[Haskell-cafe] Re: (Newbie Question) How to get the value of an "IO String" Monad

[Peter]

Hello All,

Thanks Cale, Udo, Daniel, Lemmih, Ketil, and Marc for your very helpful
posts, and Haskell resources.

Every post really helped a lot.

Thanks for all the help,
Peter

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Re: [Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Cale Gibbard]

Hi,

Apart from the other posts, you might also want to read
http://www.haskell.org/hawiki/IntroductionToIO which is a quick intro
to the way IO is handled in Haskell and
http://www.haskell.org/hawiki/UsingIo which covers similar ground, but
which also goes into a number of other common questions.

 - Cale

On 17/02/06, Peter <[EMAIL PROTECTED]> wrote:
> Hello,
>
> For the purpose of familiarizing myself with Haskell, and also because I
> love Haskell :),
> I am trying to re-make a script that I made in Python that sends a
> request to a server and extracts the list of email addresses from a
> Mailman web-page by using an XML Parser on the page's HTML that has been
> converted to XHTML by "HTML Tidy".
>
> However, I cannot seem to figure out a way to get the state of a Monad;
> Specifically I cannot get the value of an "IO String" Monad.
>
> I have read some tutorials on Monads but I guess I must have missed
> something.
>
> I have read that the >>= operator is the only way to extract the state
> of an action as a string, and pipe it to a function. So far so good.
> But, That does not seem to work, because as I understand the >>=
> operator, it expects the function on the right hand side to return an IO
> Monad, which completely defeats the purpose here.
>
> So, How am I supposed to get the value of an IO Monad, such as "IO
> String", without returning an IO Monad?
>
> If this is of any help, here is the function I am stuck on:
> recv_headers' :: Socket.Socket -> String -> IO [[String]]
> recv_headers' sock bulk
> | received == ""= error "Connection died unexpectedly."
> | received == "\n"
>   && endswith bulk "\r\n\r" = return [["foo", "bar"]]
> | otherwise = recv_headers' sock (bulk ++ received)
> where received = (Socket.recv sock 1)
> --- End code ---
>
> And here is the (expected) error I get from trying to compare "IO
> String" to "String":
> MemberBackup.hs:29:18:
> Couldn't match `IO String' against `[Char]'
>   Expected type: IO String
>   Inferred type: [Char]
> In the second argument of `(==)', namely `""'
> In a pattern guard for
>the definition of `recv_headers'':
> received == ""
> Failed, modules loaded: none.
> --- End error ---
>
> Thanks for the help,
> Peter
>
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[Haskell-cafe] Re: Unexpected results with simple IO

[Maurício]

  You're right... I was running the example in rxvt, in cygwin. Now I 
tried in Windows command shell and it works.


  Thanks,
  Maurício

Cale Gibbard wrote:

That doesn't happen for me at all, it works just fine. Maybe it's
something wrong with your terminal? You could possibly try playing
with the buffering settings on stdout, using hSetBuffering in
System.IO.

 - Cale

On 17/02/06, Maurício <[EMAIL PROTECTED]> wrote:


  Dear Haskell users,

  I have a problem using IO. The small test program below asks the user
to guess from a list of random numbers between 1 and 10. Everything
works well excepts for one problem: all the messages ("Guess a
number...", "Right..." and "Wrong...") are printed after the program
finishes, i.e., I have to use it blind. I'm afraid I misunderstand
something important about lazyness or monads... What am I doing wrong?

  Thanks,
  Maurício

module Main where
import Random

main = do
   r_gen <- getStdGen --random generator
   let r_list = (randomRs (1,10) r_gen) --random list
   guess_loop (r_list)

guess_loop (r:r_others) = do
   putStrLn "Guess a number between 1 and 10:"
   n <- readLn
   if n==r
  then do
 putStrLn "Right! :)"
 return ()
  else do
 putStrLn "Wrong... :("
 guess_loop r_others

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Re: [Haskell-cafe] Problems instancing a class

[Benjamin Franksen]

On Friday 17 February 2006 21:03, Juan Carlos Arevalo Baeza wrote:
>What really got me is that if I don't use a type synonym:
>
> instance MyClass (HT.HashTable String String) where
> htLookup h var =
> do  result <- HT.lookup h var
> case result of
> Nothing -> return ""
> Just s -> return s
>
>it still complains:
>
> Illegal instance declaration for `MyClass (HashTable String String)'
> (The instance type must be of form (T a b c)
>  where T is not a synonym, and a,b,c are distinct type variables)
> In the instance declaration for `MyClass (HashTable String String)'
>
>even though the documentation in GHC states that HashTable is
> declared as "data".

