in this case). I would certainly want to forget about it whenever it goes out of scope, but in this case I would really want to somehow ensure that the list is never held entirely in memory (forgetting it as it is built in f !! n).

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mzero
by Feuer 18 Jan '02

18 Jan '02

Why is mzero in MonadPlus rather than a separate MonadZero class?

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functions not in type classes
by Cagdas Ozgenc 18 Jan '02

18 Jan '02

Greetings. Why does Haskell let you write functions that are not a part of type class? For example instead of elem :: Eq a => a -> [a] -> Bool map :: (a -> b) -> [a] -> [b] class Container a where elem :: Eq b => b -> a b -> Bool map :: (b -> c) -> a b -> a c would define methods that can work on all containers, and create a discipline to write more reusable and generic functions. Thanks

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Regarding finding the number of unique elements in a list.
by Amrit K Patil ;012;VKSF6; 18 Jan '02

18 Jan '02

Hi, I am newbie at Haskell. I am not able to write a program to find the number of unique elments in a list in haskell. I am also not able to write a porgram to find the elements in the innermost list in a list within lists. Can anybody guide me as to g\how to go about it or better still send me the program. It would be of great help. Have a good day.. cheers, amrit. peace....

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Re: Regarding finding the number of unique elements in a list.
by Ashley Yakeley 18 Jan '02

18 Jan '02

At 2002-01-17 15:14, Amrit K Patil ;012;VKSF6; wrote: >I am not able to write a program to find the number of unique elments in a >list in haskell. Have a look in the List library for a function that finds the unique elements of a list. >I am also not able to write a porgram to find the elements in the >innermost list in a list within lists. Well, this you'll have to do yourself. -- Ashley Yakeley, Seattle WA

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Precision of `Double's in Hugs
by Liyang Hu 17 Jan '02

17 Jan '02

Hi, (I'm just a new convertee to the ways of Functional Programming, so please go easy on me! ^_^;;) Why is it that `Double's in Hugs only seem to have the same precision as a `Float'? I've some code here that only iterates a few hundred times, and the amount of accuracy lost is getting a bit ridiculous ... Exactly[0] the same code produces the much more accurate results in GHCi. I'd switch over, but at the very least I'd like to know why this is happening. BTW: From the description at http://haskell.cs.yale.edu/communities/ , I seem to get the impression that haskell(a)haskell.org should only be used for announcements, yet the archive shows quite a bit of general discussion going on. Should I have posted this to the aforementioned list as well / instead? /Liyang [0] Minus the non-Haskell 98 compliant bits, like the missing fromInt ... -_- Took me a while to figure that one out. -- .--| Liyang Hu |--| http://nerv.cx/ |--| Caius@Cam |--| ICQ: 39391385 |--. | A [programming] language that doesn't affect the way you think | | about programming is not worth knowing. |

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newtype/datatype (was efficiency)
by Iavor Diatchki 17 Jan '02

17 Jan '02

hello, it looks like a lot of Haskell experts are at POPL (prog. language conference) and are not checking their email... so here is my attempt at explaining the differences between data and newtype. data T -- has 1 vale: _|_ data T = T -- has 2 values: _|_, T data T = T () -- has 3 values: _|_, T _|_, T () newtype T = T () -- has 2 values: _|_, T () basically the "data" declaration in Haskell "lifts" a type, by adding a new _|_ element. the reason for this is that in general a data may have different shapes (hence the constructors) and while trying to make a value of the type one might non-terminate before they computed what constructor should be applied. the annoying thing is that this happens even for datatypes which dont have different shapes, i.e. data declarations with only one different shape. Even if there is only one possible shape we always remember the shape (constructor) in the representation and when pattern matching check to see if the value is of this shape. i dont know why. to (kind of) avoid this, if you have a datatype with only one shape and one field you should use a newtype. unfortunatelly if you have multiple fields you are stuck with the additional _|_. i dont think there should be any difference in efficiency between: data P a b = P a b and newtype P a b = P (a,b) unless there are some hard-coded tuple specific optimizations done by the current implementations. i cannot see any reasons why these optimizations shouldnt happen for the data declarartion as well (i am not experienced compiler writer however...) which of the above is better is a matter of personal preference - i prefer the 1st one as i think it leads to more readable patterns. finaly there was the question about strictness annotations - they seem to be quite orthogonal to the whole newtype/data issue and enable you to specify that some constructors of your datatype are strict - i.e. when you construct something with a strict constructor it will first be evalued (until it becomes a function or an application of a lazy constructor to some arguments) and only then will the disired value be created. since this evaluation might not terminate of course, some "different" values in the lazy version of the datatype all collapse to teh same thing. so in some sense newtype makes things more lazy, while ! makes them more eager. sorry for the long email hope it helped iavor -- ---------------------------------------+---------------------------------------- Iavor S. Diatchki | email: diatchki(a)cse.ogi.edu Dept. of Computer Science | web: http://www.cse.ogi.edu/~diatchki OGI School of Science and Engineering | work phone: 5037481631 OHSU | home phone: 5036434085 ---------------------------------------+----------------------------------------

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monad binding and garbage collection
by Christian Schuhegger 17 Jan '02

17 Jan '02

hi all, i'm new to haskell and i have a question on a general problem concerning monad binding and garbage collection. my question will be on how to approach this type of problem? consider the following simple program: >------ buildIOList :: [a] -> IO [a] buildIOList li = return li main = do {li <- buildIOList [1..]; putStr (show (take 100000000 li)); } <------ if i compile this program with ghc and run it it has constant memory usage because of the lazy nature of haskell. on the other hand if i use runhugs then i get: ERROR - Garbage collection fails to reclaim sufficient space i think this is because the compiler can examine the code and sees that the binding li is not used after putStr any more and therefore the garbage collector can safely collect all the list elements that were already put onto the screen, but hugs as a interpreter does not have this knowledge and therefore has to keep all the memory that the binding li refers to. if i modify this program a bit: >------ main = do {li <- buildIOList [1..]; putStr (show (take 100000000 li)); putStr (show (take 100000000 li)); } <------ i get the same memory problem with a ghc compiled file, because now all the cells in the list bound by li that are evaluated stay in the system, because the gc cannot collect them as the third line in the do expression uses li again. with a little modification: >------ main = do {li <- buildIOList [1..]; putStr (show (take 100000000 li)); li <- buildIOList [1..]; putStr (show (take 100000000 li)); } <------ the program has again constant memory usage, because the name li is bound again later in the program so the gc can collect the elements of the first li. again a modification would be: >------ buildIOList :: [a] -> [IO a] buildIOList [] = [] buildIOList (hd : tl) = (return hd) : buildIOList tl main = do {printString (buildIOList [1..]); printString (buildIOList [1..]); } printString :: Show a => [IO a] -> IO () printString [] = return () printString (hd : tl) = do {c <- hd; putStr (show c); printString tl} <------ this program has also constant memory usage, but here i have to modify a lot to propagate the "little bits of monads" in the program. my question is now how an experienced haskell programmer would attack the problem? i would also like to have a solution that works for hugs so that my code is independent of the environment it is run in. many thanks for any suggestions! -- Christian Schuhegger The most likely way for the world to be destroyed, most experts agree, is by accident. That's where we come in; we're computer professionals. We cause accidents. -- Nathaniel Borenstein

