- Glasgow-haskell-users

unfold deforestation
by Carsten Schultz 02 Sep '04

02 Sep '04

[If this should have gone to a different list, please tell me and I will subscribe to that.] Hi! There is no rule in standard libs related to unfoldr. From short googling I know that there may be several approaches to this, but as long as nothing fancy is introduced, the following might be a simple improvement: unfoldr :: (b -> Maybe (a,b)) -> b -> [a] unfoldr u x = build (g x) where g x c n = f x where f x = case u x of Just (h,t) -> h `c` f t Nothing -> n {-# INLINE unfoldr #-} It makes unfoldr a good producer and works nicely with the following example from `The Under-Appreciated Unfold' (Gibbons&Jones): === module Tree(Tree(..), bftf) where import Unfold data Tree a = Nd a [Tree a] root (Nd r _) = r kids (Nd _ ks) = ks bftf = concat . levelf levelf = unfold null (map root) (concat . map kids) unfold :: (b -> Bool) -> (b -> a) -> (b -> b) -> b -> [a] unfold p h t = unfoldr u where u x | p x = Nothing | otherwise = Just (h x, t x) === Greetings, Carsten -- Carsten Schultz (2:38, 33:47), FB Mathematik, FU Berlin http://carsten.codimi.de/ PGP/GPG key on the pgp.net key servers, fingerprint on my home page.

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Marshalling Haskell String <-> UTF-8
by Bayley, Alistair 02 Sep '04

02 Sep '04

I want to call a foreign C function that takes a UTF-8 encoded string as one of its arguments (and there's also a version of the function that receives UTF-16). Can someone point me to documentation or examples of how this would be done? AFAICT (reading the FFI spec) marshalling a String to a CString is locale-dependent, whereas I know that I want UTF-8/16. Also, if a C function returns a UTF-8 (or UTF-16) encoded string, how do I marshall this reliably into a Haskell String? Can I use the UTF-16 functions directly with CWStrings? (I'm not sure exactly what wchar_t is, as it's apparently dependent on the locale at compile-time, and could be 8, 16, or 32 bits). Thanks, Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: Marshalling Haskell String <-> UTF-8
by Bayley, Alistair 01 Sep '04

01 Sep '04

> From: George Russell [mailto:ger@informatik.uni-bremen.de] > > http://www.haskell.org//pipermail/glasgow-haskell-users/2004-April/006 > 564.html Thanks George, this looks useful. There are some things I want to clarify... module UTF8( toUTF8, -- :: String -> String -- Converts a String (whose characters must all have codes <2^31) into -- its UTF8 representation. fromUTF8WE, -- :: Monad m => String -> m String -- Converts a UTF8 representation of a String back into the String, -- catching all possible format errors. Does toUTF8 return a String whose Chars are all code-points < 256, which, when converted to bytes, will represent a UTF-8 string? Likewise, does fromUTF8WE expect a String whose Chars are all code-points < 256 i.e. they are the result of saying "chr n" for each byte in the UTF-8 stream? > From: Simon Marlow [mailto:simonmar@microsoft.com] > > In any case, none of this allows you to specify a UTF-8 conversion. Are there plans to add UTF-8 (and UTF-16) conversion functions to the libraries? I imagine they would be useful... > Your best bet is to marshal it yourself. We're a bit behind in this > area: 6.2.x doesn't have CAString and CWString, and CString is just > char*. The HEAD has CAString and CWString, and will hopefully follow > the FFI spec by the time we release 6.4 (we still have to do the > locale encoding/decoding between CString and String, IIRC). Again, I want to clarify some things... - will any encoding/decoding be performed by the peekCString/withCString/newCString(Len) functions? i.e. if I want to avoid encoding/decoding (because I know my string is already in UTF-8) then I simply have to avoid using these functions? - can I still declare the foreign functions with CString types without worrying that encoding/decoding might be attempted? - when the CAString functions are available then I should use them, but for now I will have to write something that uses castCharToCChar/castCCharToChar + peekArray0/pokeArray0. Thanks, Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: Marshalling Haskell String <-> UTF-8
by George Russell 01 Sep '04