(Guessing) Maybe because 'String' is not a type variable?

>How exactly is type undecidable if this is allowed? I found here:
>
> http://hackage.haskell.org/trac/haskell-prime/wiki/FlexibleInstances
>
>an example of a non-terminating instance:
>
> instance C b b => C (Maybe a) b
>
>I don't see how/where/when this would be non-terminating. I
> mean... if you now define an instance "C Int Int", then you actually
> also have "C (Maybe Int) Int", C (Maybe (Maybe Int)) Int" and so on,
> I can see that. But how exactly is that a problem? Can you show me a
> use of C that cannot work with that instance?

What is (or better: might be) non-terminating is the process (algorithm) 
of /type inference/. This:

http://www.haskell.org//pipermail/haskell-prime/2006-February/000609.html

message contains a detailed explanation.

Ben
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Re: [Haskell-cafe] Unexpected results with simple IO

[Cale Gibbard]

That doesn't happen for me at all, it works just fine. Maybe it's
something wrong with your terminal? You could possibly try playing
with the buffering settings on stdout, using hSetBuffering in
System.IO.

 - Cale

On 17/02/06, Maurício <[EMAIL PROTECTED]> wrote:
>Dear Haskell users,
>
>I have a problem using IO. The small test program below asks the user
> to guess from a list of random numbers between 1 and 10. Everything
> works well excepts for one problem: all the messages ("Guess a
> number...", "Right..." and "Wrong...") are printed after the program
> finishes, i.e., I have to use it blind. I'm afraid I misunderstand
> something important about lazyness or monads... What am I doing wrong?
>
>Thanks,
>Maurício
>
> module Main where
> import Random
>
> main = do
> r_gen <- getStdGen --random generator
> let r_list = (randomRs (1,10) r_gen) --random list
> guess_loop (r_list)
>
> guess_loop (r:r_others) = do
> putStrLn "Guess a number between 1 and 10:"
> n <- readLn
> if n==r
>then do
>   putStrLn "Right! :)"
>   return ()
>else do
>   putStrLn "Wrong... :("
>   guess_loop r_others
>
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[Haskell-cafe] Unexpected results with simple IO

[Maurício]

  Dear Haskell users,

  I have a problem using IO. The small test program below asks the user 
to guess from a list of random numbers between 1 and 10. Everything 
works well excepts for one problem: all the messages ("Guess a 
number...", "Right..." and "Wrong...") are printed after the program 
finishes, i.e., I have to use it blind. I'm afraid I misunderstand 
something important about lazyness or monads... What am I doing wrong?


  Thanks,
  Maurício

module Main where
import Random

main = do
   r_gen <- getStdGen --random generator
   let r_list = (randomRs (1,10) r_gen) --random list
   guess_loop (r_list)

guess_loop (r:r_others) = do
   putStrLn "Guess a number between 1 and 10:"
   n <- readLn
   if n==r
  then do
 putStrLn "Right! :)"
 return ()
  else do
 putStrLn "Wrong... :("
 guess_loop r_others

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Re: [Haskell-cafe] Infinite loop?