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Re: efficiency
by Marcin 'Qrczak' Kowalczyk 16 Jan '02

16 Jan '02

Tue, 15 Jan 2002 13:06:58 +0100, Rijk-Jan van Haaften <rjchaaft(a)cs.uu.nl> pisze: > This feature is one I consider among the worst in Haskell. If > a datatype with just a single constructor can be optimized away, It can't. newtype has a different semantics than data, as others explained. It would be weird if data had non-uniform semantics for the one-constructor-with-one-field case, even though usually newtype is the variant we prefer. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ QRCZAK

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efficiency
by Feuer 16 Jan '02

16 Jan '02

In http://www.cs.columbia.edu/~cdo/icfp99.ps Okasaki does a couple things I'm not sure I quite understand: He has a datatype newtype Empty a = E () I cannot for the life of me figure out why he didn't just do data Empty a = E The other weird thing was: newtype Pair v w a = P (v a, w a) I can't see why he didn't do data Pair v w a = P (v a) (w a) I was wondering if anyone on this list knew of any reasons the types he chose would be more efficient than mine.... David

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Re: efficiency
by Mike Gunter 15 Jan '02

15 Jan '02

> By the way, I don't understand why Haskell98 provides both strictness flags > and newtype declarations.... it seems to me that > newtype M [a1 a2 ....] = MC (...) > should be exactly the same as > data N [b1 b2 ....] = NC !(...) The Haskell report (in the "Datatype Renamings" section, 4.2.3, -- the one on newtype) explains the difference. Translating some of the examples there to your names, with > newtype M = M Int > m (M i) = 42 > data N = N !Int > n (N i) = 42 we get Main> m undefined 42 but Main> n undefined *** Exception: Prelude.undefined mike

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Re: efficiency
by Ashley Yakeley 15 Jan '02

15 Jan '02

At 2002-01-15 00:00, Ketil's local user wrote: >When recently reading the GHC docs, the chapter about profiling, I >think, I came across something about this. From Chapter 6: > >| Newtypes are better than datatypes: >| >| If your datatype has a single constructor with a single field, >| use a newtype declaration instead of a data declaration. The newtype >| will be optimised away in most cases Does that help? data (,) a b = (,) a b; newtype Pair v w a = P (v a, w a); vs. data Pair v w a = P (v a) (w a); and data () = (); newtype Empty a = E (); vs. data Empty a = E; -- Ashley Yakeley, Seattle WA

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A question from a newbie
by Chen-Chieh Feng 14 Jan '02

14 Jan '02

hi, I just started to write a few lines using hugs98. I ran into a program error that reads like this: Program error: {instDirectPositions_v1683_v1697 dir_pt} Could anybody give me a direction on how to read this error message and the possible cause of this error? Thank you very much. Chen-Chieh

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RE: Help using a state monad
by Chris Angus 14 Jan '02

14 Jan '02

your pattern match of (s:sx) seems to be the cause of the delay try --Process the input mProcess :: String -> State String mProcess sx = do b <- isOn if ( head sx == 't' ) then toggle else return () appOut (tuUpper b (head sx)) (mProcess (tail sx)) instead -----Original Message----- From: Tom Bevan [mailto:tom@regex.com.au] Sent: 14 January 2002 05:58 To: Haskell Cafe List Subject: Help using a state monad Hi all, I'm trying to get a feel for how monads might be used in a Haskell program and develop an approach that would be useful for larger projects. I have tried to model in miniture a programme which takes input from 'standard in' and processes it against an internal state and then returns the processed result to 'standard out'. My problem is that, becuase of the way I have constructed the programme 'standard out' lags behind by one character. I'm hoping that someone might have a better design they could share with me. Tom.

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Re: differentiation. Reply
by Jerzy Karczmarczuk 14 Jan '02

14 Jan '02

Ketil Z Malde adds a comment concerning the query of Zhe Fu: > > >>> Is there any built-in functions in Haskell to implement > >>> diffential operation and partial diffential operation? > >>> Or can anyone give me some advices about how to implement them > >>> with Haskell? Thanks. > Jerzy Karczmarczuk has some interesting papers on his web site using a > different approach to differentiation, maintaining, IIRC, a lazy list > of all derivatives of functions. I don't have the URL ready, but a > your favorite search engine should be able to help you out. === 1. First, I suggest to move this to Haskell-café. I leave the original address, though. 2. My work is about the "Automatic Differentiation" in Haskell. The paper http://users.info.unicaen.fr/~karczma/arpap/diffalg.pdf has been published in HOSC last year. But somebody might get interested in some geometric extensions thereof (differential forms) http://users.info.unicaen.fr/~karczma/arpap/ltforms.pdf Jerzy Karczmarczuk

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Help using a state monad
by Tom Bevan 14 Jan '02

14 Jan '02

Hi all, I'm trying to get a feel for how monads might be used in a Haskell program and develop an approach that would be useful for larger projects. I have tried to model in miniture a programme which takes input from 'standard in' and processes it against an internal state and then returns the processed result to 'standard out'. My problem is that, becuase of the way I have constructed the programme 'standard out' lags behind by one character. I'm hoping that someone might have a better design they could share with me. Tom.