01 Sep '04

I have implemented code to do this which I think is better than John Meacham's, because it (a) handles all UTF8 sequences (up to 6 bytes); (b) checks for errors as UTF8 decoders are supposed to do; (c) lets you determine if there is an error without having to seq the entire list. Here is a link: http://www.haskell.org//pipermail/glasgow-haskell-users/2004-April/006564.h…

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RE: Marshalling Haskell String <-> UTF-8
by Simon Marlow 01 Sep '04

01 Sep '04

On 01 September 2004 10:16, Bayley, Alistair wrote: > I want to call a foreign C function that takes a UTF-8 encoded string > as one of its arguments (and there's also a version of the function > that receives UTF-16). Can someone point me to documentation or > examples of how this would be done? AFAICT (reading the FFI spec) > marshalling a String to a CString is locale-dependent, whereas I know > that I want UTF-8/16. > > Also, if a C function returns a UTF-8 (or UTF-16) encoded string, how > do I marshall this reliably into a Haskell String? > > Can I use the UTF-16 functions directly with CWStrings? (I'm not sure > exactly what wchar_t is, as it's apparently dependent on the locale at > compile-time, and could be 8, 16, or 32 bits). Your best bet is to marshal it yourself. We're a bit behind in this area: 6.2.x doesn't have CAString and CWString, and CString is just char*. The HEAD has CAString and CWString, and will hopefully follow the FFI spec by the time we release 6.4 (we still have to do the locale encoding/decoding between CString and String, IIRC). In any case, none of this allows you to specify a UTF-8 conversion. wchar_t varies from platform to platform: on Windows it is 16 bits, on Linux with glibc it is 32 bits, for example. CWString is only useful for talking to C interfaces that are expressed in terms of wchar_t. Cheers, Simon

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HaXml 1.12 <-> HXmlToolbox 4.01 incompatibility?
by Jan Scheffczyk 30 Aug '04

