June 2005 - Haskell-Cafe

Processing a file with HaXml without invoking processXmlWith?
by Echo Nolan 02 Jun '05

02 Jun '05

Hi all, I've an application which needs to process some XML, but the only way I can see in the Haddock-generated documentation is using the processXmlWith wrapper, which reads it's input and output arguments from the command line. I'd prefer a pure function which took a Document or a String and returned the same, processed. I hope I've overlooked something. Regards, Echo Nolan

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LC online references
by Matthew Roberts 02 Jun '05

02 Jun '05

Hi, I hope I am not too OT here. I am in the process to coming to terms with various calculi based on the lambda calculus. I end up returning to LC books all the time to put things in context. However, I have taken to living out of my laptop and certainly can't carry around the laptop, the book I am reading plus Barendregt's book (plus my phone, plus ....). Besides, I can't afford to buy any books and the library is going to want it back sometime. Does anyone have any pointers to useful online references for the LC? Matt

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RE: Re[2]: [Haskell-cafe] ghc 6.4 import problem
by Simon Marlow 02 Jun '05

02 Jun '05

On 01 June 2005 07:24, Bulat Ziganshin wrote: > Hello Fergus, > > Tuesday, May 31, 2005, 10:24:41 PM, you wrote: > >>> import Data.Set hiding (map) >>> import qualified Data.Set as Set >>> >>> will do fine. > >> That code only compiles with ghc 6.4, and won't compile with ghc 6.2: >> you'll get an error for the "hiding (map)" part, because in 6.2 >> Data.Set does not contain a "map" function. > > btw, question to Simon Marlow and Simon PJ - how about disabling this > error message? You mean allowing import M hiding (x) even if x is not exported by M? Or making it a warning? That's certainly possible, but it constitutes an extension to Haskell 98, so it would at least require the -fglasgow-exts flag. I think you're better off keeping your code portable. Cheers, Simon

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G machine in FORTH
by Andrew Harris 02 Jun '05

02 Jun '05

Hi - Brace yourself... I work in an environment where FORTH is still used. I've been thinking about writing a G-machine interpreter in FORTH so that one could write Haskell like programs that would compile down and run "graph-reduction" style on the FORTH machine. Many developers think FORTH is nice, but the language is so, shall we say, "terse". I'm curious about what people think about this; having the expressiveness of a Haskell-like language that compiles to this environment might provide the best of both worlds, simple hardware architecture and an advanced programming language. let me know what you think, -andrew

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Type extensions
by Thomas Davie 02 Jun '05

02 Jun '05

Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not... Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. So I would eno up with three data types data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag data MyBwTaggedTree = Branch BwTag Int MyBwTaggedTree MyBwTaggedTree | Node BwTag Int and several functions to move from one to another. (Or define the most complex and not always use all the attrdbutes). What I would prefer is to be able to spocify something like: data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree extends MyTree with BwTag=True data MyBwTaggedTree extends MyBwTree with Int=0 Specifying that myBwTree is the same as MyTree but with an extra argument on each node, and to generate functions to translate between them that fill in True as a default value for the extra tag. Is this possible? Thanks Tom Davie

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Wash broken with GHC 6.4
by John Goerzen 01 Jun '05

01 Jun '05

Hi, I'm trying to use Wash with GHC 6.4. I have applied the patch from http://article.gmane.org/gmane.comp.lang.haskell.libraries/3160, and now it at least compiles. However, despite using -package WASH -package WASH-CGI -package WASHHTML, ghc is not automatically sending the input files through wash2hs. GHC 6.2 did this. Incidentally, is Wash dead upstream? I am surprised that there is no upstream version yet that even tries to work with GHC 6.4. -- John

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ghc 6.4 import problem
by Shiqi Cao 01 Jun '05

01 Jun '05

Hi, I tried to port some code from ghc 6.2 to ghc 6.4, but I got the following error PrelExts.lhs:41:25: Ambiguous occurrence `map' It could refer to either `GHC.Base.map', imported from Data.List at PrelExts.lhs:11:0-15 or `Data.Set.map', imported from Data.Set at PrelExts.lhs:10:0-14 The following is the first part of the code module PrelExts where import Data.FiniteMap import Data.Set import Data.List import IO There is no problem under ghc 6.2. What should I do? Shiqi Cao

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Simulating OO programming with type classes; writing a factory fu nction
by Bayley, Alistair 01 Jun '05

01 Jun '05

I've been experimenting with simulating OO programming in Haskell (why, you might ask? Because I'm trying to port a non-trivial piece of software, and I want to respect the original design, at least to begin with). I've been trying the technique illustrated by Ralf Laemmel: http://homepages.cwi.nl/~ralf/OOHaskell/src/interpreter/extensible.hs There's a small problem: how to write a factory function that returns values of various subtypes. The makeSubType function below won't compile, obviously because the returns types are different (they're not the same 'm'). I realise that I'm looking for something that Haskell doesn't natively do, so my question is: is there some kind of workaround that'll give me the ability to write factory functions? (No doubt there's some obvious trick that I've missed :-) I'm aware of http://homepages.cwi.nl/~ralf/OOHaskell/ , which will be my technique of last resort. I was fond of Ralf's extensible interpreter example because it's quite lightweight, but if there's no way around the factory problem, so be it. Alistair. ------------------------------------------- -- Toy OO example with subtyping module Main where main = print "boo" -- class to represent OO base class class BaseClass c -- each subtype of BaseClass is a separate datatype data SubBase1 = SubBase1 Int data BaseClass c => SubBase2 c = SubBase2 c instance BaseClass SubBase1 instance BaseClass (SubBase2 c) -- methods of BaseClass defined with new class class BaseClass c => Method c where -- illegal: can't return values of type c; can only consume them method1 :: c -> Int -> c method2 :: c -> Int -- install each subtype (datatype) as instance of Method instance Method SubBase1 where method1 _ i = SubBase1 i method2 (SubBase1 i) = i instance Method c => Method (SubBase2 c) where -- This one fails, because must return polymorphic value, not concrete --method1 _ i = SubBase2 (SubBase1 5) method1 x i = x method2 x = 2 -- Also, cannot make factory function: --makeSubType :: (Method m) => String -> m makeSubType s = if s == "SubBase1" then SubBase1 3 else SubBase2 (SubBase1 4) ----------------------------------------- ***************************************************************** 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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