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Haskell syntax files in Vim 6.0 has a bug !
by 안기영 14 Jan '02

14 Jan '02

The vim syntax file for literate haskell - lhaskell.vim - in my VIM. The version of my VIM is VIM - Vi IMproved 6.0 (2001 Sep 26, compiled Sep 26 2001 17:12:46) MS-Windows 32 bit GUI version with OLE support, Self extracting binary. you should change the line source syntax/haskell.vim to runtime! syntax/haskell.vim Also I edited the haskell syntax file - haskell.vim - to work with the \begin{code} \end{code} literate style too. It's a little tricky but this shall work good for most of the cases. I attached two files haskell.vim, lhaskell.vim. " Vim syntax file " Language: Haskell with literate comments " Maintainer: John Williams <jrw(a)pobox.com> " Last Change: 2001 May 09 " Enable literate comments let b:hs_literate_comments=1 " Include standard Haskell highlighting if version < 600 source <sfile>:p:h/haskell.vim else runtime! syntax/haskell.vim endif " vim: ts=8 " Vim syntax file " Language: Haskell " Maintainer: John Williams <jrw(a)pobox.com> " Last Change: 2001 Sep 02 " Thanks to Ryan Crumley for suggestions and John Meacham for " pointing out bugs. " " Options-assign a value to these variables to turn the option on: " " hs_highlight_delimiters - Highlight delimiter characters--users " with a light-colored background will " probably want to turn this on. " hs_highlight_boolean - Treat True and False as keywords. " hs_highlight_types - Treat names of primitive types as keywords. " hs_highlight_more_types - Treat names of other common types as keywords. " hs_highlight_debug - Highlight names of debugging functions. " Remove any old syntax stuff hanging around if version < 600 syn clear elseif exists("b:current_syntax") finish endif " (Qualified) identifiers (no default highlighting) syn match ConId "$\<[A-Z][a-zA-Z0-9_']*\.$\=\<[A-Z][a-zA-Z0-9_']*\>" syn match VarId "$\<[A-Z][a-zA-Z0-9_']*\.$\=\<[a-z][a-zA-Z0-9_']*\>" " Infix operators--most punctuation characters and any (qualified) identifier " enclosed in `backquotes`. An operator starting with : is a constructor, " others are variables (e.g. functions). syn match hsVarSym "$\<[A-Z][a-zA-Z0-9_']*\.$\=[-!#$%&\*\+/<=>\?(a)\\^|~.][-!#$%&\*\+/<=>\?@\\^|~:.]*" syn match hsConSym "$\<[A-Z][a-zA-Z0-9_']*\.$\=:[-!#$%&\*\+./<=>\?@\\^|~:]*" syn match hsVarSym "`$\<[A-Z][a-zA-Z0-9_']*\.$\=[a-z][a-zA-Z0-9_']*`" syn match hsConSym "`$\<[A-Z][a-zA-Z0-9_']*\.$\=[A-Z][a-zA-Z0-9_']*`" " Reserved symbols--cannot be overloaded. syn match hsDelimiter "(\|)\|\[\|\]\|,\|;\|_\|{\|}" " Strings and constants syn match hsSpecialChar contained "\\$[0-9]\+\|o[0-7]\+\|x[0-9a-fA-F]\+\|[\"\\'&\\abfnrtv]\|^[A-Z^_\[\\\]]$" syn match hsSpecialChar contained "\\$NUL\|SOH\|STX\|ETX\|EOT\|ENQ\|ACK\|BEL\|BS\|HT\|LF\|VT\|FF\|CR\|SO\|SI\|DLE\|DC1\|DC2\|DC3\|DC4\|NAK\|SYN\|ETB\|CAN\|EM\|SUB\|ESC\|FS\|GS\|RS\|US\|SP\|DEL$" syn match hsSpecialCharError contained "\\&\|'''\+" syn region hsString start=+"+ skip=+\\\\\|\\"+ end=+"+ contains=hsSpecialChar syn match hsCharacter "[^a-zA-Z0-9_']'$[^\\]\|\\[^']\+\|\\'$'"lc=1 contains=hsSpecialChar,hsSpecialCharError syn match hsCharacter "^'$[^\\]\|\\[^']\+\|\\'$'" contains=hsSpecialChar,hsSpecialCharError syn match hsNumber "\<[0-9]\+\>\|\<0[xX][0-9a-fA-F]\+\>\|\<0[oO][0-7]\+\>" syn match hsFloat "\<[0-9]\+\.[0-9]\+$[eE][-+]\=[0-9]\+$\=\>" " Keyword definitions. These must be patters instead of keywords " because otherwise they would match as keywords at the start of a " "literate" comment (see lhs.vim). syn match hsModule "\<module\>" syn match hsImport "\<import\>.*"he=s+6 contains=hsImportMod,hsVarSym,hsConSym syn match hsImportMod contained "\<$as\|qualified\|hiding$\>" syn match hsInfix "\<$infix\|infixl\|infixr$\>" syn match hsStructure "\<$class\|data\|deriving\|instance\|default\|where$\>" syn match hsTypedef "\<$type\|newtype$\>" syn match hsStatement "\<$do\|case\|of\|let\|in$\>" syn match hsConditional "\<$if\|then\|else$\>" " Not real keywords, but close. if exists("hs_highlight_boolean") " Boolean constants from the standard prelude. syn match hsBoolean "\<$True\|False$\>" endif if exists("hs_highlight_types") " Primitive types from the standard prelude and libraries. syn match hsType "\<$Int\|Integer\|Char\|Bool\|Float\|Double\|IO\|Void\|Addr\|Array\|String$\>" endif if exists("hs_highlight_more_types") " Types from the standard prelude libraries. syn match hsType "\<$Maybe\|Either\|Ratio\|Complex\|Ordering\|IOError\|IOResult\|ExitCode$\>" syn match hsMaybe "\<Nothing\>" syn match hsExitCode "\<$ExitSuccess$\>" syn match hsOrdering "\<$GT\|LT\|EQ$\>" endif if exists("hs_highlight_debug") " Debugging functions from the standard prelude. syn match hsDebug "\<$undefined\|error\|trace$\>" endif " Comments syn match hsLineComment "--.*" syn region hsBlockComment start="{-" end="-}" contains=hsBlockComment syn region hsPragma start="{-#" end="#-}" " Literate comments--any line not starting with '>' is a comment. if exists("b:hs_literate_comments") syn region hsLiterateComment start="^" end="^>" contains=hsLiterateCodeBlock syn region hsLiterateCodeBlock start="begin{code}" end="end{code}" contains=hsVarSym,hsConSym,hsString,hsCharacter,hsNumber,hsFloat,hsModule,hsImport,hsImportMod,hsInfix,hsStructure,hsStructure,hsTypedef,hsStatement,hsConditional,hsBoolean,hsType,hsMaybe,hsExitCode,hsOrdering,hsDebug,hsLineComment,hsBlockComment,hsPragma endif if !exists("hs_minlines") let hs_minlines = 50 endif exec "syn sync lines=" . hs_minlines if version >= 508 || !exists("did_hs_syntax_inits") if version < 508 let did_hs_syntax_inits = 1 command -nargs=+ HiLink hi link <args> else command -nargs=+ HiLink hi def link <args> endif hi link hsModule hsStructure hi link hsImport Include hi link hsImportMod hsImport hi link hsInfix PreProc hi link hsStructure Structure hi link hsStatement Statement hi link hsConditional Conditional hi link hsSpecialChar SpecialChar hi link hsTypedef Typedef hi link hsVarSym hsOperator hi link hsConSym hsOperator hi link hsOperator Operator if exists("hs_highlight_delimiters") " Some people find this highlighting distracting. hi link hsDelimiter Delimiter endif hi link hsSpecialCharError Error hi link hsString String hi link hsCharacter Character hi link hsNumber Number hi link hsFloat Float hi link hsConditional Conditional hi link hsLiterateComment hsComment hi link hsBlockComment hsComment hi link hsLineComment hsComment hi link hsComment Comment hi link hsPragma SpecialComment hi link hsBoolean Boolean hi link hsType Type hi link hsMaybe hsEnumConst hi link hsOrdering hsEnumConst hi link hsEnumConst Constant hi link hsDebug Debug delcommand HiLink endif let b:current_syntax = "haskell" " Options for vi: ts=8 sw=2 sts=2 nowrap noexpandtab ft=vim