[rgo]

On Fri, 17 Feb 2006 17:19:53 +
Jon Fairbairn <[EMAIL PROTECTED]> wrote:

> On 2006-02-17 at 20:12GMT rgo wrote:
> > Hi all,
> > my program probably goes into infinite loop... But i cannot understand 
> > where and why.
> 
> getDirectoryContents will include "." and "..", so if you
> follow those, you're bound to loop.
> 
> -- 
> Jón Fairbairn  Jon.Fairbairn at cl.cam.ac.uk
> 
> 

Thanks.
It's the third time in my life when do this error. :-[

here working code:

import System.Directory

data MyFile = MyDir {
dir_name :: FilePath,
dir_files :: [MyFile]
}
| MyFile {
file_name :: FilePath
}

read_dir_entries :: FilePath -> [FilePath] -> IO [MyFile]
read_dir_entries _ [] = return []
read_dir_entries dirname (name:names) = do
isdir <- doesDirectoryExist name;
entry <- if (isdir && (name /= ".") && (name /= ".."))
then read_dir (dirname ++ "/" ++ name)
else return (MyFile {file_name = name});
entries <- read_dir_entries dirname names;
return (entry:entries)

read_dir :: FilePath -> IO MyFile
read_dir name = do 
content <- getDirectoryContents name;
files <- read_dir_entries name content;
return MyDir {
dir_name = name,
dir_files = files
}

instance Show MyFile where
show (MyDir {dir_name = name, dir_files = files}) = 
"\n" ++ name ++ ":\n" ++ show files
show (MyFile {file_name = name}) = "\t" ++ name ++ "\n"

main = do
cwd <- getCurrentDirectory;
dir <- read_dir cwd;
print dir;
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Re: [Haskell-cafe] Problems instancing a class

[Juan Carlos Arevalo Baeza]

Gerrit van den Geest wrote:

Mark
Mark Jones has (some time ago) also written a very detailed e-mail 
about this topic:


http://www.haskell.org/pipermail/haskell/2000-October/006128.html


  I really don't understand anything spoken about in this message. I 
guess I need it translated into plain English :)


  I'd contend that, if Haskell is so good (and I believe it is), it 
should be more accessible to your average Joe, even if some might disagree.



Grt


"type" introduce a type synonym, and Haskell98 forbids these in
instances, so GHC complains. GHC also lifts this restriction when
invoked with -fglasgow-exts .
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#type-synonyms 



  Thanx all for the replies. And here I thought I was compiling with 
extensions enabled. Blame it on using a compile script. Not that I had 
made the connection with this error...


  What really got me is that if I don't use a type synonym:

instance MyClass (HT.HashTable String String) where
   htLookup h var =
   do  result <- HT.lookup h var
   case result of
   Nothing -> return ""
   Just s -> return s

  it still complains:

Illegal instance declaration for `MyClass (HashTable String String)'
   (The instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type variables)
In the instance declaration for `MyClass (HashTable String String)'

  even though the documentation in GHC states that HashTable is 
declared as "data".


  I haven't had time to read all the documentation you guys have 
forwarded to me, but still, I have one question. I was reading here:


http://hackage.haskell.org/trac/haskell-prime/wiki/TypeSynonymInstances

  and it says "The proposal is to allow type synonyms (fully applied, 
as ever) in instance heads. These would be fully expanded before any 
other restrictions on instance heads were checked", which is what sounds 
logical to me. And then it says "Not very useful without either 
FlexibleInstances  or 
UndecidableInstances ". I 
don't understand why. This extension would not depend on those other 
more involved extensions. Later down it says "Cons - Since constraints 
on the instance head refer to the expanded type, errors can be more 
obscure". Is that the reason? Because the other extensions would make 
errors clearer?


  I'd like to ask someone in the know to provide a good example that 
shows the reasons behind those statements are there. Preferably added to 
the Wiki :)


  How exactly is type undecidable if this is allowed? I found here:

http://hackage.haskell.org/trac/haskell-prime/wiki/FlexibleInstances

  an example of a non-terminating instance:

instance C b b => C (Maybe a) b

  I don't see how/where/when this would be non-terminating. I mean... 
if you now define an instance "C Int Int", then you actually also have 
"C (Maybe Int) Int", C (Maybe (Maybe Int)) Int" and so on, I can see 
that. But how exactly is that a problem? Can you show me a use of C that 
cannot work with that instance?


  Thanx!

