[Haskell-cafe] Re: Question on rank-N polymorphism
Ryan Ingram discussed a question of writing
fs f g = (f fst, g snd)
so that fs ($ (1, 2)) type checks.
This is not that difficult:
{-# LANGUAGE RankNTypes, MultiParamTypeClasses -#}
{-# LANGUAGE FunctionalDependencies, FlexibleInstances #-}
class Apply f x y | f x - y where
apply :: f - x - y
instance Apply (x-y) x y where
apply = ($)
data Fst = Fst
data Snd = Snd
instance Apply Fst (x,y) x where
apply _ = fst
instance Apply Snd (x,y) y where
apply _ = snd
The function in question:
fs3 f = (apply f Fst, apply f Snd)
-- One of Wouter Swierstra's examples
-- examples = (fs id, fs repeat, fs (\x - [x]), fs ((,)id))
data Id a = Id
instance Apply (Id a) Fst ((a,a) - a) where
apply _ _ = fst
instance Apply (Id a) Snd ((a,a) - a) where
apply _ _ = snd
ex1 = fs3 Id
Now, Ryan's main example
newtype Pair a b = Pair (forall w. (((a,b) - w) - w))
instance Apply (Pair a b) Fst a where
apply (Pair f) _ = f fst
instance Apply (Pair a b) Snd b where
apply (Pair f) _ = f snd
ex4 = fs3 (Pair ($ (1, 2)))
-- (1,2)
Incidentally, a different variation of this example is discussed in
http://okmij.org/ftp/Computation/extra-polymorphism.html
Indeed, such a selection from a pair occurs quite often...
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Re: [Haskell-cafe] Deprecated packages on Hackage?
Erik de Castro Lopo mle...@mega-nerd.com writes: Finally, if a package is deprecated it might be usefult to have a reason as well so the hackage entry might say: Deprecated : true (replaced by package XXX) or Deprecated : true (needs maintainer) Or just Deprecated: (reason)?. Couldn't the presence of a Deprecated field be sufficient - the true seems gratuitious to me. One could also have something like Superseeds and Superseeded-by, of course, if that turns out to be the usual reasons for deprecation. And in a later post: Well there is at least one package (network-dns) where the maintainter doesn't want to maintain it any more but would be happy for someone else to take it over. It would be nice if something like this could be represented in the package metadata. Absence of a Maintainer field? One problem is that the last uploaded package is likely to have an active maintainer, and when the maintainer disappears, he or she is unlikely to do a last update changing the status. Perhaps we could have automated emails to maintainers twice a year or so? -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to compile base?
Henk-Jan van Tuyl wrote: I tried to compile base-4.0.0.0 (on Windows XP) as follows: [...]\base\4.0.0.0runhaskell Setup configure command line: module `Prelude' is not loaded It seems that Base needs another way to compile, how? AFAIK base is shipped with GHC, and cannot be compiled separately. GHC 6.10.1 ships with base-4.0.0.0, later versions in the 6.10.x series may have a somewhat later version. Regards, -- Jochem Berndsen | joc...@functor.nl GPG: 0xE6FABFAB ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Incremental XML parsing with namespaces?
On Mon, Jun 8, 2009 at 20:39, John Millikinjmilli...@gmail.com wrote:
I'm trying to convert an XML document, incrementally, into a sequence
of XML events. A simple example XML document:
doc xmlns=org:myproject:mainns xmlns:x=org:myproject:otherns
titleDoc title/title
x:refabc1234/x:ref
html xmlns=http://www.w3.org/1999/xhtml;bodyHello world!/body/html
/doc
The document can be very large, and arrives in chunks over a socket,
so I need to be able to feed the text data into a parser and receive
a list of XML events per chunk. Chunks can be separated in time by
intervals of several minutes to an hour, so pausing processing for the
arrival of the entire document is not an option. The type signatures
would be something like:
type Namespace = String
type LocalName = String
data Attribute = Attribute Namespace LocalName String
data XMLEvent =
EventElementBegin Namespace LocalName [Attribute] |
EventElementEnd Namespace LocalName |
EventContent String |
EventError String
parse :: Parser - String - (Parser, [XMLEvent])
I've looked at HaXml, HXT, and hexpat, and unless I'm missing
something, none of them can achieve this:
+ HaXml and hexpat seem to disregard namespaces entirely -- that is,
the root element is parsed to doc instead of
(org:myproject:mainns, doc), and the second child is x:ref
instead of (org:myproject:otherns, ref). Obviously, this makes
parsing mixed-namespace documents effectively impossible. I found an
email from 2004[1] that mentions a filter for namespace support in
HaXml, but no further information and no working code.
I would recommend hexpat to do the job. Contrary to what you are
saying, hexpat does offer namespace handling:
http://hackage.haskell.org/packages/archive/hexpat/0.8/doc/html/Text-XML-Expat-Namespaced.html
Perhaps you need more than that?
Personally I found hexpat to be fast, space efficient and easy to use.
Here is the representation I got for your example. Please note the
namespaces in right places.
* (toNamespaced ( toQualified t'))
Element {eName = NName {nnNamespace = Just org:myproject:mainns,
nnLocalPart = doc}, eAttrs = [(NName {nnNamespace = Just
http://www.w3.org/2000/xmlns/;, nnLocalPart =
x},org:myproject:otherns),(NName {nnNamespace = Just
org:myproject:mainns, nnLocalPart =
xmlns},org:myproject:mainns)], eChildren = [Text \n,Text
,Element {eName = NName {nnNamespace = Just org:myproject:mainns,
nnLocalPart = title}, eAttrs = [], eChildren = [Text Doc
title]},Text \n,Text ,Element {eName = NName {nnNamespace =
Just org:myproject:otherns, nnLocalPart = ref}, eAttrs = [],
eChildren = [Text abc1234]},Text \n,Text ,Element {eName =
NName {nnNamespace = Just http://www.w3.org/1999/xhtml;, nnLocalPart
= html}, eAttrs = [(NName {nnNamespace = Just
http://www.w3.org/1999/xhtml;, nnLocalPart =
xmlns},http://www.w3.org/1999/xhtml;)], eChildren = [Element {eName
= NName {nnNamespace = Just http://www.w3.org/1999/xhtml;,
nnLocalPart = body}, eAttrs = [], eChildren = [Text Hello
world!]}]},Text \n]}
Best regards
Krzysztof Skrzętnicki
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[Haskell-cafe] Dutch HUG meeting tonight in Amsterdam
Hi everyone, Tonight there will be another meeting of the Dutch Haskell Users' Group! This time we'll meet in Amsterdam, in the library. On the wiki [1] you can find the details of how to reach it. We'll be at the top floor and shouldn't be hard to recognize. The meeting is set to begin at 19:30. Everybody's welcome, from beginners to advanced haskellers. Even if you never programmed in Haskell before it'll probably be a lot of fun. See you tonight! There are a lot of international people joining, so no knowledge of Dutch is necessary. If you're joining, also be sure to subscribe to our mailinglist [2]. See you tonight! -chris [1]: http://www.haskell.org/haskellwiki/Dutch_HUG [2]: http://groups.google.com/group/dutch-hug ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Incremental XML parsing with namespaces?