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Re: Haskell-Cafe digest, Vol 1 #293 - 4 msgs
by Jay Cox 11 Jan '02

11 Jan '02

> Message: 4 > From: Jorge Adriano <jadrian(a)mat.uc.pt> > To: haskell-cafe(a)haskell.org, glasgow-haskell-users(a)haskell.org > Subject: State Transformer > Date: Mon, 7 Jan 2002 15:41:57 +0000 > <snip> > Till then I had just read what I needed to be able to use the IO Monad. > Seems to me like having a State Transformer monad its the best way to do it. > Now I've read a great deal of Richard Birds Book chap 10 (Monads), as well as > the "Monads for the Haskell Working Programmer"[1] by Theodore Norvell. > > I was going to try to make my own simple examples using a ST. > A State Monad seemed something like would most probably be in some Standard > Library, or at least in some GHC library. > And it was (section 4.31.ST in the hslibs documentation) > I wanted to use this ST, but then I noticed it was different from the one > described in tutorial[1]. > > I was expecting the ST Monad ghc module to provide an apply function, > analogue to the > > applyST :: StateTrans s a -> s -> (s, a) > > applyST (ST p) s = p s I believe you want runST. its type is not > in the tutorial. > I also expected to have general functions to access and change State. I can't > implement them myself since the ST constructor is (obviously) not exported. > But this ST module seems to work in a completely diferent way. > >From what I can tell it is not suposed to be applyed to an initial state, > instead it starts with an 'empty' state... > State is controled with Referencies (mutable variables). yes, you can initialize any number of mutable variables from within the state monad. > Ok, now my problem, how do I use this? > > ---------------------------------------- > stupidFunc > runST ( > do { > initstate <- newSTRef ('x',0) > -- and now what? > }) > ----------------------------------------- > ----- import ST main = print (runST (do { x <-newSTRef 3;add1 x; y<-newSTRef "Elvis"; saysomething y; a<-readSTRef x; b<-readSTRef y; return (a,b)})) add1 a = do l <-readSTRef a writeSTRef a (l+1) saysomething y = modifySTRef y leftthebuilding leftthebuilding::String->String leftthebuilding x = x ++ " has left the building." ----- compile the above code with -package lang perhaps that'll give you some idea how to deal with STRefs and all. Jay Cox

