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A question about dynamic typing
by Nick Name 18 Jan '03

18 Jan '03

Or maybe the subject has nothing to do with my question :) I was thinking of an "haskell desktop interface", i.e. just a desktop interface, but written in haskell. I thought this: one can define a type class with the "view" method: data Draw -- the drawing monad class Visible t where view :: t -> Time -> Draw And so one can have a list of viewable objects and by mapping some "runDrawIntoIO" function wich spawns a thread for each element of a list, obtain animated icons and applets where one expects just icons. If I use the "Visible" class, I can use existential types to create a non-homogeneous list and "view" all the objects. The same thing could be achieved, of course, with record types: data Visible = { view :: Time -> Draw } and I can achieve the same result I get with existential types. My problem is: an user putting someway different "visible" elements in a list (wich could be represented in a side panel) will want to retrieve an object with its full type when clicking on it (to be able to perform specific applications), but this isn't allowed by the type system. It's not a problem of functional programming: even in object oriented programming I see the same problem if one doesn't use some form of typecast. Where am I wrong? Are there alternatives? Vincenzo

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non-question, instance decl
by ketil＠ii.uib.no 14 Jan '03

14 Jan '03

Hi, experimenting with QuickCheck, I write > instance (Arbitrary e) => Arbitrary (Array Int e) where > arbitrary = undefined and I get: | Illegal instance declaration for `Arbitrary (Array Int e)' | (The instance type must be of form (T a b c) | where T is not a synonym, and a,b,c are distinct type variables) | In the instance declaration for `Arbitrary (Array Int e)' |Failed, modules loaded: QuickCheck. Okay, after experimenting a bit (more fun than reading documentation) I see what's happening: I can't instantiate a halfway specialized type, but 'Ix i => Arbitrary (Array i e)' is Okay. Unfortunately, I really depend on i being Int in my (real) definition of 'arbitrary'. I got around it by liberal doses of 'fromInteger' and a 'Num' context -- ugly, but it seems to work. I suppose there's a solid reason why my previous attempt is illegal? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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A newbie question about Arrows.
by Nicolas Oury 14 Jan '03

14 Jan '03

Hello, I read some of the material about Arrows on www.haskell.org/arrows and I have some questions : * Why is made the choice to use (,) as Cartesian in first? Can't we write something like : class Cartesian p where pair :: a -> b -> (p a b) projLeft :: (p a b) -> a projRight :: (p a b) -> b class Cartesian pair => Arrow ar where arr ::( ar pair a b) -> (ar pair a b) (>>>) :: (ar pair a b) -> (ar pair b c) -> (ar pair a c) first :: (ar pair a b) -> (ar (pair a d) (pair b d)) (The same could be said for ArrowChoice) I think this could be more powerful and could allow a smarter management of inputs and outputs of arrows than tuples. I think for example to event-driven arrows : we could make a pair of inputs without mixing the "event happened" information. * Why is made the choice of first as being a base function? We have first f = f *** (arr id) second f = (arr id) *** f So they could be define from (***). Moreover, the definition of f *** with first and second creates an order in the use of the f and g (first f >>> second g) whereas it seems that (***) is a "parallel" operator. In an event-driven arrow we have to give the same events to f and g even if they are independent. Best regards, Nicolas Oury

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Re: Web Application Development
by Dominic Steinitz 11 Jan '03

11 Jan '03

I have summarised the responses I have received and cross posted this to Haskell-Cafe in case anyone wishes to carry on the conversation. > In a moment of weakness, I offered to help a friend develop a website. It's > a simple database application which carries out very little calculation and > then displays the results via HTML. > > The original code was in PHP and MySQL so rather than re-invent the wheel I > > So my question is: what should I do to re-write this in Haskell? > 1. SQL 1.a. MySQL 1.a.i Use MySQL-HS which requires the use of H/Direct. Available via www.haskell.org. 1.a.ii Use http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/hbase/Source/Foreign/. This is pre-alpha. 1.b Oracle 1.b.i Use C2HS and libsqlora8 (a simple C library for interfacing to Oracle http://www.poitschke.de/libsqlora8/) There is no publicly available code. 1.c Database Independent 1.c.i Use the DBI model as found in Perl and the libpq binding as listed on Haskell.org (http://www.astercity.net/~khaliff/haskell/dbi/dbi.tgz) This currently has a PostgreSQL binding but is not listed on SourceForge. 1.c.ii Use the code at http://sourceforge.net/projects/htoolkit/. This is planning (that is pre pre-alpha). There appear to be bindings for MySQL, ODBC and PostgreSQL. 2. HTML I have tried out Wash/CGI and this seems satisfactory for my purposes at the moment. Many thanks to everyone who replied, Dominic.