30 Aug '04

Hi all, I try to use HaXml and HXmlToolbox together with GHC 6.2.1. I am, however, a bit confused since some versions seem to "influence" each other. So far I tried: HaXml 1.11 + HXmlToolbox 4.01 and HaXml 1.12 + HXmlToolbox 4.01. While HaXml 1.11 appears to work fine with HXmlToolbox 4.01, I get some strange errors for the combination 1.12 + 4.01 when using HXmlToolbox. You find examples below. Any ideas? Best regards, Jan --------------------------------------------------------- All examples below are for the combination HaXml 1.12 + HXmlToolbox 4.01: The TreeVisualization from HXmlToolbox: ghci -package hxt TreeVisualisation.hs Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Loading package lang ... linking ... done. Loading package unix ... linking ... done. Loading package posix ... linking ... done. Loading package network ... linking ... done. Loading package net ... linking ... done. Loading package parsec ... linking ... done. Loading package hxt ... linking ... done. Compiling Main ( TreeVisualisation.hs, interpreted ) TreeVisualisation.hs:11: Failed to find interface decl for `AssocList' from module `AssocList' TreeVisualisation.hs:11: Failed to find interface decl for `AssocList' from module `AssocList' tcLookup: `AssocList' is not in scope When checking kinds in `AssocList String String' In the type synonym declaration for `Attributes' Failed, modules loaded: none. ----------------- Invoking ghci with just -package hxt appears to work at first sight, but then: ghci -package hxt ... Loading package hxt ... linking ... done. Prelude> :b AssocList data FM k a = adjust :: forall a k. (Eq k) => (a -> a) -> k -> FM k a -> FM k a adjustAll :: forall a k. (Eq k) => (a -> a) -> k -> FM k a -> FM k a count :: forall k a. (Eq k) => FM k a -> k -> Int delete :: forall k a. (Eq k) => k -> FM k a -> FM k a deleteAll :: forall k a. (Eq k) => k -> FM k a -> FM k a deleteSeq :: forall seq :: (* -> *) k a. (Eq k, Sequence.Sequence seq) => seq k -> FM k a -> FM k a .... Strange, thats not the AssocList from HXmlToolbox (see below)! Prelude> :b HdomParser Failed to find interface decl for `AssocList.AssocList' from module `AssocList' Failed to find interface decl for `AssocList.addEntries' from module `AssocList' Failed to find interface decl for `AssocList.addEntry' from module `AssocList' Failed to find interface decl for `AssocList.delEntries' from module `AssocList' Failed to find interface decl for `AssocList.delEntry' from module `AssocList' Failed to find interface decl for `AssocList.hasEntry' from module `AssocList' Failed to find interface decl for `AssocList.lookup1' from module `AssocList' Failed to find interface decl for `AssocList.lookupDef' from module `AssocList' Failed to find interface decl for `AssocList.AssocList' from module `AssocList' Failed to find interface decl for `AssocList.AssocList' from module `AssocList' tcLookup: `AssocList.AssocList' is not in scope When checking kinds in `AssocList.AssocList GHC.Base.String GHC.Base.String' In the type synonym declaration for `XmlTreeTypes.Attributes' -------------------------------------------------------------------- Example for the combination HaXml 1.11 + HXmlToolbox 4.01 ghci -package hxt Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Loading package lang ... linking ... done. Loading package unix ... linking ... done. Loading package posix ... linking ... done. Loading package network ... linking ... done. Loading package net ... linking ... done. Loading package parsec ... linking ... done. Loading package hxt ... linking ... done. Prelude> :b AssocList type AssocList k v = [(k, v)] addEntries :: forall k v. (Eq k) => AssocList k v -> AssocList k v -> AssocList k v addEntry :: forall k v. (Eq k) => k -> v -> AssocList k v -> AssocList k v delEntries :: forall k v. (Eq k) => [k] -> AssocList k v -> AssocList k v delEntry :: forall k v. (Eq k) => k -> AssocList k v -> AssocList k v hasEntry :: forall k v. (Eq k) => k -> AssocList k v -> Bool lookup1 :: forall k e. (Eq k) => k -> AssocList k [e] -> [e] lookupDef :: forall v k. (Eq k) => v -> k -> AssocList k v -> v Prelude>

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RE: Problem with Unboxed Types
by Simon Peyton-Jones 27 Aug '04

27 Aug '04

Just to be sure, I've just compiled and run this code with GHC 6.2.1, on a Linux system. I can't account for your difficulty I'm afraid. Simon {-# OPTIONS -fglasgow-exts #-} module Main where import GHC.Exts showUnboxedInt :: Int# -> String showUnboxedInt n = (show $ I# n) ++ "#" main = print (showUnboxedInt 3#) | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell-users- | bounces(a)haskell.org] On Behalf Of Jorge Adriano Aires | Sent: 27 August 2004 02:55 | To: glasgow-haskell-users(a)haskell.org | Subject: Problem with Unboxed Types | | Hello, | I'd like to try using Unboxed types, but I always get a "parse error on input | `#'" error. To make sure I wasn't making some mistake I tried the example in | the wiki: | | http://www.haskell.org/hawiki/UnboxedType | -------------------------------------------------------- | module Main where | import GHC.Exts | | showUnboxedInt :: Int# -> String | showUnboxedInt n = (show $ I# n) ++ "#" | -------------------------------------------------------- | | Even tried adding fglasgow-exts, nothing works though. I'm using GHC 6.2.1 on | a SuSE 8.2 system (glibc 2.3) installed using the available RPMs. Any idea? | | Thanks in advance, | J.A. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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Problem with Unboxed Types
by Jorge Adriano Aires 27 Aug '04

27 Aug '04

Hello, I'd like to try using Unboxed types, but I always get a "parse error on input `#'" error. To make sure I wasn't making some mistake I tried the example in the wiki: http://www.haskell.org/hawiki/UnboxedType -------------------------------------------------------- module Main where import GHC.Exts showUnboxedInt :: Int# -> String showUnboxedInt n = (show $ I# n) ++ "#" -------------------------------------------------------- Even tried adding fglasgow-exts, nothing works though. I'm using GHC 6.2.1 on a SuSE 8.2 system (glibc 2.3) installed using the available RPMs. Any idea? Thanks in advance, J.A.