And just to provide an example of working program:
---
module Main where
import Text.XML.Expat.Qualified
import Text.XML.Expat.Namespaced
import Text.XML.Expat.Tree
import qualified Data.ByteString.Lazy as BSL
main = do
f - BSL.readFile doc1.xml
let (tree,error) = parseTree Nothing f
case error of
Nothing - putStrLn Here you are: (print . toNamespaced .
toQualified $ (tree :: Node String String))
Just err - putStrLn Error! print err
---
$ ./hexpat-test
Here you are:
Element {eName = NName {nnNamespace = Just org:myproject:mainns,
nnLocalPart = doc}, eAttrs = [(NName {nnNamespace = Just
http://www.w3.org/2000/xmlns/;, nnLocalPart =
x},org:myproject:otherns),(NName {nnNamespace = Just
org:myproject:mainns, nnLocalPart =
xmlns},org:myproject:mainns)], eChildren = [Text \n,Text
,Element {eName = NName {nnNamespace = Just org:myproject:mainns,
nnLocalPart = title}, eAttrs = [], eChildren = [Text Doc
title]},Text \n,Text ,Element {eName = NName {nnNamespace =
Just org:myproject:otherns, nnLocalPart = ref}, eAttrs = [],
eChildren = [Text abc1234]},Text \n,Text ,Element {eName =
NName {nnNamespace = Just http://www.w3.org/1999/xhtml;, nnLocalPart
= html}, eAttrs = [(NName {nnNamespace = Just
http://www.w3.org/1999/xhtml;, nnLocalPart =
xmlns},http://www.w3.org/1999/xhtml;)], eChildren = [Element {eName
= NName {nnNamespace = Just http://www.w3.org/1999/xhtml;,
nnLocalPart = body}, eAttrs = [], eChildren = [Text Hello
world!]}]},Text \n]}
# we mess with doc1.xml and exchange /doc for /do
$ ./hexpat-test
Error!
XMLParseError mismatched tag (XMLParseLocation {xmlLineNumber = 5,
xmlColumnNumber = 2, xmlByteIndex = 205, xmlByteCount = 0})
Best regards
Krzysztof Skrzętnicki
2009/6/9 Krzysztof Skrzętnicki gte...@gmail.com:
On Mon, Jun 8, 2009 at 20:39, John Millikinjmilli...@gmail.com wrote:
I'm trying to convert an XML document, incrementally, into a sequence
of XML events. A simple example XML document:
doc xmlns=org:myproject:mainns xmlns:x=org:myproject:otherns
titleDoc title/title
x:refabc1234/x:ref
html xmlns=http://www.w3.org/1999/xhtml;bodyHello
world!/body/html
/doc
The document can be very large, and arrives in chunks over a socket,
so I need to be able to feed the text data into a parser and receive
a list of XML events per chunk. Chunks can be separated in time by
intervals of several minutes to an hour, so pausing processing for the
arrival of the entire document is not an option. The type signatures
would be something like:
type Namespace = String
type LocalName = String
data Attribute = Attribute Namespace LocalName String
data XMLEvent =
EventElementBegin Namespace LocalName [Attribute] |
EventElementEnd Namespace LocalName |
EventContent String |
EventError String
parse :: Parser - String - (Parser, [XMLEvent])
I've looked at HaXml, HXT, and hexpat, and unless I'm missing
something, none of them can achieve this:
+ HaXml and hexpat seem to disregard namespaces entirely -- that is,
the root element is parsed to doc instead of
(org:myproject:mainns, doc), and the second child is x:ref
instead of (org:myproject:otherns, ref). Obviously, this makes
parsing mixed-namespace documents effectively impossible. I found an
email from 2004[1] that mentions a filter for namespace support in
HaXml, but no further information and no working code.
I would recommend hexpat to do the job. Contrary to what you are
saying, hexpat does offer namespace handling:
http://hackage.haskell.org/packages/archive/hexpat/0.8/doc/html/Text-XML-Expat-Namespaced.html
Perhaps you need more than that?
Personally I found hexpat to be fast, space efficient and easy to use.
Here is the representation I got for your example. Please note the
namespaces in right places.
* (toNamespaced ( toQualified t'))
Element {eName = NName {nnNamespace = Just org:myproject:mainns,
nnLocalPart = doc}, eAttrs = [(NName {nnNamespace = Just
http://www.w3.org/2000/xmlns/;, nnLocalPart =
x},org:myproject:otherns),(NName {nnNamespace = Just
org:myproject:mainns, nnLocalPart =
xmlns},org:myproject:mainns)], eChildren = [Text \n,Text
,Element {eName = NName {nnNamespace = Just org:myproject:mainns,
nnLocalPart = title}, eAttrs = [], eChildren = [Text Doc
title]},Text \n,Text ,Element {eName = NName {nnNamespace =
Just org:myproject:otherns, nnLocalPart = ref}, eAttrs = [],
eChildren = [Text abc1234]},Text \n,Text ,Element {eName =
NName {nnNamespace = Just http://www.w3.org/1999/xhtml;, nnLocalPart
= html}, eAttrs = [(NName {nnNamespace = Just
http://www.w3.org/1999/xhtml;, nnLocalPart =
xmlns},http://www.w3.org/1999/xhtml;)], eChildren = [Element {eName
= NName {nnNamespace = Just http://www.w3.org/1999/xhtml;,
nnLocalPart = body}, eAttrs = [], eChildren = [Text Hello
world!]}]},Text \n]}
Best regards
Krzysztof Skrzętnicki
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[Haskell-cafe] Unification for rank-N types
Hi,
I have the following code:
---
{-# LANGUAGE RankNTypes #-}
f :: ((forall a. a - a) - b) - b
f x = x id
g :: (forall c. Eq c = [c] - [c]) - ([Bool],[Int])
g y = (y [True], y [1])
h :: ([Bool],[Int])
h = f g
---
GHC rejects it:
Couldn't match expected type `forall a. a - a'
against inferred type `forall c. (Eq c) = [c] - [c]'
Expected type: forall a. a - a
Inferred type: forall c. (Eq c) = [c] - [c]
In the first argument of `f', namely `g'
In the expression: f g
But, intuitively, this code is type-safe, and actually I can convince
the typechecker in it with the following workaround:
---
h :: ([Bool],[Int])
h = let g' = (\(x :: forall a. a - a) - g x) in f g'
---
So, is the current behavior of GHC correct ot it is a bug?
How unification for rank-N types should proceed?
Thanks,
Vladimir
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[Haskell-cafe] Convert IO Int to Int
Hi, I am using the System.Random method randomRIO. How can I convert its output to an Int? Thanks... -- View this message in context: http://www.nabble.com/Convert-IO-Int-to-Int-tp23940249p23940249.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
You can not convert an IO Int to Int, or at least, you shouldn't. However, you can do as follows: test :: IO () test = do int - randomRIO -- or whatever it is called print $ useInt int useInt :: Int - Int useInt x = x+10 //Tobias 2009/6/9 ptrash ptr...@web.de: Hi, I am using the System.Random method randomRIO. How can I convert its output to an Int? Thanks... -- View this message in context: http://www.nabble.com/Convert-IO-Int-to-Int-tp23940249p23940249.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tobias Olausson tob...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
ptrash wrote: Hi, I am using the System.Random method randomRIO. How can I convert its output to an Int? Thanks... You cannot [1], you should read up on monads and I/O in Haskell, for example http://haskell.org/haskellwiki/IO_inside [1] Yes, you can, but no, you don't want to. Regards, -- Jochem Berndsen | joc...@functor.nl GPG: 0xE6FABFAB ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
On 2009/06/09, at 19:33, Tobias Olausson wrote: You can not convert an IO Int to Int, or at least, you shouldn't. However, you can do as follows: test :: IO () test = do int - randomRIO -- or whatever it is called print $ useInt int useInt :: Int - Int useInt x = x+10 Or, you can lift pure function into IO. the below test' function almost same as above test function. (But I used randomIO instead of randomRIO because it seemed to be a typo :-) test' = print = fmap useInt randomIO I think it is more handy than using do notation, when you want to do something simple with monads. And converting IO Int to IO anything is much easier and safer than converting IO Int to Int. ghci :m +System.Random Data.Char ghci :t fmap (+1) randomIO fmap (+1) randomIO :: (Num a, Random a) = IO a ghci :t fmap show randomIO fmap show randomIO :: IO String ghci :t fmap chr randomIO fmap Data.Char.chr randomIO :: IO Char ghci :t fmap (+) randomIO fmap (+) randomIO :: (Num a, Random a) = IO (a - a) Thanks, Hashimoto //Tobias 2009/6/9 ptrash ptr...@web.de: Hi, I am using the System.Random method randomRIO. How can I convert its output to an Int? Thanks... -- View this message in context: http://www.nabble.com/Convert-IO-Int- to-Int-tp23940249p23940249.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tobias Olausson tob...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Unification for rank-N types
One more example:
This does not type-check:
---
{-# LANGUAGE RankNTypes, ImpredicativeTypes #-}
f :: [forall a. t a - t a] - t b - t b
f = foldr (.) id
---
Couldn't match expected type `forall a. f a - f a'
against inferred type `b - c'
In the first argument of `foldr', namely `(.)'
