January 2007 - Haskell-Cafe

RE: [Haskell-cafe] How did you stumble on Haskell?
by Bob Davison 29 Jan '07

29 Jan '07

>From: "Alexy Khrabrov" <deliverable(a)gmail.com> >To: haskell-cafe(a)haskell.org >Subject: [Haskell-cafe] How did you stumble on Haskell? >Date: Sun, 28 Jan 2007 19:01:57 -0800 > >How do people stumble on Haskell? -- snip >What's folks most interesting ways to arrive at FP? > >Cheers, >Alexy I have been programming other stuff for years (APL, C, Assembly, Visual Basic, Java, C#) and a year or two ago I started thinking of going back to study computing a bit more formally. Reading web pages of lecturers on courses that looked interesting I found a few whose favorite language was a functional programming language called Haskell. I didn't know anything about functional programming so I just moved on. More recently I started to work my way through 'Modern compiler implementation in Java' by Andrew Appel and he seemed keen on functional programming and the book covers extending the basic language to a functional programming language. (I have have read the relevant chapter but not got as far as implementing it yet!) Just before Christmas I decided to investigate Haskell. I got hold of a copy of Graham Hutton's new book 'Programming in Haskell', which I am thoroughly enjoying. Haskell is great fun but as I dig deeper I am finding my lack of mathematical sophistication to be a problem. This leads me off thread to ask if anyone could recommend reading for someone who has done mathematics to college level, but nearly 30 years ago when many English schools didn't cover 20th century mathematics. I thought calculus was about differentiation and integration and was very surprised to discover that there were such things as 'predicate calculus', 'propositional calculus', and various flavours of 'lambda calculus'. I also have little or no idea of set theory, group theory, domain theory, combinatory logic, ... (I can just imagine the surprised looks on the faces of the mathematicians reading this. You never know computer programmers could be so ignorant, did you?) I have no idea how much of this stuff I need to know but I would certainly like to be able to learn more of this facinating new world and not just be content with learning how to write a Haskell program. I just don't know where to start. Thanks, Bob Davison _________________________________________________________________ Turn searches into helpful donations. Make your search count. http://click4thecause.live.com/search/charity/default.aspx?source=hmemtagli…

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type and class problems compiling code on GHC 6.6
by José Miguel Vilaça 29 Jan '07

29 Jan '07

Dear Haskellers, I had used some code which worked fine on GHC 6.4 and now it dont compile on GHC 6.6. Can anyone please give me some workarounds and/or explanations about these 2 errors? instance InfoKind a b => InfoKind (Maybe a) b where blank = Nothing check n _ Nothing = ["No info value stored with "++n] check n g (Just a) = check n g a GHC complains that Illegal instance declaration for `InfoKind (Maybe a) b' (the Coverage Condition fails for one of the functional dependencies) In the instance declaration for `InfoKind (Maybe a) b' multiListViewGetTSelections :: MultiListView x () -> IO [x] multiListViewGetTSelections multiListView = do { Just ((model, _) :: (Var [x], x -> String)) <- unsafeObjectGetClientData multiListView ; -- more and more code } GHC complains that A pattern type signature cannot bind scoped type variables `x' unless the pattern has a rigid type context In the pattern: (model, _) :: (Var [x], x -> String) I would really appreciate some help. Best regards Miguel Vilaça

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c2hs failed to process __extension__
by 向井淳 29 Jan '07

29 Jan '07

Hello all, I have a trouble when compiling a program with c2hs. The program can be seen at http://www.city5.org/prog/hstraier/, which is a binding with HyperEstraier (a fulltext search system). When I compile it at FreeBSD 6.2, I encountered the following error: | % runghc Setup.hs build | Preprocessing library hstraier-0.2... | c2hs: Error in C header file. | | /usr/include/machine/endian.h:146: (column 25) [FATAL] | >>> Syntax error! | The symbol `{' does not fit here. From the dumped file Node.i (you can see from http://www.city5.org/ Node.i), the error line is as follows: | return (__extension__ ({ register __uint32_t __X = (_x); __asm ("bswap %0" : "+r" (__X)); __X; })); c2hs fails to process __extension__? From the changelog, c2hs now can understand __extension. Is this an exceptional case? Best Regards, Jun Mukai mukai(a)jmuk.org