1 0

differential operation (Typing Question!)
by Jay Cox 11 Jan '02

11 Jan '02

|> Message: 1 |> From: "Zhe Fu" <fuzh(a)cs.orst.edu> |> To: <haskell-cafe(a)haskell.org> |> Subject: how to implement the differential operation with Haskell? |> Date: Wed, 9 Jan 2002 00:37:20 -0800 |> |> Is there any built-in functions in Haskell to implement diffential |> operation? Or can anyone give me some advices about how to implement |them |> with Haskell? Thanks. If you happen to be talking about symbolically differentiating expressions, I happen to be in the early stage of developing a extendible module to do such a thing right now! However I seem to have come across some typing problems. But to understand my problem I may need to try to summarize the details thereof I started out with a recursive datatype >data BasDif r = Ext (r (BasDif r)) | > Symbol Sym | Constant Rational | > Sum (BasDif r) (BasDif r) | > Prod (BasDif r) (BasDif r) | > Divide (BasDif r) (BasDif r) | > Negate (BasDif r) | D (BasDif r) Notice the Ext( r (BasDif r)). I use this to extend the datatype. I define a basic differentiating function "d" on all of these constructs except Ext. When d pattern matches (Ext x) it calls some instance of the diff class on x. diff on some function g(u) where u is an expression returns a pair (u,g') back to "d" so that "d" can complete the chain rule d(g(u))=g'(u)*d(u). (technically it returns a function g' which takes two arguments: a data constructor and a BasDif expression of some sort the current expected typing of diff (forgive me for not knowing proper vernacular) is given below. >class Diff x where > diff:: Sym -> x a -> (a, (x a1 -> r (BasDif r)) -> a1 -> BasDif r) Its rather complex but I'll admit I stole it from the ghci interpreter giving me the type of function which did conceptually what I wanted it to do. the types are below. Differentiate> :type diffTrig forall t a a1 r. t -> Trig a -> (a, (Trig a1 -> r (BasDif r)) -> a1 -> BasDif r) Differentiate> :type diffNe forall t a a1 r. t -> NumExp a -> (a, (NumExp a1 -> r (BasDif r)) -> a1 -> BasDif r) Before I expound upon why the type needs to be so complicated, let me tell you what I'm trying to do with these diff functions that follow. (In the following definitions, prod, sum_, negate_, are more or less helper functions which make some trivial simplifications as they make products, sums, and additive inverses) >data NumExp a = N Rational a >diffNe s (N i x) = > let j = i-1 > in if (j /= 0 ) > then (x,\f-> \v -> prod (Ext(f (N (i-1) v))) (Constant i)) > else (x,\_-> \_->Constant 1) > >instance Diff NumExp where > diff=diffNe >data Trig a = Sin a | Cos a | Tan a | Cot a| Sec a |Csc a deriving Show >diffTrig s (Sin x) = (x, \f -> Ext . f . Cos) >diffTrig s (Cos x) = (x, \f -> negate_ . Ext . f . Sin) ... more defs >instance Diff Trig where > diff = diffTrig With these definitions, I have enough to take derivatives of (BasDif Trig) expressions or (BasDif NumExp) expressions, but not both. Therefore I defined the above expressions (as complicated as they are) so that I could make a instance of diff by combining the two without copy/pasting code (and without explicitly calling diffTrig and diffNe) thus being able to differentiate expressions of BasDif TrigNumExp (and thus being able to differentiate expressions with + - * / sine,cosine,tangent,their reciprocals and any combination thereof with a rational exponent) >data TrigNumExp a= T (Trig a) | NE (NumExp a) >instance Show a => Show (TrigNumExp a) where > showsPrec _ (T x) = shows x > showsPrec _ (NE x) = shows x > >diffTrigNumExp s (T x) = let (k,f)= diff s x in (k,\g->f (g .T)) >diffTrigNumExp s (NE x) = let (k,f)= diff s x in (k,\g->f (g .NE)) > >instance Diff TrigNumExp where > diff = diffTrigNumExp In this I have succeeded. THEN I tried to do the same for inverse trig functions. These are of a rather different nature. the derivative of inverse sine requires something to represent square roots of expressions, hence I use the constructor for NE expressions. >diffArcT s (ArcSin x) =(x,\f-> \y->Ext(f ( N (-1/2) > (sum_ (Constant 1) (negate_ (prod y y)))))) >diffArcT s _ = error "incomplete" the type of diffArcT is Differentiate> :type diffArcT forall t a r1 r. t -> ArcTrig a -> (a, (NumExp (BasDif r1) -> r (BasDif r)) -> BasDif r1 -> BasDif r) I suppose the error message I get when adding this function to the module is understandable given the class constraints.. ... Differentiate.v2.lhs:177: Couldn't match `ArcTrig a1' against `NumExp (BasDif r)' Expected type: Sym -> ArcTrig a -> (a, (ArcTrig a1 -> r1 (BasDif r1)) -> a1 -> BasDif r1) Inferred type: Sym -> ArcTrig a -> (a, (NumExp (BasDif r) -> r2 (BasDif r2)) -> BasDif r -> BasDif r2) In the definition of `diff': diffArcT ... but seems to me to be a reasonable relaxing of types for the class doesn't work either (in fact it produces errors for every instance of diff ... Old class >class Diff x where > diff:: Sym -> x a -> (a, (x a1 -> r (BasDif r)) -> a1 -> BasDif r) New class class Diff x where diff:: Sym -> x a -> (a, (x1 a1 -> r (BasDif r)) -> a1 -> BasDif r) New errors with new class Cannot unify the type-signature variable `x1' with the type `TrigNumExp' Expected type: Sym -> TrigNumExp a -> (a, (x1 a1 -> r (BasDif r)) -> a1 -> BasDif r) Inferred type: Sym -> TrigNumExp a -> (a, (TrigNumExp a11 -> r1 (BasDif r1)) -> a11 -> BasDif r1) In the definition of `diff': diffTrigNumExp Differentiate.v2.lhs:177: Cannot unify the type-signature variable `x1' with the type `NumExp' Expected type: Sym -> ArcTrig a -> (a, (x1 a1 -> r (BasDif r)) -> a1 -> BasDif r) Inferred type: Sym -> ArcTrig a -> (a, (NumExp (BasDif r1) -> r2 (BasDif r2)) -> BasDif r1 -> BasDif r2) ... etc. What I don't understand is WHY NumExp cannot be unified with x1. what I would personally would like to understand is if Haskell's Hindley-Miller type system can even deal with what I want to do (basically I want a type for diff such that the definition of diffArcT would be tolerable to the compiler.) I realize I am a bit over my head here so it might be concievable that just some pointer to a reference on how haskell deals with type resolution might help as well, or perhaps some general method for concieving of a general type for a member of a class, or perhaps some ideas to re-engineer this module. Anyway, I suppose this module might be of some interest to some people so I have put it in a temporary place so that some people may play with it. It doesnt have any functions for reading general expresssions yet, you could enter some funky multivariable expression and see what comes out. (I must warn you, if you happen to use the "d" function, it doesnt do much but the simplest simplifications. some of the less trivial simplifications have been commented out because they do not help the runtime. It is at (temporarely) http://home.flash.net/~flygal/Jay/Haskell/Differentiate/v0.1/Differentiate.… Have fun and thanks for any help Jay Cox

1 0

Re: State Transformer
by Ketil Z Malde 11 Jan '02

11 Jan '02

Jorge Adriano <jadrian(a)mat.uc.pt> writes: > Anyway, I was coding some simple GA, and as you probably know I need to use > random values. The most elegant way I could think of was to generate some [...] > Monads! (right?) Well, I suppose so. Generally speaking. But, you might want to consider using the standard random generation routines from the (IO) top level of your program, and just split the random generator for each function that uses it. IOW, passing each function its own random generator, instead of worrying about returning the "rest" of a global random sequence. (I don't have any good example code, I'm afraid, but at leat have a look at the chapter on Random in the library report on http://haskell.org) -kzm -- If I haven't seen further, it is by standing in the footprints of giants

2 1

how to implement the differential operation with Haskell?
by Zhe Fu 09 Jan '02

09 Jan '02

Is there any built-in functions in Haskell to implement diffential operation? Or can anyone give me some advices about how to implement them with Haskell? Thanks.