But this, very similar, does type-check:
---
{-# LANGUAGE RankNTypes, ImpredicativeTypes #-}
f :: [forall a. t a - t a] - t b - t b
f = foldr (\g - (.) g) id
---
What is the reason for this?
Thanks,
Vladimir
On 6/9/09, Vladimir Reshetnikov v.reshetni...@gmail.com wrote:
Hi,
I have the following code:
---
{-# LANGUAGE RankNTypes #-}
f :: ((forall a. a - a) - b) - b
f x = x id
g :: (forall c. Eq c = [c] - [c]) - ([Bool],[Int])
g y = (y [True], y [1])
h :: ([Bool],[Int])
h = f g
---
GHC rejects it:
Couldn't match expected type `forall a. a - a'
against inferred type `forall c. (Eq c) = [c] - [c]'
Expected type: forall a. a - a
Inferred type: forall c. (Eq c) = [c] - [c]
In the first argument of `f', namely `g'
In the expression: f g
But, intuitively, this code is type-safe, and actually I can convince
the typechecker in it with the following workaround:
---
h :: ([Bool],[Int])
h = let g' = (\(x :: forall a. a - a) - g x) in f g'
---
So, is the current behavior of GHC correct ot it is a bug?
How unification for rank-N types should proceed?
Thanks,
Vladimir
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[Haskell-cafe] Re: A generics question
Sterling Clover s.clover at gmail.com writes: Try it with the following type signature and it should work fine: convert :: (Data a) = Int - a - a Of course, as has been noted, SYB is a rather big sledgehammer for the insect in question. Cheers, S. Thank you Sterling. That is the answer I was looking for. I was trying out things from the various Scrap your Boilerplate papers, and got stuck doing something that I thought should be easy. Now it makes sense. Thanks again. Best wishes, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
Ok, thanks for the information. -- View this message in context: http://www.nabble.com/Convert-IO-Int-to-Int-tp23940249p23942344.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Combine to List to a new List
Hi, I have the following two lists: a = [1,2,3] b = [A,B,C] I want a combination of the to lists: c = [(1,A), (2, B), (3, C)] How can I do this? I have tried c = [(x,y) | x - a, y - b] But this just returns me a list with every possible combination of the 2 lists. Thanks... -- View this message in context: http://www.nabble.com/Combine-to-List-to-a-new-List-tp23942440p23942440.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combine to List to a new List
Try c = zip a b On Tue, Jun 9, 2009 at 9:05 AM, ptrash ptr...@web.de wrote: Hi, I have the following two lists: a = [1,2,3] b = [A,B,C] I want a combination of the to lists: c = [(1,A), (2, B), (3, C)] How can I do this? I have tried c = [(x,y) | x - a, y - b] But this just returns me a list with every possible combination of the 2 lists. Thanks... -- View this message in context: http://www.nabble.com/Combine-to-List-to-a-new-List-tp23942440p23942440.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Combine to List to a new List
Probably you might need the zip function.Check here:http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:zip Date: Tue, 9 Jun 2009 06:05:57 -0700 From: ptr...@web.de To: haskell-cafe@haskell.org Subject: [Haskell-cafe] Combine to List to a new List Hi, I have the following two lists: a = [1,2,3] b = [A,B,C] I want a combination of the to lists: c = [(1,A), (2, B), (3, C)] How can I do this? I have tried c = [(x,y) | x - a, y - b] But this just returns me a list with every possible combination of the 2 lists. Thanks... -- View this message in context: http://www.nabble.com/Combine-to-List-to-a-new-List-tp23942440p23942440.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _ Windows Live™: Keep your life in sync. Check it out! http://windowslive.com/explore?ocid=TXT_TAGLM_WL_t1_allup_explore_012009___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combine to List to a new List
Hey, cool. Thanks! -- View this message in context: http://www.nabble.com/Combine-to-List-to-a-new-List-tp23942440p23942633.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Applying Data.Map
Hi Toby,
Thanks for the helpful comments. I'd gotten used to arithmetic operator
sections (+2), (*2), etc. but hadn't picked up on the generality of using them
with *any* infix function. I can also see the benefit of using List.Group.
However, I'm uncertain about how to import just fromList and ! from with the
imports I'm using
import Data.Map (Map) (fromList,!) ???
import qualified Data.Map as Map (fromList,!) ???
Michael
--- On Mon, 6/8/09, Toby Hutton toby.hut...@gmail.com wrote:
From: Toby Hutton toby.hut...@gmail.com
Subject: Re: [Haskell-cafe] Applying Data.Map
To: michael rice nowg...@yahoo.com
Cc: haskell-cafe@haskell.org
Date: Monday, June 8, 2009, 8:57 PM
Although in this example using Data.Map is overkill, if the alphabet was very
large then Data.Map probably would be the way to go. In that case I'd use:
map head . group . sort instead of nub . sort
since it's noticeably quicker for large lists. This is because nub needs to
preserve the order of input, removing redundancies, but you're sorting it
anyway.
Also, in map (\c - m Map.! c) s you can use the 'section' (m Map.!) instead.
e.g., map (m Map.!) s
The Map.! is ugly though. As you're only using fromList and (!) from Data.Map,
I'd just import those explicitly since they don't clash with Prelude. Then
you'd have map (m !) s
Toby.
On Tue, Jun 9, 2009 at 4:59 AM, michael rice nowg...@yahoo.com wrote:
I wrote a Haskell solution for the Prolog problem stated below. I had written a
function SQUISH before discovering that NUB does the same thing. While the
solution works, I thought maybe I could apply some functions in the Data.Map
module, and so wrote a second version of SERIALIZE, one no longer needing
TRANSLATE. Using the Data.Map module is probably overkill for this particular
problem, but wanted to familiarize myself with Map type. Suggestions welcome.
Prolog code also included below for those interested.
Michael
===
{-
From Prolog By Example, Coelho, Cotta, Problem 42, pg. 63
Verbal statement:
Generate a list of serial numbers for the items of a given
list,
the members of which are to be numbered in alphabetical order.
For example, the list [p,r,o,l,o,g] must generate [4,5,3,2,3,1]
-}
{-
Prelude :l
serialize
[1 of 1] Compiling Main ( serialize.hs, interpreted )
Ok, modules loaded: Main.