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Re: [Haskell-cafe] Simple HTTP lib for Windows?
by brad clawsie 29 Jan '07

29 Jan '07

hi, i have popped in on this thread before to mention my own extension to Network.HTTP (http://www.b7j0c.org/content/haskell-http.html, providing get() and head()). i would like to thank bjorn for his work on Network.HTTP and echo his observation that this package needs some work and active maintainence. i would also suggest that people needing http functionality extend his package instead of writing their own or providing quick and dirty hacks that cover 80% of the problem space, as i think others have discussed. http is a simple protocol, but it is not trivial, and reference implementations need to address the standard as completely as possible. no one is going to take haskell seriously as a practical tool if the libraries don't address the corner cases, and http has some. reading through the Network.HTTP code, it does appear that the original authors and present maintainers are concerned with standards conformance, which means reading the standard. http is becoming as integral to the development environment as file access. haskell needs an authoritative native implementation or a ffi wrapper to libcurl. developers also need to know that there is *one* reference library for http support (like perl's LWP)...likewise i think the lack of a single reference library for sql/db access is also hurting haskell adoption. perl wins here again with DBI. p.s. i would gladly volunteer to maintain the http package if i thought my haskell was "export quality". it isn't. thanks brad

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Error and warning messages with -O
by Marco Túlio Gontijo e Silva 29 Jan '07

29 Jan '07

Hello, I was trying to compile a library I wrote with cabal, but I couldn't because of -O. I tried removing it and it worked ok. Then I noticed that -O was giving some warnings and an error that are not present without it: marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ /usr/bin/ghc -package-name HaskellGPC-0.0.1 --make -hide-all-packages -i -idist/build/autogen -i. -odir dist/build -hidir dist/build -package base-2.0 -O -Wall -Werror -fffi Algebra.Geometric.Gpc [1 of 1] Compiling Algebra.Geometric.Gpc ( Algebra/Geometric/Gpc.hs, dist/build/Algebra/Geometric/Gpc.o ) /tmp/ghc5871_0/ghc5871_0.hc:5:35: error: Algebra/Geometric/Gpc.h: Arquivo ou diretório não encontrado /tmp/ghc5871_0/ghc5871_0.hc: In function ‘r2Hk_entry’: /tmp/ghc5871_0/ghc5871_0.hc:290:0: warning: implicit declaration of function ‘gpc_polygon_clip’ /tmp/ghc5871_0/ghc5871_0.hc: In function ‘r2Hm_entry’: /tmp/ghc5871_0/ghc5871_0.hc:327:0: warning: implicit declaration of function ‘gpc_free_polygon’ marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ /usr/bin/ghc -package-name HaskellGPC-0.0.1 --make -hide-all-packages -i -idist/build/autogen -i. -odir dist/build -hidir dist/build -package base-2.0 -Wall -Werror -fffi Algebra.Geometric.Gpc [1 of 1] Compiling Algebra.Geometric.Gpc ( Algebra/Geometric/Gpc.hs, dist/build/Algebra/Geometric/Gpc.o ) marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ Why can't I use -O? Thanks. -- malebria Marco Túlio Gontijo e Silva Correio (MSN): malebria(a)riseup.net Jabber (GTalk): malebria(a)jabber.org Ekiga: malebria(a)ekiga.net IRC: malebria(a)irc.freenode.net malebria(a)irc.oftc.org Skype: marcotmarcot Telefone: 33346720 Celular: 98116720 Endereço: Rua Paula Cândido, 257/201 Gutierrez 30430-260 Belo Horizonte/MG Brasil

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Happy outputs "parE" - ideas on what to change in my .y file?
by Mark Wassell 29 Jan '07

29 Jan '07

Sometimes happy outputs 'parE' when I run it on my .y file? I believe it is coming from a part of Grammer.lhs which has the comment line "This bit is a real mess, mainly because of the error message support." Are there any suggestions on what to change in my .y file to get over this? Thanks Mark

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Multiparamater class type-inference error
by Alfonso Acosta 29 Jan '07