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Re: Interpret haskell within haskell.
by cwitty＠newtonlabs.com 10 Jan '03

10 Jan '03

Matt Hellige <matt(a)immute.net> writes: > So it seems natural, especially to new users, to hope that Haskell and > the ML variants, sharing many other features common to these > languages, would offer this facility as well. The answer, in general, > is that they do not. (Sorry for the belated reply.) Evidently, some ML variants do include eval or eval-like facilities. When reading through the sources of the Isabelle 2002 proof verification system, I noticed the "use_text" eval-like function which is implemented using the following system-specific functions: smlnj-0.93: System.Compile.use_stream polyml: PolyML.Compiler mlworks, mosml: use Carl Witty

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newbie questions
by Max Ischenko 09 Jan '03

09 Jan '03

Hello! I'm started to learn Haskell and have some questions. After reading a "Gentle introduction..." I've decided to write a simple program -- a complement to Literate.hs from hugs demos which reads a plain haskell source file and converts it to literate haskell source. The source code of this program follows. Now the questions: - To be able to run it both in Hugs and by GHC I need an `import Data.Char(isSpace) for GHC`. Can this be conditionally included in a source for GHC only? - Am I reinventing the wheel with my `pad` function? Could I use some standard function(s) instead? - What if I'd want to do a more complex processing, like handling {--} comments? The `classify` function analize one line at a time and I'd want too see a surronding context? Should I use another pass, just like for `addblanks`? module Lit where import System(getArgs) -- classification of the haskell source lines data Classified = Program String | Blank | Comment String deriving (Show) classify :: String -> Classified classify ('-':'-':xs) = Comment (dropWhile isSpace xs) classify s | all isSpace s = Blank | otherwise = Program s unclassify :: Classified -> String unclassify (Program s) = "> " ++ s unclassify (Comment s) = s unclassify Blank = [] process :: String -> String process hs = unlines (map unclassify (addblanks (clines hs))) clines hs = map classify (lines hs) -- inserts a blank between a program line and a comment line where needed addblanks :: [Classified] -> [Classified] addblanks = pad needBlank Blank needBlank :: Classified -> Classified -> Bool needBlank (Program p) (Comment c) = True needBlank (Comment c) (Program p) = True needBlank _ _ = False -- inserts a value between those adjanced elements for which -- function p returns True pad :: (a -> a -> Bool) -> a -> [a] -> [a] pad p value [] = [] pad p value (x:[]) = [x] pad p value (x:xs) | p x (head xs) = x:value:rest | otherwise = x:rest where rest = pad p value xs usage err = err ++ "\n" ++ "USAGE: lit <filename>" main :: IO () main = do strs <- getArgs case strs of [] -> ioError (userError (usage "Not enough arguments.")) [str] -> mklit str _ -> ioError (userError (usage "Too many arguments.")) -- takes a filename without suffix, reads a plain haskell source -- and creates a literate haskell source. mklit :: String -> IO () mklit f = do hs <- readFile (f ++ ".hs") writeFile (f ++ ".lhs") (process hs) -- Bst rgrds, M.A.X.: Mechanical Artificial Xenomorph.

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Re: A newbie question about Arrows.
by Ashley Yakeley 09 Jan '03

09 Jan '03

At 2003-01-06 03:14, Ross Paterson wrote: >class PreArrow ar where > arr :: (a -> b) -> ar a b > (>>>) :: ar a b -> ar b c -> ar a c ... > class (PreArrow ar, Monoidal p u) => GenArrow ar p u where > first :: ar a b -> ar (p a c) (p b c) My own preference is something like this: class (PreArrow ar) => GenArrow' ar where arrApply :: ar p (q -> r) -> ar p q -> ar p r My GenArrow' is actually equivalent to Hughes' Arrow and I assume your GenArrow. In an ideal world, I could leverage my FunctorApply class to have something like this: class (Functor f) => FunctorApply f where -- first arg (fab) "executed" first fApply :: f (a -> b) -> (f a -> f b) -- first arg (fa) "executed" first fPassTo :: f a -> f (a -> b) -> f b fPassTo fa fab = fApply (fmap (\a ab -> ab a) fa) fab class (PreArrow ar,forall p. FunctorApply (ar p)) => GenArrow'' ar Unfortunately, GHC does not yet allow this kind of superclassing. <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hbase/Source/HBa se/Category/Functor.hs?rev=HEAD&content-type=text/plain> <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hbase/Source/HBa se/Category/Arrow.hs?rev=HEAD&content-type=text/plain> -- Ashley Yakeley, Seattle WA

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OT: Project Idea: Aristo - A Framework for Embedding and Integrating High-Level Languages
by Shlomi Fish 08 Jan '03