*Main serialize prolog
[4,5,3,2,3,1]
*Main
-}
===Haskell code==
import Data.Char
import Data.List
import Data.Map (Map)
import qualified Data.Map as
Map
{-
translate :: [Char] - [(Char,Int)] - [Int]
translate [] _ = []
translate (x:xs) m = (fromJust (lookup x m)) : (translate xs m )
-}
{-
serialize :: [Char] - [Int]
serialize s = let c = nub $ sort s
n = [1..(length c)]
in translate s (zip c n)
-}
serialize :: [Char] - [Int]
serialize s = let c = nub $ sort s
n = [1..(length c)]
m = Map.fromList $ zip c n
in map (\c - m Map.! c) s
Prolog code
serialize(L,R) :- pairlists(L,R,A),arrange(A,T),
numbered(T,1,N).
? - typo?
pairlists([X|L],[Y|R],[pair(X,Y)|A]) :- pairlist(L,R,A).
pairlists([],[],[]).
arrange([X|L],tree(T1,X,T2)) :- partition(L,X,L1,L2),
arrange(L1,T1),
arrange(L2,T2).
arrange([],_).
partition([X|L],X,L1,L2) :- partition(L,X,L1,L2).
partition([X|L],Y,[X|L1],L2) :- before(X,Y),
partition(L,Y,L1,L2).
partition([X|L],Y,L1,[X|L2]) :- before(Y,X),
partition(L,Y,L1,L2).
partition([],_,[],[]).
before(pair(X1,Y1),pair(X2,Y2)) :- X1X2.
numbered(tree(T1,pair(X,N1),T2),N0,N) :- numbered(T1,N0,N1),
N2 is N1+1,
numbered(T2,N2,N).
numbered(void,N,N).
Prolog examples
Execution:
?- serialize([p,r,o,l,o,g]).
[4,5,3,2,3,1]
?- serialize ([i,n,t,.,a,r,t,i,f,i,c,i,a,l]).
[5,7,9,1,2,8,9,5,4,5,3,5,2,6]
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Re: [Haskell-cafe] Combine to List to a new List
On Tue, 2009-06-09 at 06:05 -0700, ptrash wrote: Hi, I have the following two lists: a = [1,2,3] b = [A,B,C] I want a combination of the to lists: c = [(1,A), (2, B), (3, C)] How can I do this? What you want is a function with the following type signature: [t1] - [t2] - [(t1,t2)] Search for that in hoogle[1] and you get the function zip as a result[2] (as already told in this thread). Hoogle is your friend (and helps you help yourself)! :) 1: http://haskell.org/hoogle/ 2: http://haskell.org/hoogle/?hoogle=%5Ba%5D+-%3E+%5Bb%5D+-%3E+%5B%28a% 2Cb%29%5D ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Applying Data.Map
On Tue, Jun 9, 2009 at 15:23, michael ricenowg...@yahoo.com wrote: import Data.Map (Map) (fromList,!) ??? import qualified Data.Map as Map (fromList,!) ??? Because ! is an operator, you need to enclose it in parentheses. Also, the (Map) in the import is already the list of things you are importing; you can just add to that. So do the following: Import these without qualification: import Data.Map (Map, fromList, (!)) Import everything else (actually including Map, fromList and (!)) with qualification Map: import qualified Data.Map as Map Cheers, Thomas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
Hmm...it am not getting through it. I just want to generate a random number and then compare it with other numbers. Something like r = randomRIO (1, 10) if (r 5) then... else ... -- View this message in context: http://www.nabble.com/Convert-IO-Int-to-Int-tp23940249p23943301.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
On Tue, Jun 9, 2009 at 2:52 PM, ptrashptr...@web.de wrote: Hmm...it am not getting through it. I just want to generate a random number and then compare it with other numbers. Something like r = randomRIO (1, 10) if (r 5) then... else ... You have to do it inside the IO monad, something like myFunc = do r - randomRIO (1, 10 if r 5 then ... else ... /M -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
On Tue, 9 Jun 2009, ptrash wrote: I am using the System.Random method randomRIO. How can I convert its output to an Int? in general: http://haskell.org/haskellwiki/How_to_get_rid_of_IO about randomIO: http://haskell.org/haskellwiki/Avoiding_IO#State_monad ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
Am Dienstag 09 Juni 2009 15:57:24 schrieb Magnus Therning: On Tue, Jun 9, 2009 at 2:52 PM, ptrashptr...@web.de wrote: Hmm...it am not getting through it. I just want to generate a random number and then compare it with other numbers. Something like r = randomRIO (1, 10) if (r 5) then... else ... You have to do it inside the IO monad, something like myFunc = do r - randomRIO (1, 10 if r 5 then ... else ... /M Or make the source of the pseudo-random numbers explicit: import System.Random function :: (RandomGen g, Random a) = g - other args - result function gen whatever | r 5 = blah newgen something | r 3 = blub newgen somethingElse | otherwise = bling where (r,newgen) = randomR (lo,hi) gen and finally, when the programme is run: main = do args - getArgs sg - getStdGen foo - thisNThat print $ function sg foo If you're doing much with random generators, wrap it in a State monad. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Incremental XML parsing with namespaces?
John Millikin wrote: On Mon, Jun 8, 2009 at 3:39 PM, Henning Thielemannlemm...@henning-thielemann.de wrote: I think you could use the parser as it is and do the name parsing later. Due to lazy evaluation both parsers would run in an interleaved way. I've been trying to figure out how to get this to work with lazy evaluation, but haven't made much headway. Tips? The only way I can think of to get incremental parsing working is to maintain explicit state, but I also can't figure out how to achieve this with the parsers I've tested (HaXml, HXT, hexpat). Can you please look at http://code.haskell.org/~thielema/tagchup/example/Escape.hs http://code.haskell.org/~thielema/tagchup/example/Strip.hs You just have to replace Text.XML.Basic.Name.LowerCase by Text.XML.Basic.Name.Qualified for use of qualified names. -- Mit freundlichen Gruessen Henning Thielemann Viele Gruesse Henning Martin-Luther-Universitaet Halle-Wittenberg, Institut fuer Informatik Tel. +49 - 345 - 55 24773 Fax +49 - 345 - 55 27333 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Applying Data.Map
In the import statements, it wasn't clear to me that I could import types as well as functions, and Map is a type. All clear now. Thanks. Michael --- On Tue, 6/9/09, Thomas ten Cate ttenc...@gmail.com wrote: From: Thomas ten Cate ttenc...@gmail.com Subject: Re: [Haskell-cafe] Applying Data.Map To: michael rice nowg...@yahoo.com Cc: haskell-cafe@haskell.org Date: Tuesday, June 9, 2009, 9:40 AM On Tue, Jun 9, 2009 at 15:23, michael ricenowg...@yahoo.com wrote: import Data.Map (Map) (fromList,!) ??? import qualified Data.Map as Map (fromList,!) ??? Because ! is an operator, you need to enclose it in parentheses. Also, the (Map) in the import is already the list of things you are importing; you can just add to that. So do the following: Import these without qualification: import Data.Map (Map, fromList, (!)) Import everything else (actually including Map, fromList and (!)) with qualification Map: import qualified Data.Map as Map Cheers, Thomas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to improve below code?
Andy Stewart wrote:
So have a better solution to avoid write above ugly code
How about:
data Page a =
Page {pageName :: IORef String
,pageId:: Int
,pageBuffer:: a
,pageBox :: VBox
}
class PageBuffer a where
pageBufferClone :: a - IO (a, VBox)
pageClone :: PageBuffer a = Page a - IO (Page a)
pageClone page = do
-- Get common information for clone page.
name - pageGetName page
let id = pageId page
pb = pageBuffer page
-- Get clone information for dynamic interface.