29 Jan '07

Hi all, Sorry if this is too GHC-specific. I'm getting this strange instantiation error when compiling the following code with GHC 6.6: No instance for (Synchronous s ((a, b) -> a) (a, b) a) arising from use of `mapSY' at SynchronousLib.lhs:342:8-16 Possible fix: add an instance declaration for (Synchronous s ((a, b) -> a) (a, b) a) In the expression: mapSY fst In the definition of `fstSY': fstSY = mapSY fst --- data Signal a = NullS | a :- Signal a deriving (Eq) class Synchronous s f1 a b | f1 -> a , f1 -> b where mapSY :: f1 -> s a -> s b delaySY :: a -> s a -> s a instance Synchronous Signal (a->b) a b where mapSY _ NullS = NullS mapSY f (x:-xs) = f x :- (mapSY f xs) delaySY e es = e:-es fstSY = mapSY fst {-- No instance for (Synchronous s ((a, b) -> a) (a, b) a) arising from use of `mapSY' at SynchronousLib.lhs:342:8-16 Possible fix: add an instance declaration for (Synchronous s ((a, b) -> a) (a, b) a) In the expression: mapSY fst In the definition of `fstSY': fstSY = mapSY fst Failed, modules loaded: AbsentExt, Vector, Queue, Signal. --} -- No error arises if fstSY is declared as fstSY a = mapSY fst a Is it GHC bug or am I supposed to declare it like that to assist type inference? Thanks in advance, Alfonso Acosta

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Re: Extensible static checking of dimensions?
by Björn Buckwalter 28 Jan '07