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Extensible Serialization class with type classes
by Simon David Foster 27 Aug '04

27 Aug '04

I've been trying to put together a type-class based serialization (XML) for types in GHC. Essentially the serializer class takes the form class Seriliazer a where serialize :: a -> XmlFilter This works very well, but I then wanted to make this system extensible, so that for example for types which are also typed by equivalent XSD Types (i.e. types with constraint XSDType a => a) could have their type-data encoded into the tree. The problem is, it seems to me there is no viable method of adding an extra parameter function to the serialiaze function, which will enforce extra constraints on a and allow recursive serialization. Obviously simply passing a function (C a => a -> XmlFilter) doesn't work, since the type a becomes unified to the top-level type and so can't be passed down. The second attempt was to add another type class called Hook, with two type variables; the first being a dummy type and the second the variable to represent the type being serialized; class Hook a t where hookAttr :: t -> a -> [(String, String)] hookElem :: t -> a -> [XmlFilter] and rewriting serialize as class Hook a t => Serializer a t where serialize :: t -> a -> XmlFilter Then, passing the dummy type to the serialized function as the t value would force the extra constraints from its Hook instance on a. This method though doesn't really work that well; first of all it requires undecidable-instances and second each new instance on serialize requires a humongous context since each type used directly and indirectly needs inferring as an instance of Hook. The idea was that this code could be generated automatically, and that really makes it too difficult to do. The Third attempt was to use existential types to encapsulate a constrained polymorphic type and use Typeable + Generics to choose between a number of functions. This though doesn't work either, since each value down the type tree needs to be of the appropriate monomorphic existentially quantified type since polymorphic types aren't Typeable. I seem to have pretty much exhausted every option for building this extensible Serializer, and so my question is this; is there anyway of passing a hook function to the serializer class instances, such that it will remain polymorphic down the serialization tree but will also enforce the constraints on the input parameter on the type being serialized. For example is it every likely to be possible to reify polymorphic types? -Si.

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RE: GHC and MPTCs
by Simon Peyton-Jones 26 Aug '04

26 Aug '04

Your problem is that next_state :: forall s v. VertexState s v => s -> s So an application of next_state to an argument of type sty will require the constraint VertexState sty v but that says NOTHING about v. It's a bit like calling a function with type forall a b. Foo a => b -> b Anyway, I think what you want is a functional dependency class VertesState s v | s -> v Then fixing s will fix v, which is what you want I think Simon | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell-users- | bounces(a)haskell.org] On Behalf Of Ketil Malde | Sent: 25 August 2004 14:45 | To: glasgow-haskell-users(a)haskell.org | Subject: GHC and MPTCs | | | Hi, | | I recently tried to do the following code: | | class VertexState s v where | new_point :: s -> v -> v -> v | next_state :: s -> s | | -- interleave :: (VertexState s a) => s -> [a] -> [a] -> [a] | interleave s (t:ts) (b:bs) = | new_point s t b : interleave (next_state s) bs (t:ts) | interleave _ [] [b] = [] | | Firstly, GHC doesn't deal all too gracefully with this if the type | signature is uncommented, it tells me: | | Could not deduce (VertexState s v) | from the context (VertexState s a) | arising from use of `next_state' at Grid.lhs:25 | Probable fix: | Add (VertexState s v) to the type signature(s) for `interleave' | | Unfortunately, doing the probable fix just results in a new request | for (VertexState s v1), and so on. | | I suppose the real problem is that GHC can't decide from next_state | that it is supposed to use exactly this instance of VertexState, is | there a good way to do that without adding an explicit dummy parameter | of type v? | | (I really would like to be able to have instances with the same s but | different v's, which I think adding functional deps would make | difficult.) | | -kzm | -- | If I haven't seen further, it is by standing in the footprints of giants | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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