(pBuffer, pBox) - pageBufferClone pb
-- Return clone page.
pageNewInternal name id pBuffer pBox
I'm not totally sure if that will work without seeing the rest of your
code. But it seems neater, and no GADTs in sight. If you need to store
Page StringBuffer in a list with Page ImageBuffer you will have a
problem, so perhaps you could spell out what else you need to do with
these Page items in your application?
Thanks,
Neil.
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[Haskell-cafe] How to improve below code?
Hi all,
I have below source code, i use Dynamic for `pageBuffer`.
In implement of function `pageClone`, after `case pt of`, i need write
like this, and this code looks ugly, if `PageTyep` have 100 type, i need
write those ugly code 100 times.
case pt of
TStringBuffer - pageBufferClone (x :: StringBuffer)
TImageBuffer - pageBufferClone (x :: ImageBuffer)
TVideoBuffer - pageBufferClone (x :: TVideoBuffer)
TMixBuffer- pageBufferClone (x :: TMixBuffer)
So have a better solution to avoid write above ugly code?
Someone suggestion me use GADTs instead, but i don't know how to write correct
GADTs code replace current version, if GADTs is best way, someone can
explain it detail? It's better if someone give me demo code.
-- Source Code start
--
data PageType = TStringBuffer | TImageBuffer | TVideoBuffer | TMixBuffer
deriving (Eq, Show, Read)
data Page =
Page {pageName :: IORef String
,pageId:: Int
,pageType :: PageType
,pageBuffer:: Dynamic
,pageBox :: VBox
}
class PageBuffer a where
pageBufferClone :: a - IO (Dynamic, VBox)
-- | Page clone interface.
pageClone :: Page - IO Page
pageClone page = do
-- Get common information for clone page.
name - pageGetName page
let id = pageId page
pt = pageType page
pb = pageBuffer page
-- Get clone information for dynamic interface.
(pBuffer, pBox) -
case fromDynamic pb of
Just x -
case pt of
TStringBuffer - pageBufferClone (x :: StringBuffer)
TImageBuffer - pageBufferClone (x :: ImageBuffer)
TVideoBuffer - pageBufferClone (x :: TVideoBuffer)
TMixBuffer- pageBufferClone (x :: TMixBuffer)
Nothing - pageCloneEmpty
-- Return clone page.
pageNewInternal name id pt pBuffer pBox
-- Source Code end
--
Thanks!
-- Andy
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Re: [Haskell-cafe] Convert IO Int to Int
On Tue, Jun 9, 2009 at 16:14, Daniel Fischerdaniel.is.fisc...@web.de wrote: Am Dienstag 09 Juni 2009 15:57:24 schrieb Magnus Therning: On Tue, Jun 9, 2009 at 2:52 PM, ptrashptr...@web.de wrote: Hmm...it am not getting through it. I just want to generate a random number and then compare it with other numbers. Something like r = randomRIO (1, 10) if (r 5) then... else ... You have to do it inside the IO monad, something like myFunc = do r - randomRIO (1, 10 if r 5 then ... else ... /M Or make the source of the pseudo-random numbers explicit: import System.Random function :: (RandomGen g, Random a) = g - other args - result function gen whatever | r 5 = blah newgen something | r 3 = blub newgen somethingElse | otherwise = bling where (r,newgen) = randomR (lo,hi) gen and finally, when the programme is run: main = do args - getArgs sg - getStdGen foo - thisNThat print $ function sg foo If you're doing much with random generators, wrap it in a State monad. To avoid reinventing the wheel one can use excellent package available on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MonadRandom The die function simulates the roll of a die, picking a number between 1 and 6, inclusive, and returning it in the Rand monad. Notice that this code will work with any source of random numbers g. die :: (RandomGen g) = Rand g Int die = getRandomR (1,6) The dice function uses replicate and sequence to simulate the roll of n dice. dice :: (RandomGen g) = Int - Rand g [Int] dice n = sequence (replicate n die) To extract a value from the Rand monad, we can can use evalRandIO. main = do values - evalRandIO (dice 2) putStrLn (show values) Best regards Krzysztof Skrzętnicki ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
Magnus Therning writes: ptrash wrote: ...am not getting through it. I just want to generate a random number and then compare it with other numbers. Something like r = randomRIO (1, 10) if (r 5) then... else ... You have to do it inside the IO monad, something like myFunc = do r - randomRIO (1, 10 This may continue forever... With nice references to monads, to uns...@#*!, etc. ... We may say, as many tutorials do : this is not what you want! (which I hate ; you are not my conscience, whoever you are...), or just give some code, not always readable... Perhaps I belong to a minority here, but I strongly believe that at THIS level, the first thing to do - unless I am dead wrong - is to explain to our friend ptrash (who could find a less gothic pseudo) that in a pure functional programming, the construction r = whatEver(par1,par2) being a function call, cannot give just a random number, something which is not (intuitively) determined, and changes with every call, despite the constancy of the arguments. For most of us, acquainted with the stuff, it becomes trivial, but if somebody doesn't know that a classical pseudo-random generator modifies a seed, and in such a way involves a side effect, then sending him to the monadic heaven is dangerous. Please, tell him first about random streams, which he can handle without IO. Or, about ergodic functions (hashing contraptions which transform ANY parameter into something unrecognizable). When he says : I know all that, THEN hurt him badly with monads. Jerzy Karczmarczuk ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Convert IO Int to Int
Hello jerzy, Tuesday, June 9, 2009, 8:23:04 PM, you wrote: Please, tell him first about random streams, which he can handle without IO. Or, about ergodic functions (hashing contraptions which transform ANY parameter into something unrecognizable). When he says : I know all that, THEN hurt him badly with monads. i think that for someone coming from imperative programming teeling about IO monad is the easiest way. and then he will learn how to do it FP way -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] please comment on my parser, can I do this cleaner?
All I want to do is split on commas, but not the commas inside () or tags.
I have been wanting to master parsec for a long time and this simple
exercise looked like a good place to start.
The code below does the right thing. Am I missing any tricks to make
this simpler/neater?
Thanks, thomas.
thart...@ubuntu:~/perlArenacat splitEm.
splitEm.hs splitEm.hs~ splitEm.pl splitEm.pl~
thart...@ubuntu:~/perlArenacat splitEm.hs
{-# LANGUAGE ScopedTypeVariables #-}
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Char
import Text.PrettyPrint (vcat, render, text)
import Data.List.Split hiding (sepBy, chunk)
import Text.ParserCombinators.Parsec.Token
import Debug.Trace
import Debug.Trace.Helpers
-- this works, but is there a neater/cleaner way?