28 Jan '07

Mike Gunter <m <at> ryangunter.com> wrote: | The very nice Buckwalter and Denney dimensional-numbers packages both | work on a fixed set of base dimensions. This is a significant | restriction for me--I want to avoid adding apples to oranges as well | as avoiding adding meters to grams. Is it possible to have an | extensible set of base dimensions? If so, how usable can such a | system be made? Is it very much worse than a system with a fixed set | of base dimensions? Mike, I apologize for not having replied to your message earlier, I have not been subscribing to the café and only recently noticed your post in the archives. As for you question, please see the literate haskell module below. It allows you to extend the set of SI base dimensions in 'Buckwalter.Dimensional' with an arbitrary number of extra dimensions. My hope is that it lifts the significant restriction you saw previously. As for the usability of such a system, once you have set up the dimensions relevant to your problem domain usage should be identical to (and seamless with) the original base dimensions. You will have to be the judge as to whether it is usable enough for you. I would certainly appreciate comments on usability. In fact I would also be interested to hear how you envision applying the functionality you requested, regardless of this modules usefulness. Also, I can't promise that the module is rock solid -- I haven't done a whole lot of testing. If you don't have the 'Buckwalter.NumTypes' and 'Buckwalter.Dimensional' modules already you can download the latest tarball with all three modules from the project web site at: http://code.google.com/p/dimensional/ Thanks, Björn Buckwalter ~~~~~~~~~~ BEGIN 'Buckwalter/Dimensional/Extensible.lhs' ~~~~~~~~~~ Buckwalter.Dimensional.Extensible -- Extensible physical dimensions Bjorn Buckwalter, bjorn.buckwalter(a)gmail.com License: BSD3 = Summary = On January 3 Mike Gunter asked[1]: | The very nice Buckwalter and Denney dimensional-numbers packages | both work on a fixed set of base dimensions. This is a significant | restriction for me--I want to avoid adding apples to oranges as | well as avoiding adding meters to grams. Is it possible to have | an extensible set of base dimensions? If so, how usable can such | a system be made? Is it very much worse than a system with a fixed | set of base dimensions? In this module we facilitate the addition an arbitrary number of "extra" dimensions to the seven base dimensions defined in 'Buckwalter.Dimensional'. A quantity or unit with one or more extra dimensions will be referred to as an "extended dimensional". = Preliminaries = Similarly with 'Buckwalter.Dimensional' this module requires GHC 6.6 or later. > {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} > module Buckwalter.Dimensional.Extensible > where > import Prelude hiding > ((*), (/), (+), (-), (^), sqrt, negate, pi, sin, cos, exp) > import qualified Prelude as P ((*), sin, cos, exp) > import Buckwalter.NumType (NumType, Add, Sub, Halve, Negate, Zero, Pos1) > import Buckwalter.Dimensional hiding (square, cubic, sin, cos, exp) = DExt, Apples and Oranges = We define the datatype 'DExt' which we will use to increase the number of dimensions from the seven SI base dimensions to an arbitrary number of dimensions. We make 'DExt' an instance of 'Dims' allowing us to use the 'Dimensional' type without change. > data (NumType n, Dims d) => DExt n d > instance Dims (DExt n d) Using 'DExt' we can define type synonyms for extended dimensions applicable to our problem domain. For exampel, Mike Gunter could define the 'Apples' and 'Oranges' dimensions and the corresponding quantities. ] type DApples = DExt Pos1 (DExt Zero DOne) ] type DOranges = DExt Zero (DExt Pos1 DOne) ] type Apples = Quantity DApples ] type Oranges = Quantity DOranges And while he was at it he could define corresponding units. ] apple :: Num a => Unit DApples a ] apple = Dimensional 1 ] orange :: Num a => Unit DOranges a ] orange = Dimensional 1 = Arithmetic = We get negation, addition and subtracton for free with extended dimensionals. However, we will need instances of the 'Mul', 'Div' and 'Sqrt' classes for the corresponding operations to work. > instance (Add n n' n'', Mul d d' d'') > => Mul (DExt n d) (DExt n' d') (DExt n'' d'') > instance (Sub n n' n'', Div d d' d'') > => Div (DExt n d) (DExt n' d') (DExt n'' d'') > instance (Halve n n', Sqrt d d') => Sqrt (DExt n d) (DExt n' d') Now, in order to work seamlessly with the quantities and units defined in 'Buckwalter.Dimensional' we must be able to automatically extend their dimensions when multiplying or dividing by an extended dimensional. > instance (Mul d (Dim l m t i th n j) d') => Mul (DExt x d) > (Dim l m t i th n j) > (DExt x d') > instance (Mul (Dim l m t i th n j) d d') => Mul (Dim l m t i th n j) > (DExt x d) > (DExt x d') > instance (Div d (Dim l m t i th n j) d') => Div (DExt x d) > (Dim l m t i th n j) > (DExt x d') > instance (Div (Dim l m t i th n j) d d', Negate x x') > => Div (Dim l m t i th n j) > (DExt x d) > (DExt x' d') The above instances also enable taking integer powers of extended dimensionals. To allow dimensionless quantities to be raised to the power of any other dimensionless quantity (where either or both of the quantities may be an extended dimensional) some new 'Power' instances are required. > instance (Floating a, Power a d (Dimensionless a) DOne) > => Power a (DExt Zero d) (Dimensionless a) DOne > where (Dimensional x) ^ (Dimensional y) = Dimensional (x ** y) > instance (Floating a, Power a DOne (Quantity d a) DOne) > => Power a DOne (Quantity (DExt Zero d) a) DOne > where (Dimensional x) ^ (Dimensional y) = Dimensional (x ** y) > instance (Floating a, Power a d (Quantity d' a) DOne) > => Power a (DExt Zero d) (Quantity (DExt Zero d') a) DOne > where (Dimensional x) ^ (Dimensional y) = Dimensional (x ** y) Note that the extra dimensions are dropped from the result. This is acceptable since they will be added again as necessary when multiplying or dividing by extended dimensionals. = Elementary functions, 'square' and 'cubic' = In 'Buckwalter.Dimensional' the elementary functions where restricted to dimensionals with the physical dimension 'DOne'. The same applies to the 'square' and 'cubic' functions and the 'DLength' dimension respectively. Unfortunately this implementation is inflexible in that it will not accomodate extended dimensionals even if they have no extend into the extra dimensions (i.e. the powers are all 'Zero'). To solve this problem we must provide new, generalized implementations of such functions. We start by defining the class 'BaseDim' which relates extended dimensions to the corresponding base dimension if applicable. An extended dimension has a corresponding base dimension if the powers of all extra dimensions are 'Zero'. Two 'BaseDim' instances are required. > class BaseDim d d' | d -> d' > instance BaseDim (Dim l m t i th n j) (Dim l m t i th n j) > instance (BaseDim d d') => BaseDim (DExt Zero d) d' Using 'BaseDim' in the constraints we can now redefine the elementary functions. As with the 'Power' instances above we drop the extra dimensions from the results. > sin, cos :: (Floating a, BaseDim d DOne) => Quantity d a -> Dimensionless a > sin (Dimensional x) = Dimensional (P.sin x) > cos (Dimensional x) = Dimensional (P.cos x) > exp :: (Floating a, BaseDim d DOne) => Quantity d a -> Dimensionless a > exp (Dimensional x) = Dimensional (P.exp x) Ditto for 'square' and 'cubic'. > square :: (Num a, BaseDim d DLength) => Unit d a -> Unit DArea a > square (Dimensional x) = Dimensional (x P.* x) > cubic :: (Num a, BaseDim d DLength) => Unit d a -> Unit DVolume a > cubic (Dimensional x) = Dimensional (x P.* x P.* x) (As a side note we could probably use 'BaseDim' to clean up the constraints and/or reduce the instances of 'Power' above, and perhaps for 'Mul' and 'Div'.) = References = [1] http://www.haskell.org/pipermail/haskell-cafe/2007-January/021069.html