1 0

State Transformer
by Jorge Adriano 07 Jan '02

07 Jan '02

Hi, I'm studying, among other things, Genetic Algorithms and Neural Networks and I decided I'd use haskell to code some simple GAs and NNs along with my study. Well, maybe it was not such a good idea after all, because I've been spending way more time learning more Haskell then GAs and NNs :( Anyway, I was coding some simple GA, and as you probably know I need to use random values. The most elegant way I could think of was to generate some infinite list of random values and pass them around as arguments to the functions that need those values. I called data which wraped this list Environment, and at first it seemed a nice way to solve the problem. Well, now I think it gets kind of weird because some functions will end up to have type something -> (otherthing, Environment), to update those lists... it's just ugly. Beside those lists I'd also like to control some statistics like the number of mutations, n. of crossovers, best fitness value in each generation, etc... I figured out that there should be a better way to do this then just chaging all the signatures and passing all this values around. Monads! (right?) Till then I had just read what I needed to be able to use the IO Monad. Seems to me like having a State Transformer monad its the best way to do it. Now I've read a great deal of Richard Birds Book chap 10 (Monads), as well as the "Monads for the Haskell Working Programmer"[1] by Theodore Norvell. I was going to try to make my own simple examples using a ST. A State Monad seemed something like would most probably be in some Standard Library, or at least in some GHC library. And it was (section 4.31.ST in the hslibs documentation) I wanted to use this ST, but then I noticed it was different from the one described in tutorial[1]. I was expecting the ST Monad ghc module to provide an apply function, analogue to the > applyST :: StateTrans s a -> s -> (s, a) > applyST (ST p) s = p s in the tutorial. I also expected to have general functions to access and change State. I can't implement them myself since the ST constructor is (obviously) not exported. But this ST module seems to work in a completely diferent way. >From what I can tell it is not suposed to be applyed to an initial state, instead it starts with an 'empty' state... State is controled with Referencies (mutable variables). Ok, now my problem, how do I use this? I can't really see how to change this referencies from within some function. (Got an example in the end to explain better waht I mean with that [Example1]) I'd also appreciate some coments on: Using a ST monad (good idea, bad?) Using the Ghc ST monad? Chromosomes defined as arrays? - either IArray or Diff array got to give it some more thought... (don't want Ints + bitwise operations right now...) Well, any other comments or hints that you think that might be usefully are welcome. I've already checked out the paper from the TAIGA project[2], it's not exactly done the way I'm thinking about doing it, but I got some usefull tips from there, like the use of a Monad to control statistics. One of my main problems so far as been *knowing what do I need to know*! I don't know anyone that codes in haskell, not having anyone to talk to and share ideas doesn't helps much either. Things get complicated where you (you - the guy that comes from the imperative paradigm) less expects it too... the space leaks, using monads to control state... if you still have not read about this stuff, IMO, it is easy to feel like you already know enough to do some solve some kind of problems when you actually don't. Any newbie to C or Pascal can make a few randoms here and there, and keep track of statistics... when you already spent some time with haskell you don't even question whether you already know enough to do something like *that*. Only when you start to work, and thing start to get messy, you begin to think that *maybe you need something you still don't know about*, and then you got to find out what it is... Documentation, I also feel like it could be more and better... the ST module in ghc for instance... would it be that hard to put at least some simple example there? No, just the type signatures... Well, this is just my opinion anyway. Thanks for your atention, and happy 2002 ;-) J.A. [Example1] How can I do this for instance, with the Ghc ST Monad: ------------------------------------------------------------------------ -- the State Trans defined as in the tutorial newtype StateTrans s a = ST( s -> (s, a) ) instance Monad (StateTrans s) where -- (>>=) :: StateTrans s a -> (a -> StateTrans s b) -> StateTrans s b (ST p) >>= k = ST( \s0 -> let (s1, a) = p s0 (ST q) = k a in q s1 ) -- return :: a -> StateTrans s a return a = ST( \s -> (s, a) ) applyST :: StateTrans s a -> s -> (s, a) applyST (ST p) s = p s -- just change the state putST :: (Char,Int) -> StateTrans (Char,Int) () putST (c',n') = ST(\(c,n)->((c',n'),())) -- get the state getST :: StateTrans (Char,Int) (Char,Int) getST = ST(\(x,y)-> ((x,y), (x,y))) testfunc :: StateTrans (Char,Int) Int testfunc = do foo bar (c,n)<-getST return n foo = do (c,n)<-getST putST ('a',n+1) bar = do (c,n)<-getST putST (c,n+2) tryTestFunc = applyST testfunc ('x',0) ------------------------------------------------------------------------ I is something like this? ---------------------------------------- stupidFunc runST ( do { initstate <- newSTRef ('x',0) -- and now what? }) ----------------------------------------- [1] http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm [2] http://www.scms.rgu.ac.uk/staff/db/Taiga/ http://www.scms.rgu.ac.uk/staff/db/Taiga/APGA2000.ps.gz

2 1

RE: Compiler error using IO
by Ashley Yakeley 07 Jan '02

07 Jan '02

>| while :: IO Bool -> IO () -> IO () >| >| while test action = do res <- test >| if res then do action >| while test action >| else return () > >Haskell uses "layout" to reduce the need for parentheses. For your code to >parse, "if" should align with "res" vertically, as in: > > ... = do res <- test > if res ... The "layout" rules drive me nuts. You might prefer using parentheses and semicolons, as I do: -- (not tested) while :: IO Bool -> IO () -> IO (); while test action = do { res <- test; if res then do { action; while test action; } else return (); }; -- ...and then you can put the whitespace wherever you want. -- Ashley Yakeley, Seattle WA

6 5

(no subject)
by Noriyuki Sugita 05 Jan '02

05 Jan '02

1 0

Compiler error using IO
by Tom Bevan 04 Jan '02

04 Jan '02

When trying to right IO I continually run across this sort of compile error which I just don't understand. Any help would be very muich appreciated. Function: while :: IO Bool -> IO () -> IO () while test action = do res <- test if res then do action while test action else return () Error: io.hs: 112 lines, 2463 characters. Reading file "io.hs": Parsing ERROR io.hs:110 - Last generator in do {...} must be an expression Tom

2 1

Intro to Functional Prog. by Bird, Section 2.5
by Shawn P. Garbett 02 Jan '02

02 Jan '02

I've been working on understanding Haskell using hugs98 and playing with some of the examples in Bird's book. In section 2.5, page 46 of the second edition, he gives an example of dependent types: But I get this error I've been unable to solve: ---------------------------------------------------------------------------------------------------------------- --Type checking --ERROR sec2-5.hs:8 - Type error in application --*** Expression : case' (Left' f,Right' g) --*** Term : (Left' f,Right' g) --*** Type : (Either' (d -> e) f,Either' g (h -> i)) --*** Does not match : (a -> b,c -> b) data Either' a b = Left' a | Right' b case' :: (a->g, b->g) -> Either' a b -> g case' (f, g) (Left' x) = f x case' (f, g) (Right' y) = g y plus' :: (a->b , g->d) -> Either' a g -> Either' b d plus' (f,g) = case' (Left' f, Right' g) ----------------------------------------------------------------------------- Please note that the version I've entered is correct by the errata published for this section.