main = ripInputsXs (toEof splitter) splitter [ goodS, badS ]
-- I need a way to split on commas, but not the commas inside '' or
'()' characters
goodS = *2FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
badS = *2)FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
-- the first matches a ), so reject this
splitter = do
chunks :: [String] - toEof (many chunk)
let pieces = map concat $ splitOn [,] chunks
return pieces -- chunks
where
atom = string ,
| ( many1 $ noneOf () )
chunk = parenExpr | atom
parenExpr :: GenParser Char st [Char]
parenExpr = let paren p = betweenInc (char '(' ) (char ')' ) p
| betweenInc (char '' ) (char '' )
p
in paren $ option $ do ps - many1 $ parenExpr | atom
return . concat $ ps
betweenInc o' c' p' = do
o - o'
p - p'
c - c'
return $ [o] ++ p ++ [c]
toEof p' = do
r - p'
eof
return r
ripInputs prs prsName xs = mapM_ (putStrLn . show . parse prs prsName ) xs
ripInputsXs prs prsName xs = mapM_ (putStrLn . showXs . parse prs prsName ) xs
where showXs v = case v of
Left e - show e
Right xs - render . vcat . map text $ xs
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Re: [Haskell-cafe] nubBy seems broken in recent GHCs
2009/6/6 Bertram Felgenhauer bertram.felgenha...@googlemail.com: Interesting. This was changed in response to http://hackage.haskell.org/trac/ghc/ticket/2528 | Tue Sep 2 11:29:50 CEST 2008 Simon Marlow marlo...@gmail.com | * #2528: reverse the order of args to (==) in nubBy to match nub | This only makes a difference when the (==) definition is not | reflexive, but strictly speaking it does violate the report definition | of nubBy, so we should fix it. It turns out that 'elem' differs from the report version and should have its comparison reversed. Of course that would only ever matter for broken Eq instances. However, the report also states that the nubBy function may assume that the given predicate defines an equivalence relation. http://haskell.org/onlinereport/list.html#sect17.6 So I'm not sure there's anything to be fixed here - although backing out the above patch probably won't hurt anybody. Bertram Yeah, while most Eq instances really do define an equivalence relation (at least extensionally), and the Report authors probably thought in terms of an equivalence relation (even going so far as to say that nubBy can assume its parameter is one, which I think was a mistake), I think nubBy has a much more general use. It does a generalised kind of sieving, specifically, nubBy f xs is the unique subsequence of xs that: 1) Has the property that if x and y are elements such that x occurs before y in it, then f x y is False. 2) The sequence of indices of selected elements is lexicographically minimum for all subsequences satisfying condition 1. (That is, it always picks the earliest elements possible.) I think this is how it ought to be specified. Similarly, groupBy f xs is (and should be) the unique list of contiguous sublists of xs such that: 1) concat (groupBy f xs) = xs 2) If x is the head of any of the sublists and y is any other element of that sublist, then f x y 3) The sequence of lengths of the sublists is lexicographically maximum for all lists satisfying the first two properties (That is, it always prefers adding elements to an earlier group to starting a new group.) - Cale ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Convert IO Int to Int
Bulat Ziganshin wrote: Hello jerzy, Tuesday, June 9, 2009, 8:23:04 PM, you wrote: Please, tell him first about random streams, which he can handle without IO. Or, about ergodic functions (hashing contraptions which transform ANY parameter into something unrecognizable). When he says : I know all that, THEN hurt him badly with monads. i think that for someone coming from imperative programming teeling about IO monad is the easiest way. and then he will learn how to do it FP way I came from a imperative programming background. I didn't feel like this help me at all back then. At least in the beginning you want to detach yourself from an imperative style, not try to simulate it with some weird structure that you don't really understand. More generally I really wish IO hadn't been the first Monad I played with. It's so close to a Functor, yet in my mind Functors were simple, just structures that could be mapped, and Monads were these mysterious things that allowed you to get away with side effects and that once you were inside you could never get out. Jorge ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] please comment on my parser, can I do this cleaner?
Am Dienstag 09 Juni 2009 20:29:09 schrieb Thomas Hartman:
All I want to do is split on commas, but not the commas inside () or
tags.
I have been wanting to master parsec for a long time and this simple
exercise looked like a good place to start.
The code below does the right thing. Am I missing any tricks to make
this simpler/neater?
Thanks, thomas.
thart...@ubuntu:~/perlArenacat splitEm.
splitEm.hs splitEm.hs~ splitEm.pl splitEm.pl~
thart...@ubuntu:~/perlArenacat splitEm.hs
{-# LANGUAGE ScopedTypeVariables #-}
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Char
import Text.PrettyPrint (vcat, render, text)
import Data.List.Split hiding (sepBy, chunk)
import Text.ParserCombinators.Parsec.Token
import Debug.Trace
import Debug.Trace.Helpers
-- this works, but is there a neater/cleaner way?
main = ripInputsXs (toEof splitter) splitter [ goodS, badS ]
-- I need a way to split on commas, but not the commas inside '' or
'()' characters
goodS = *2FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
badS = *2)FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
-- the first matches a ), so reject this
splitter = do
chunks :: [String] - toEof (many chunk)
let pieces = map concat $ splitOn [,] chunks
return pieces -- chunks
where
atom = string ,
| ( many1 $ noneOf () )
chunk = parenExpr | atom
I think that does not do what you want.
For input FOO,BAR,BAZ, chunks is [FOO,BAR,BAZ], that won't be split; as far
as I can
see, it splits only on commas directly following a parenExpr (or at the
beginning of the
input or directly following another splitting comma).
parenExpr :: GenParser Char st [Char]
parenExpr = let paren p = betweenInc (char '(' ) (char ')' ) p
| betweenInc (char '' ) (char '' )
p
in paren $ option $ do ps - many1 $ parenExpr | atom
return . concat $ ps
betweenInc o' c' p' = do
o - o'
p - p'
c - c'
return $ [o] ++ p ++ [c]
toEof p' = do
r - p'
eof
return r
ripInputs prs prsName xs = mapM_ (putStrLn . show . parse prs prsName ) xs
ripInputsXs prs prsName xs = mapM_ (putStrLn . showXs . parse prs prsName )
xs where showXs v = case v of
Left e - show e
Right xs - render . vcat . map text $ xs
I can offer (sorry for the names, and I don't know if what that does is really
what you
want):
keepSepBy :: Parser a - Parser a - Parser [a]
keepSepBy p sep = (do
r - p
(do s - sep
xs - keepSepBy p sep
return (r:s:xs)) | return [r])
| return []
twain :: Parser a - Parser a - Parser [a] - Parser [a]
twain open close list = do
o - open
l - list
c - close
return (o:l++[c])
comma :: Parser String
comma = string ,
simpleChar :: Parser Char
simpleChar = noneOf (),
suite :: Parser String
suite = many1 simpleChar
atom :: Parser String
atom = fmap concat $ many1 (parenExp | suite)
parenGroup :: Parser String
parenGroup = fmap concat $ keepSepBy atom comma
parenExp :: Parser String
parenExp = twain (char '') (char '') parenGroup
| twain (char '(') (char ')') parenGroup
chunks :: Parser [String]
chunks = sepBy atom comma
splitter = do
cs - chunks
eof
return cs
goodS = *2FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
badS = *2)FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
goodRes = parse splitter splitter goodS
badRes = parse splitter splitter badS
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Re: [Haskell-cafe] Convert IO Int to Int
2009/6/9 Krzysztof Skrzętnicki gte...@gmail.com On Tue, Jun 9, 2009 at 16:14, Daniel Fischerdaniel.is.fisc...@web.de wrote: If you're doing much with random generators, wrap it in a State monad. To avoid reinventing the wheel one can use excellent package available on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MonadRandom Please do! Prefer MonadRandom to explicit generator passing: http://lukepalmer.wordpress.com/2009/01/17/use-monadrandom/. Keep computations in MonadRandom, and pull them out with evalRandomIO at the last second. Luke http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MonadRandom The die function simulates the roll of a die, picking a number between 1 and 6, inclusive, and returning it in the Rand monad. Notice that this code will work with any source of random numbers g. die :: (RandomGen g) = Rand g Int die = getRandomR (1,6) The dice function uses replicate and sequence to simulate the roll of n dice. dice :: (RandomGen g) = Int - Rand g [Int] dice n = sequence (replicate n die) To extract a value from the Rand monad, we can can use evalRandIO. main = do values - evalRandIO (dice 2) putStrLn (show values) Best regards Krzysztof Skrzętnicki ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to improve below code?
