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Today's Topics:
1. help with a class definition (Rustom Mody)
2. Re: help with a class definition (Daniel Fischer)
3. Multi-parameter type classes (David Beacham)
4. Re: Multi-parameter type classes (Stephen Tetley)
5. Re: Cabal on mac (Tim Pizey)
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Message: 1
Date: Mon, 28 Nov 2011 19:51:56 +0530
From: Rustom Mody <[email protected]>
Subject: [Haskell-beginners] help with a class definition
To: Beginners Haskell <[email protected]>
Message-ID:
<CAG3g5nkHPScLk3rVHiPKNc5Bz+ZLcKY0Oy9id=gbj7mpxw3...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
The Data.Vector.Unboxed page on hackage has the following line
class (Vector Vector a, MVector MVector a) => Unbox a
Can someone explain whats the repeating Vector (and MVector) there?
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Message: 2
Date: Mon, 28 Nov 2011 16:01:27 +0100
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] help with a class definition
To: [email protected]
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="utf-8"
On Monday 28 November 2011, 15:21:56, Rustom Mody wrote:
> The Data.Vector.Unboxed page on hackage has the following line
>
> class (Vector Vector a, MVector MVector a) => Unbox a
>
> Can someone explain whats the repeating Vector (and MVector) there?
Vector and MVector both appear as the names of two different things here.
The first occurrence is as the name of the (two-parameter) class,
class [M]Vector v a where ...
and the second occurrence is as the name of a datatype constructor
data [M]Vector a = [M]Vector ...
In instance declarations, the name can also appear as the name of a third
thing, the value constructor ;)
------------------------------
Message: 3
Date: Mon, 28 Nov 2011 19:35:11 +0000
From: David Beacham <[email protected]>
Subject: [Haskell-beginners] Multi-parameter type classes
To: Beginners Haskell <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi all,
I've been reading through Okasaki's purely functional data structures
and noticed that the source he provides for the Set implementations uses
multi-parameter type classes, whereas those for the Heap structures does
not.
class Set s a where
empty :: s a
insert :: a -> s a -> s a
...
class Heap h where
empty :: Ord a => h a
insert :: Ord a => a -> h a -> h a
...
The only difference I can see is that the instance declarations for Set
contain the Ord constraint, whereas they are defined on the class
functions for Heap.
instance (Ord a) => Set RedBlackSet a where
...
I realise that the type of `a` needs to be constrained to Ord instances
at some point in the code, but is there any particular reason for doing
it differently in each case? If not, should one method be preferred over
the other?
Thanks for the help, David.
------------------------------
Message: 4
Date: Mon, 28 Nov 2011 22:38:34 +0000
From: Stephen Tetley <[email protected]>
Subject: Re: [Haskell-beginners] Multi-parameter type classes
To: David Beacham <[email protected]>
Cc: Beginners Haskell <[email protected]>
Message-ID:
<CAB2TPRBR7b6KmProLJ1pvWs=z5naza3mjfig0jj7jeq+q7p...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Maybe some instances of Sets can be built with just equality (Eq)
rather than Ordering (Ord) on the element?
The multi-param class makes the element type tangible so you can use a
different constraint:
-- List - the poor man's set.
instance (Eq a) => Set [] a where
empty = []
insert a as = nub (a:as)
...
------------------------------
Message: 5
Date: Mon, 28 Nov 2011 23:32:51 +0000
From: Tim Pizey <[email protected]>
Subject: Re: [Haskell-beginners] Cabal on mac
To: Daniel Fischer <[email protected]>
Cc: [email protected]
Message-ID:
<caardntionkdd5aecb1abcwkndj4ratpohh-wyxfj0kzhwlq...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Hi Daniel,
Thanks for your reply:
On 25 November 2011 00:48, Daniel Fischer wrote:
> On Friday 25 November 2011, 01:32:13, Tim Pizey wrote:
>> Hi,
>>
>> I have installed cabal and HaxMl
>>
>> cabal install HaXml
>>
>> but my .hs file cannot find
>>
>> import Text.XML.HaXml.Xml2Haskell (readXml)
>>
>> giving
>>
>> *** Chasing dependencies:
>> Chasing modules from: *ch.hs
>>
>> ch.hs:3:8:
>> ? ? Could not find module `Text.XML.HaXml.Xml2Haskell':
>> ? ? ? locations searched:
>> ? ? ? ? Text/XML/HaXml/Xml2Haskell.hs
>> ? ? ? ? Text/XML/HaXml/Xml2Haskell.lhs
>>
>> It would seem only to be looking in the current directory.
>>
>> Hoping I have just missed something obvious.
>
> In a way.
>
> Text.XML.HaXml.Xml2Haskell was in HaXml-1.13, but is no longer in HaXml
> since version 1.19 (current is 1.22.5). Old tutorial, I guess.
>
> There are two functions with the name readXml in the new versions, in
> Text.XML.HaXml.XmlContent and Text.XML.HaXml.XmlContent.Haskell
> (probably the same function, just exported from both modules).
>
> The type has changed, instead of XmlContent a => String -> Maybe a, it is
> now XmlContent a => String -> Either String a, presumably specifying the
> parse failure now if there is one, otherwise I expect the behaviour to be
> the same.
I am trying to use XsdToHaskell but am getting stuck.
The page http://projects.haskell.org/HaXml/
has dead links for DtdToHaskell and XsdToHaskell
There is an apparently dead project at http://code.google.com/p/xsdtohaskell/
I have a set of .xsd file which reference each other and the command
XsdToHaskell chassis.xsd > Chassis.xsd.hs
generates a lot of haskell, but seems to include debugging eg
Parse Success!
Do you or others know of some uptodate documentation?
cheers
Tim
--
Tim Pizey
http://pizey.net/~timp
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