3 2

RE: haskell sockets
by Simon Marlow 02 Jan '02

02 Jan '02

> i want to write/read data to the soccerserver of the robocup. > the sample > code below works, but i want to have a handle for writing to > the server > instead of SocketPrim.sendTo. how can i do that ? SocketPrim.socketToHandle gives you back a Handle which you can use to read/write to/from using Haskell's normal IO operations. Note that you need to call SocketPrim.connect, even for a Datagram socket, to fix the endpoint for the socket. Cheers, Simon

1 0

PHOTOS XXX GRATUITES !!!
by sandy000＠hotmail.com 29 Dec '01

29 Dec '01

1 0

haskell sockets
by Thomas Rabe 22 Dec '01

22 Dec '01

hello, i want to write/read data to the soccerserver of the robocup. the sample code below works, but i want to have a handle for writing to the server instead of SocketPrim.sendTo. how can i do that ? import IO import Socket import SocketPrim local = "127.0.0.1" portnum = 6000 local' = iNADDR_ANY portnum' = aNY_PORT main = withSocketsDo $ do -- create udp socket s1 <- socket AF_INET Datagram 0 ia <- inet_addr local -- bind socket to first free port given by system bindSocket s1 (SockAddrInet ( portnum') local') -- write to soccerserver on port 6000 SocketPrim.sendTo s1 "(init myteam)" (SockAddrInet ( portnum) ia) h' <- socketToHandle s1 ReadWriteMode hSetBuffering h' LineBuffering -- hPutsStr not works: Reason: Transport endpoint is not connected ?! -- hPutStr h' "foo" -- bad style infinity loop, just for testing testloop h' testloop h = do b <- hWaitForInput h 2000 if b == True then do line <- hGetChar h hFlush h putChar line else putStrLn "End of Input" testloop h Greetings, Thomas Rabe. -- mail: raven at drehmoment dot org -- home: http://hal9000.drehmoment.org -- more: http://drehmoment.org

1 0

Haskell
by J Brown 21 Dec '01

21 Dec '01

hi, I am only a beginner of programming with Haskell, how do I define each of the following: * maxFirst :: (Int,Int) -> (Int,Int) -> (Int,Int) which returns, of its two input tuples, the one whose first component is the highest. * Define a function maximumFirst :: [(Int,Int)] -> (Int,Int) which returns, from its list of input tuples, the tuple whose first component is highest, assuming input list is non-empty. A function is given: maxf :: [Int] -> Int such that if maxf [x1,....,xn] = xi then elem xi [x1,...,xn] == True f xi >= f x1 f xi >= f x2 ...... fxi >= f xn * How do you define the function maxf (above) which returns the element of its non-empty input list for which f is maximal. *and to define a function minf which returns the element from its non-empty input list for which f is minimal. *how do I define the function maxIndexf :: [Int] -> Int such that if maxf xs = i for non-empty xs, then 0 <= i < length xs f (x s !! i) >= f (x s !! 0) f (x s !! i) >= f (x s !! 1) .... f (x s !! i) >= f (last xs) that is it returns the index of the element of its input list for which f is maximal. __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com

2 1

Haskell
by Law 20 Dec '01

20 Dec '01

hi, how do I define this function: tupleWithf :: [Int] -> [(Int, Int)] such that tupleWithf [x1,x2,.....,xn] = [ (f x1, x1), (f x2 x2),...., (f xn, xn) ]

2 1

Re: We need "Documentation"
by Eray Ozkural (exa) 20 Dec '01

20 Dec '01

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Hal, On Thursday 13 December 2001 20:47, Hal Daume III wrote: > I think we should move this off the mailing list. I'm willing to > spear-head such an effort. Anyone who is interested in contributing, > please email me. I'll compile a list of people and we can figure out what > we want to do. I think it's okay to discuss in haskell-cafe. I'd like to contribute too but I have little time nowadays trying to finish writing my msc. thesis. I do have a few suggestions though: 1. Using wiki does not seem to be a bad idea. You may also consider using a word processing system such as LaTeX though. It's easy to convert LaTeX to html and it yields very quality prints as you all know. However, for true collaboration wiki is the king. There is already a nice Haskell wiki appropriate for such a project IIRC. 2. I don't think following the outline of a class based imperative language will be fitting for a high quality Haskell tutorial. I think you should look at OcaML tutorial and the python tutorial, and combine ideas from both as well as your own :) Thanks, - -- Eray Ozkural (exa) <erayo(a)cs.bilkent.edu.tr> Comp. Sci. Dept., Bilkent University, Ankara www: http://www.cs.bilkent.edu.tr/~erayo GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F 7C07 AE16 874D 539C -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE8IMf0fAeuFodNU5wRAunNAJ9eS6w6b5ho45htdzDLxhEFhK5EtACgjVdY dP0ObV/+usABuJgRL7aWV9M= =SLzH -----END PGP SIGNATURE-----

2 1

RE: space efficiency question
by Simon Peyton-Jones 18 Dec '01

18 Dec '01

[redirected to haskell-cafe] No, GHC doesn't test pointer equality first. It's unsound to do so, since it might turn bottom into non-bottom, or remove `seq` calls that kill space leaks. S -----Original Message----- From: Frank Dellaert [mailto:frank@cc.gatech.edu] Sent: 18 December 2001 11:48 To: Simon Peyton-Jones; Haskell(a)haskell.org Subject: Re: space efficiency question Thanks ! Now, a small follow-up question: if I subsequently test 2 Instances whether they were instantiated with the same Attributes value, as in test (Instance a1 _) (Instance a2 _) = (a1==a2) will this be implemented efficiently ? I.e. will it check first whether the pointers happen to be the same, and only then do a full Eq comparison ? Cheers Frank ----- Original Message ----- From: Simon <mailto:simonpj@microsoft.com> Peyton-Jones To: Frank Dellaert <mailto:frank@cc.gatech.edu> ; Haskell(a)haskell.org Sent: Monday, December 17, 2001 10:24 AM Subject: RE: space efficiency question Now: if I have a couple of thousand of these Instances, created with the same Attributes value, as in: a = Attributes ["a","bc"] i0000 = Instance a [1,2] i0001 = Instance a [1,1] ... i7896 = Instance a [2,1] can I be assured that there are not thousands of actual copies of a but that they simply have a pointer to a ? I'm interested in the answer for both ghc (compiled/interpreted) as interpreted Hugs. Yes for GHC: you'll only get one copy of 'a'. Simon

1 0

sending values between actions
by Nuno Silva 17 Dec '01

17 Dec '01

can anyone tell me how to send values from one action to another??? I have main0 = do s <- getLine writeFile "text.txt" s main1 = do .... I'd like to use main0 info on main1 action to build a nXm multisquares box... anyone understood this???? please help... __________________________________________________ Do You Yahoo!? Check out Yahoo! Shopping and Yahoo! Auctions for all of your unique holiday gifts! Buy at http://shopping.yahoo.com or bid at http://auctions.yahoo.com