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Re: Haskell Weekly News: January 02, 2007
by Björn Buckwalter 28 Jan '07

28 Jan '07

Henning Thielemann <lemming <at> henning-thielemann.de> writes: > On Tue, 2 Jan 2007, Donald Bruce Stewart wrote: > > > Dimensional: Statically checked physical dimensions. Björn Buckwalter > > [4]announced version 0.1 of [5]Dimensional, a module for statically > > checked physical dimensions. The module facilitates calculations with > > physical quantities while statically preventing e.g. addition of > > quantities with differing physical dimensions. > > > > 4. http://article.gmane.org/gmane.comp.lang.haskell.general/14691 > > 5. http://code.google.com/p/dimensional/ Henning, First, let me apologize for not answering earlier. I have been reluctant to subscribe to the café due to the volume of messages. Instead I tend to occasionally browse the archives. Needless to say your questions eluded me until now. The same is true for Mike Gunter's message[1] which I will respond to presently. > How is it related to this one: > http://www.haskell.org/haskellwiki/Dimensionalized_numbers > ? I ashamedly admit that I am guilty of inventing my own wheel. I haven't looked too closely at Aaron's code but I believe there are some conceptual similarities. Regarding completeness user my library supports all seven base dimensions while Aaron's library currently supports only length and time (I believe it is meant as a proof of concept (toy example in his own words) rather than a complete library). My ambition is to provide a reasonably complete library. > It should certainly be mentioned on > http://www.haskell.org/haskellwiki/Physical_units > http://www.haskell.org/haskellwiki/Libraries_and_tools/Mathematics#Physical… I have added it to the above pages. Thanks, Björn Buckwalter [1] http://www.haskell.org/pipermail/haskell-cafe/2007-January/021069.html

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Converting Types to Terms
by Klaus Ostermann 28 Jan '07

28 Jan '07

I would like to have a program that can synthesize programs for a given type, composing only functions from a given library. For example, suppose my library has isZero :: Int -> Bool map :: (a -> b) -> [a] -> [b] and :: Bool -> Bool -> Bool fold :: (a -> b -> a) -> a -> [b] -> a True :: Bool (.) :: (b -> c) -> (a -> b) -> a -> c then I want to ask, say, for a program of type [Int] -> Bool and get as answer (fold and True) . (map isZero) I have found two approaches in this direction. The first one is the De-Typechecker http://www.haskell.org/pipermail/haskell/2005-March/015423.html and the other one is Djinn http://permalink.gmane.org/gmane.comp.lang.haskell.general/12747 However, with none of these approaches I managed to do anything with list functions. What else is available (besides Djinn and De-Typechecker)? Are lists a problem? In general, what are the practical and theoretical limits of these program synthesizers? Are there any overview papers for this topic? Regards, Klaus

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