08 Jan '03

I just returned from a dinner a few members of the Linux Israeli scene had with Richard Stallman (of GNU fame), as he his now visiting Israel. I got the opportunity to talk with him, and asked him the GNU project insistance of using guile for extensibility. He said the reason for that was that converting any other language to Scheme was a good a choice out of the fact Scheme was so simple, powerful and generic. I said converting Perl 5 code to Scheme would not be too trivial at all, and he said that was the case for converting Perl to almost everything else. :-) (despite this joke, I'm sure converting many languages to Scheme would be incredibly problematic) Then I noted that they should try the GIMP approach instead: and use a procedural database to integrate several languages back-ends. Then, he said that the type system would not be flexible enough for that. After a while that it was a challenge: create a bridge in ANSI C, that will be flexible enough to interoperate code between many virtual machines: perl 5, python, Ruby, Java, Haskell, O'Caml, etc. The codename is dubbed "Aristo", which is the Hebrew pronouncation and spelling of "Aristotle". (it's simpler and without the "totel" at the end). Now what I need is some advice on designing a type system: OK, I can have arrays of ints, and associative arrays from strings to ints, etc. But an array of records whose one field is an array of ints, the other a linked list of strings, the other an associative array from floats to this record itself, is something I'd rather not have to do type interference with. (usually you don't need such strict typing requirements). Here are some of my ideas, so far. Let me know what you think: 1. Aristo can only be written in ANSI C. Any other language will be unacceptable, as all implementations of the back-ends are written in ANSI C. 2. Aristo should have a nice nested namespace organization, with import and export features, package-scope globals etc. The gimp flat list (in which namespace collision is maintained by prefixes) is not scalable. 3. There should be a union type which is a type identifier followed by the type value, which could assume any value. It is possible some types will not be unions and will be plain types without the type value. 4. A standard enum can be used to store the object meta-type, and a standard pointer/long union can be used to store the data. Almost anything will be done using indirection. 5. Types should be declared using a machine-readable type definition, that can be understood by humans. 6. Maybe I can take a look at GIMP PDB for code and ideas, but probably not much. From my impression it's more of an ad-hoc system for the GIMP. 7. A procedure can restrict the types that pass to it, but I'm not sure to what extent. 8. I'll have to translate between conventions of different languages inside each language back-end. 9. It's first implementation will probably not have all the functionality I'd like it to have, but better to release early and release often. Your ideas and comments are most welcome. At the moment Aristo is at the pre-coding stage. (I'm just thinking about its design). But I'd rather have something well-designed and well-scrutinized in mind before I start actual coding. Regards, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif(a)vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ He who re-invents the wheel, understands much better how a wheel works.

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Ambiguous defaults
by Ferenc Wagner 08 Jan '03

08 Jan '03

Hello, What's the way to express the following: a compound object is generally made up of two components with identical type. For this common case I'd like to provide some default methods, which take the object apart, operate on the parts and put the results back together. In other cases this default method should be overridden, eg. when the compound object isn't made of two identical components. My first try (below) doesn't work, can you recommend a solution? Thanks: Feri. \begin{code} module Test where class Component b where property :: b -> Int class Compound a where decompose :: Component b => a -> (b,b) additive :: a -> Int additive x = property l + property r where (l,r) = decompose x \end{code} ghci: test.hs:9: Ambiguous type variable(s) `b' in the constraint `Component b' arising from use of `property' at test.hs:9 hugs: ERROR test.hs:9 - Cannot justify constraints in default member binding *** Expression : additive *** Type : Compound a => a -> Int *** Given context : Compound a *** Constraints : Component b

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RE: Space explosion
by Simon Marlow 06 Jan '03

06 Jan '03

> > -- Specs contains the options for the simulation; > rollBounds simply > finds the lowest and highest possible rolls > > -- type RollValue = Int > > -- type RollValueDist = Array RollValue Int > > probDist :: Specs -> [RollValue] -> RollValueDist > > probDist specs rolls = > > accumArray (+) 0 (rollBounds specs) [(r, 1) | r <- rolls] > > It should be doing an in-place array update as it evaluates the > comprehension, but apparently something's being kept in memory longer > than it has an excuse for. It does do a sequence of in-place updates, but this is another instance of the foldl space leak. The additions aren't being performed strictly, so the array is filling up with suspensions. For example: accumArray (+) 0 (1,10) [(3,1), (3,1), (3,1), ...] would produce this sequence of arrays as accumArray processed its input list: { (3,1), ... } { (3,1+1), ... } { (3,1+1+1), ... } we should really have a strict version of accumArray (the lazy version *is* useful sometimes: you can refer recursively to the final array, for example). A quick workaround in your case (GHC only, I'm afraid) is to use UArray instead, available from Data.Array.Unboxed. These arrays contain unboxed values and are therefore strict by definition; and you can use all the usual array operations including accumArray. This workaround only applies if you're using one of the types supported by UArray though (Int, Char, and other base types). Cheers, Simon

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