3 2

Re: Pointers in Haskell??
by Jeff Dalton 17 Dec '01

17 Dec '01

The answers are making this question seem trickier than I'd thought it was, because so far they (both) make it sound like structure-sharing is tied very closely to names / variables. For instance: > In Haskell, you can arrange for a large data structure to be shared by > giving it a name, and then using the name wherever you'd use a pointer in > some other language. The idea here seems to be that you have a name that refers specifically to the struccture you wish to share (as opposed to needing only an expression whose value is that structure), and then there are two possible interpretations: One possible interpretation is: this is *a* way to get sharing, to show that it's possible to have shared structures. The other possible interpretation is: this is *the* (only) way to do it. What I'd expected was that it would suffice to have an expression (and indeed that implementationally it would be much like Lisp or Java so that values were always (except in exceptional cases) secretly "pointers to".) -- Jeff

3 2

Error message
by victoria borisovna rozhkova 16 Dec '01

16 Dec '01

Hello, what does it mean: Variable definition clashes the import. thank you Vika

2 1

modules, classes, instances
by David Feuer 16 Dec '01

16 Dec '01

Well, I've been doing some more stupid thinking, and I've decided that I am not satisfied with the module system in haskell, or the way it deals with namespaces. It seems to me that there are four kinds of things that need to be dealt with: classes, instances, types, values, and possibly some kind of ML-style structures/functors. I see no good reason not to treat a class declaration similarly to the way Haskell treats modules. For example, if I have a class class A x where f1::... f2::... I would like to access the functions by writing A.f1, A.f2, etc. There should, however, be some way to unqualify the names, the way ML allows structures to be opened. Of course, this can be done in Haskell by putting the class declaration inside a module, but doing so can be annoying, and I think this style clearly attaches the method to the class it belongs to. It thereby lets you have a List l=>List.empty::l and also a Bag b=>Bag.empty::b automatically. Then there are types and instance declarations. I have the strange feeling that they go together. So I'm thinking it might be good to have some kind of type/instance module that can export types and instances. I can't understand why it is that instance declarations can't contain nested declarations other than the required ones. I think it would be nice if they could contain other declarations, as modules can. Also, instances could be imported (not just as part of a module, but separately) ML-style structures/functors. For everything else. Finally: I want nested modules!!! They probably couldn't be compiled separately, but they'd provide some namespace control. -- /Times-Bold 40 selectfont/n{moveto}def/m{gsave true charpath clip 72 400 n 300 -4 1{dup 160 300 3 -1 roll 0 360 arc 300 div 1 1 sethsbcolor fill}for grestore 0 -60 rmoveto}def 72 500 n(This message has been)m (brought to you by the)m(letter alpha and the number pi.)m(David Feuer) m(David_Feuer(a)brown.edu)m showpage

2 1

drawing grids
by Nuno Silva 14 Dec '01

14 Dec '01

hello can anyone tell me how to draw grids?? it's like this: Imagine that I say that I want the grid to be 5X6, it will compute the distbetween the limits and then draw them. if I want a 6X10, it will do the same. thanks __________________________________________________ Do You Yahoo!? Check out Yahoo! Shopping and Yahoo! Auctions for all of your unique holiday gifts! Buy at http://shopping.yahoo.com or bid at http://auctions.yahoo.com

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overloading efficiency
by David Feuer 14 Dec '01

14 Dec '01

I have read that using overloaded functions is expensive because overloading is implemented by passing dictionaries around. These various sources seem to agree that this in necessary to allow separate compilation. I don't understand why this is... Couldn't the functions be specialized at link time? -- /Times-Bold 40 selectfont/n{moveto}def/m{gsave true charpath clip 72 400 n 300 -4 1{dup 160 300 3 -1 roll 0 360 arc 300 div 1 1 sethsbcolor fill}for grestore 0 -60 rmoveto}def 72 500 n(This message has been)m (brought to you by the)m(letter alpha and the number pi.)m(David Feuer) m(David_Feuer(a)brown.edu)m showpage

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Re: drawing costumized squared boxes
by Yoann Padioleau 12 Dec '01

12 Dec '01

Nuno Silva <nunopedrosilva(a)yahoo.com> writes: > hello I'm new to Haskell and I'm having trouble > drawing squares in the screen... it's like the game > minesweeper, we will have to define how many squares > we want the game to have and automatically draw them. > if anyone could help me I'd be gratefull borrow the book The Haskell School of Expression: Learning Functional Programming through Multimedia : Paul Hudak Cambridge university press to your library, and read it. > > __________________________________________________ > Do You Yahoo!? > Check out Yahoo! Shopping and Yahoo! Auctions for all of > your unique holiday gifts! Buy at http://shopping.yahoo.com > or bid at http://auctions.yahoo.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

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drawing costumized squared boxes
by Nuno Silva 12 Dec '01

12 Dec '01

hello I'm new to Haskell and I'm having trouble drawing squares in the screen... it's like the game minesweeper, we will have to define how many squares we want the game to have and automatically draw them. if anyone could help me I'd be gratefull __________________________________________________ Do You Yahoo!? Check out Yahoo! Shopping and Yahoo! Auctions for all of your unique holiday gifts! Buy at http://shopping.yahoo.com or bid at http://auctions.yahoo.com

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Re: haxml
by Christopher Milton 11 Dec '01

11 Dec '01

--- David Smallwood <drs(a)dmu.ac.uk> wrote: > Does anyone know of a mirror site where I can download the latest > version of HaXml. (Connection to York is - and has been for a > while - apparently down: > ftp://ftp.cs.york.ac.uk/pub/haskell/HaXml/HaXml-1.02.tar.gz, that is) It's up now, unlike Hypatia, which needs a new maintainer. [ http://hypatia.dcs.qmul.ac.uk/ ] And, no, I don't have the server space to do it. Chris Milton just noticing Google has Usenet going back to 1981... http://groups.google.com/groups?q=clock+functional+programming&hl=en&rnum=2… __________________________________________________ Do You Yahoo!? Check out Yahoo! Shopping and Yahoo! Auctions for all of your unique holiday gifts! Buy at http://shopping.yahoo.com or bid at http://auctions.yahoo.com

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Re: (no subject)
by Ashley Yakeley 11 Dec '01

11 Dec '01

At 2001-12-10 16:07, uma kompella wrote: >i am new to haskell and am having a problem to write >function which takes a boolean expression and returns >a truthvalue stating whether or not it is a tautology. If you really want to impress your tutor, see if you can find a function that does this in polynomial time. -- Ashley Yakeley, Seattle WA

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