On Tue, Jun 9, 2009 at 7:21 AM, Neil Brownnc...@kent.ac.uk wrote:
data Page a =
Page {pageName :: IORef String
,pageId :: Int
,pageBuffer :: a
,pageBox :: VBox
}
class PageBuffer a where
pageBufferClone :: a - IO (a, VBox)
pageClone :: PageBuffer a = Page a - IO (Page a)
pageClone page = do
-- Get common information for clone page.
name - pageGetName page
let id = pageId page
pb = pageBuffer page
-- Get clone information for dynamic interface. (pBuffer, pBox) -
pageBufferClone pb
-- Return clone page.
pageNewInternal name id pBuffer pBox
Actually you can avoid the type parameter on a using an existential:
{-# LANGUAGE ExistentialQuantification #-}
data Page = forall a. PageBuffer a =
Page {pageName :: IORef String
,pageId:: Int
,pageBuffer:: a
,pageBox :: VBox
}
Now you can still use [Page]. You can't do pageBuffer p, though,
you'll get this fun error message:
Cannot use record selector `pageBuffer' as a function due to
escaped type variables
Probable fix: use pattern-matching syntax instead
Instead you need to do
case p of Page{pageBuffer = x} - ...something with x...
This will bring the PageBuffer context into scope, inside of the case statement.
-- ryan
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[Haskell-cafe] FlexibleContexts and FlexibleInstances
Dear all, This post is partly a gripe about how poor the formal documentation for various GHC extensions is, partly a gripe about how GHC blurs the lines between syntactic and type-level issues as well as between various extensions, and partly a gripe about how the Haskell 98 report is sometimes similarly blurred where syntax is concerned (or not). All these things make the life of a poor parser implementor quite miserable at times. All in good jest of course, but with an edge of truth, especially regarding (lack of) formal documentation. The issue at hand which has caused my frustration is the FlexibleContexts [1] and FlexibleInstances [2] extensions, which lift restrictions imposed by Haskell 98 on the forms of contexts and instances that may be defined. Great extensions both of them - but what do they do, really really? The following toy program requires MultiParamTypeClasses OR FlexibleContexts in order to be accepted by GHC(i): f :: (T a b) = a - Int f _ = 0 This of course assumes that we import the definition of T, we *must* have MultiParamTypeClasses enabled if we want to declare T. Both extensions thus enable classes with more than one argument to appear in contexts. Changing the program to f :: (T a ()) = a - Int f _ = 0 i.e. changing the second argument to T to () instead, means we now *must* have FlexibleInstances, in order to allow the non-tyvar argument. This is nothing surprising, this is what FlexibleInstances are supposed to do. But the question is, is this a syntactic issue or a typing issue? In GHC proper this doesn't really matter much, as long as it is caught *somewhere* then all is dandy. GHC's parser lets everything pass, and it's the type checker that balks at this program. But for someone like me with *only* a parser, this is a question that needs a clear answer. Looking at the online report, the productions regarding contexts are context - class | ( class1 , ... , classn ) (n=0) class - qtycls tyvar | qtycls ( tyvar atype1 ... atypen ) (n=1) qtycls - [ modid . ] tycls tycls - conid tyvar - varid Ok, so clearly the () is a syntactic extension enabled by FlexibleContexts, as it is not a tyvar nor a tyvar applied to a sequence of types. So this is something that a parser should handle. FlexibleContexts also enables similar parses of contexts in other places, for instance in class declarations, for which the Haskell 98 report says topdecl - class [scontext =] tycls tyvar [where cdecls] scontext- simpleclass | ( simpleclass1 , ... , simpleclassn ) (n=0) simpleclass - qtycls tyvar The difference here is that the simpleclass doesn't allow the tyvar applied to a sequence of types bit. FlexibleContexts lifts that restriction too, so there should be no difference between the two kinds of contexts. So the new formal productions for flexible contexts should be something like fcontext- fclass | ( fclass1 , ... , fclassn ) (n=0) fclass - qtycls type1 ... typen (n=1) topdecl - data [fcontext =] simpletype = constrs [deriving] | newtype [fcontext =] simpletype = newconstr [deriving] | class [fcontext =] tycls tyvar [where cdecls] | instance [fcontext =] qtycls inst [where idecls] gendecl - vars :: [fcontext =] type Does this seem correct? Now let's turn to FlexibleInstances, which similarly lifts restrictions, only to instance declarations instead of contexts. The Haskell 98 report says on instance declarations: topdecl - instance [scontext =] qtycls inst [where idecls] inst- gtycon | ( gtycon tyvar1 ... tyvark )(k=0, tyvars distinct) | ( tyvar1 , ... , tyvark ) (k=2, tyvars distinct) | [ tyvar ] | ( tyvar1 - tyvar2 )(tyvar1 and tyvar2 distinct) Note the re-appearance of scontext, which is the same as above. The instance head must be a type constructor, possibly applied to a number of type variables, or one of three built-in syntactic cases. This is where I consider the Haskell 98 report blurry - the fact that the tyvars must be distinct, is that truly a syntactic issue? It might be, it's certainly something that could be checked syntactically. But when you take into account that with the proper extensions, they no longer need to be distinct, at what level would we expect such a check to happen? My gut feeling is that this check for distinctness is something that a type checker might do better than a parser, though it's not clear cut by any means. But since I don't do any other kind of name resolution or checking in my parser even if it would be possible (e.g. multiple declarations of the same symbol), I would find it a bit anomalous to check this too. Turning on FlexibleInstances, we shouldn't need to follow any of the above restrictions on
Re: [Haskell-cafe] Incremental XML parsing with namespaces?
Hi, you may also want to look at: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xml It knows about namespaces and, also, it's parser is lazy. -Iavor On Mon, Jun 8, 2009 at 11:39 AM, John Millikinjmilli...@gmail.com wrote: I'm trying to convert an XML document, incrementally, into a sequence of XML events. A simple example XML document: doc xmlns=org:myproject:mainns xmlns:x=org:myproject:otherns titleDoc title/title x:refabc1234/x:ref html xmlns=http://www.w3.org/1999/xhtml;bodyHello world!/body/html /doc The document can be very large, and arrives in chunks over a socket, so I need to be able to feed the text data into a parser and receive a list of XML events per chunk. Chunks can be separated in time by intervals of several minutes to an hour, so pausing processing for the arrival of the entire document is not an option. The type signatures would be something like: type Namespace = String type LocalName = String data Attribute = Attribute Namespace LocalName String data XMLEvent = EventElementBegin Namespace LocalName [Attribute] | EventElementEnd Namespace LocalName | EventContent String | EventError String parse :: Parser - String - (Parser, [XMLEvent]) I've looked at HaXml, HXT, and hexpat, and unless I'm missing something, none of them can achieve this: + HaXml and hexpat seem to disregard namespaces entirely -- that is, the root element is parsed to doc instead of (org:myproject:mainns, doc), and the second child is x:ref instead of (org:myproject:otherns, ref). Obviously, this makes parsing mixed-namespace documents effectively impossible. I found an email from 2004[1] that mentions a filter for namespace support in HaXml, but no further information and no working code. + HXT looks promising, because I see explicit mention in the documentation of recording and propagating namespaces. However, I can't figure out if there's an incremental mode. A page on the wiki[2] suggests that SAX is supported in the html tag soup parser, but I want incremental parsing of *valid* documents. If incremental parsing is supported by the standard arrow interface, I don't see any obvious way to pull events out into a list -- I'm a Haskell newbie, and still haven't quite figured out monads yet, let alone Arrows. Are there any libraries that support namespace-aware incremental parsing? [1] http://www.haskell.org/pipermail/haskell-cafe/2004-June/006252.html [2] http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_XML ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] please comment on my parser, can I do this cleaner?
Thanks. It seems my original parser also works against FOO,BAR,BAZ if
you only modify
atom = string ,
| ( many1 $ noneOf (), ) -- add ,
Indeed, what to call the thingies in a parser is a source of some
personal consternation.
What is a token, what is an atom, what is an expr? It all seems to be
somewhat ad hoc.
2009/6/9 Daniel Fischer daniel.is.fisc...@web.de:
Am Dienstag 09 Juni 2009 20:29:09 schrieb Thomas Hartman:
All I want to do is split on commas, but not the commas inside () or
tags.
I have been wanting to master parsec for a long time and this simple
exercise looked like a good place to start.
The code below does the right thing. Am I missing any tricks to make
this simpler/neater?
Thanks, thomas.
thart...@ubuntu:~/perlArenacat splitEm.
splitEm.hs splitEm.hs~ splitEm.pl splitEm.pl~
thart...@ubuntu:~/perlArenacat splitEm.hs
{-# LANGUAGE ScopedTypeVariables #-}
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Char
import Text.PrettyPrint (vcat, render, text)
import Data.List.Split hiding (sepBy, chunk)
import Text.ParserCombinators.Parsec.Token
import Debug.Trace
import Debug.Trace.Helpers
-- this works, but is there a neater/cleaner way?
main = ripInputsXs (toEof splitter) splitter [ goodS, badS ]
-- I need a way to split on commas, but not the commas inside '' or
'()' characters
goodS = *2FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
badS = *2)FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
-- the first matches a ), so reject this
splitter = do
chunks :: [String] - toEof (many chunk)
let pieces = map concat $ splitOn [,] chunks
return pieces -- chunks
where
atom = string ,
| ( many1 $ noneOf () )
chunk = parenExpr | atom
I think that does not do what you want.
For input FOO,BAR,BAZ, chunks is [FOO,BAR,BAZ], that won't be split; as
far as I can
see, it splits only on commas directly following a parenExpr (or at the
beginning of the
input or directly following another splitting comma).
parenExpr :: GenParser Char st [Char]
parenExpr = let paren p = betweenInc (char '(' ) (char ')' ) p
| betweenInc (char '' ) (char '' )
p
in paren $ option $ do ps - many1 $ parenExpr | atom
return . concat $ ps
betweenInc o' c' p' = do
o - o'
p - p'
c - c'
return $ [o] ++ p ++ [c]
toEof p' = do
r - p'
eof
return r
ripInputs prs prsName xs = mapM_ (putStrLn . show . parse prs prsName ) xs
ripInputsXs prs prsName xs = mapM_ (putStrLn . showXs . parse prs prsName )
xs where showXs v = case v of
Left e - show e
Right xs - render . vcat . map text $ xs
I can offer (sorry for the names, and I don't know if what that does is
really what you
want):
keepSepBy :: Parser a - Parser a - Parser [a]
keepSepBy p sep = (do
r - p
(do s - sep
xs - keepSepBy p sep
return (r:s:xs)) | return [r])
| return []
twain :: Parser a - Parser a - Parser [a] - Parser [a]
twain open close list = do
o - open
l - list
c - close
return (o:l++[c])
comma :: Parser String
comma = string ,
simpleChar :: Parser Char
simpleChar = noneOf (),
suite :: Parser String
suite = many1 simpleChar
atom :: Parser String
atom = fmap concat $ many1 (parenExp | suite)
parenGroup :: Parser String
parenGroup = fmap concat $ keepSepBy atom comma
parenExp :: Parser String
parenExp = twain (char '') (char '') parenGroup
| twain (char '(') (char ')') parenGroup
chunks :: Parser [String]
chunks = sepBy atom comma
splitter = do
cs - chunks
eof
return cs
goodS = *2FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
badS = *2)FOO2,1,*3(SigB8:0:2,BAR),*2Siga2:0,Sigb8,7,6,5,0
goodRes = parse splitter splitter goodS
badRes = parse splitter splitter badS
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[Haskell-cafe] Re: I love purity, but it's killing me.
Paul L nine...@gmail.com wrote in article 856033f20906082224s2b7d5391gdc7a4ed913004...@mail.gmail.com in gmane.comp.lang.haskell.cafe: The open question is whether there exists such a solution that's both elegant and efficient at maintain proper sharing in the object language. What is your criterion for efficient? We certainly can get rid of all interpretive overheads by either having a tagless interpreter (as in Oleg and Shan's paper), or by direct compilation. (BTW, the paper is by Jacques Carette, Oleg Kiselyov, and Chung-chieh Shan.) But so far I don't see how a tagless interpreter could handle sharing when it can't be distinguished in the host language. Indeed, I would agree with those on this thread who have stated that sharing should be distinguished in the host language. -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig We want our revolution, and we want it now! -- Marat/Sade We want our revolution, and we'll take it at such time as you've gotten around to delivering it -- Haskell programmer ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] help with a question
I'm very new to haskell hugs and would appreciate it if someone could help me
out here. I've been giving 2 questions.
1.) A and B are two sets of integers. Implement a function to obtain the
integers that belong to both sets.
Test your function fully.
2.) Define and test a function f, which, if A is a set of {x, y, z} then
f(A) = {{},{x}, {y}, {z}, {x, y}, {x,z}, {y,z}, {x, y, z}}
Manage to get the first one.
interset::[Int]-[Int]-[Int]
interset x [] = []
interset [] y = []
interset x@(xs:xt) y@(ys:yt) =
if xs == ys
then as:(interset at yt)
else interset at y
Totally have no clue for the 2nd question. could someone help me out ?
Many thanks!
--
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Re: [Haskell-cafe] help with a question
haonan21 wrote:
I'm very new to haskell hugs and would appreciate it if someone could help me
out here. I've been giving 2 questions.
1.) A and B are two sets of integers. Implement a function to obtain the
integers that belong to both sets.
Test your function fully.
2.) Define and test a function f, which, if A is a set of {x, y, z} then
f(A) = {{},{x}, {y}, {z}, {x, y}, {x,z}, {y,z}, {x, y, z}}
Manage to get the first one.
interset::[Int]-[Int]-[Int]
interset x [] = []
interset [] y = []
interset x@(xs:xt) y@(ys:yt) =
if xs == ys
then as:(interset at yt)
else interset at y
Totally have no clue for the 2nd question. could someone help me out ?
Many thanks!
Haonan,
This looks like homework, but I can offer a few suggestions. Your
interset function uses as and at where I think you mean xs and
xt and the else case is wrong (you need to test the code!). Anyway,
it looks like you're assuming that the lists are in ascending order, and
I don't see that in the problem specification -- it won't work if that
isn't the case. More interestingly, you should look at the List (or
Data.List) library; it contains a library function which can solve your
problem in one line. As for the second function, that's a classic
problem used for teaching recursion: find all subsets of a given list.
The way to solve it is to first ask what the solution is for the empty
set (which should be obvious). Then assume that you have the solution
for the tail of the list ({y, z}). How would you use this and the head
of the list (x) to generate the full solution?
HTH,
Mike
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Re: [Haskell-cafe] Re: Building network package on Windows
On Mon, Jun 8, 2009 at 10:18 PM, Iavor Diatchki iavor.diatc...@gmail.comwrote: OK, I think that I found and fixed the problem. As Thomas pointed out, the configure script is not wrong. The problem turned out to be the foreign import for getnameinfo (this was the missing symbol). So it was the name mangling! Great, thanks for the patch. It's applied. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