JCAB

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Re: [Haskell-cafe] Problems instancing a class

[Gerrit van den Geest]

Mark 


Mark Jones has (some time ago) also written a very detailed e-mail about this 
topic:

http://www.haskell.org/pipermail/haskell/2000-October/006128.html

Grt


"type" introduce a type synonym, and Haskell98 forbids these in
instances, so GHC complains. GHC also lifts this restriction when
invoked with -fglasgow-exts .
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#type-synonyms



Flexible Instances will probably be added to HaskellPrime:
 http://hackage.haskell.org/trac/haskell-prime/wiki/FlexibleInstances

This page:

http://cvs.haskell.org/Hugs/pages/users_guide/class-extensions.html#FLEXIBLE-INSTANCES

says that ultimately you would turn the type language pretty much into
Prolog (which would allow more expressive power---and less
inconvenience as we both would like--but make general type checking
undecidable). Instead they do a more conservative extension with a
fixed depth of constraints it will check so the compiler will
terminate.

 Jared.
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Re: [Haskell-cafe] Problems instancing a class

[Jared Updike]

> "type" introduce a type synonym, and Haskell98 forbids these in
> instances, so GHC complains. GHC also lifts this restriction when
> invoked with -fglasgow-exts .
> http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#type-synonyms
>

Flexible Instances will probably be added to HaskellPrime:
  http://hackage.haskell.org/trac/haskell-prime/wiki/FlexibleInstances

This page:

http://cvs.haskell.org/Hugs/pages/users_guide/class-extensions.html#FLEXIBLE-INSTANCES

says that ultimately you would turn the type language pretty much into
Prolog (which would allow more expressive power---and less
inconvenience as we both would like--but make general type checking
undecidable). Instead they do a more conservative extension with a
fixed depth of constraints it will check so the compiler will
terminate.

  Jared.
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Re: [Haskell-cafe] Problems instancing a class

[Roberto Zunino]

Juan Carlos Arevalo Baeza wrote:

> type Context = HT.HashTable String String
[snip]
> Illegal instance declaration for `MyClass Context'
>(The instance type must be of form (T a b c)
> where T is not a synonym, and a,b,c are distinct type variables)
> In the instance declaration for `MyClass Context'

"type" introduce a type synonym, and Haskell98 forbids these in 
instances, so GHC complains. GHC also lifts this restriction when 
invoked with -fglasgow-exts .


http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#type-synonyms

>   If I use "data" instead of "type", it works:
>
> data Context = C (HT.HashTable String String)

This is fine: data (and newtype) declarations do not introduce type 
synonyms, but a genuine new type.


Regards,
Roberto Zunino.
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[Haskell-cafe] Problems instancing a class

[Juan Carlos Arevalo Baeza]

  So, consider this code:

import Data.HashTable as HT

class MyClass a where
   htLookup :: a -> String -> IO String

type Context = HT.HashTable String String

instance MyClass Context where
   htLookup h var =
   do  result <- HT.lookup h var
   case result of
   Nothing -> return ""
   Just s -> return s

  This doesn't compile. GHC says:

Illegal instance declaration for `MyClass Context'
   (The instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type variables)
In the instance declaration for `MyClass Context'

  If I use "data" instead of "type", it works:

data Context = C (HT.HashTable String String)

instance MyClass Context where
   htLookup (C h) var =
   do  result <- HT.lookup h var
   case result of
   Nothing -> return ""
   Just s -> return s

  Why? What's going on here?

JCAB

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[Haskell-cafe] Re: Associated Type Synonyms question

[Stefan Wehr]

Stefan Wehr <[EMAIL PROTECTED]> wrote::

> Niklas Broberg <[EMAIL PROTECTED]> wrote::
>
>> On 2/10/06, Ross Paterson <[EMAIL PROTECTED]> wrote:
>>> On Fri, Feb 10, 2006 at 05:20:47PM +0100, Niklas Broberg wrote:
>>> > - when looking at the definition of MonadWriter the Monoid constraint
>>> > is not strictly necessary, and none of the other mtl monads have
>>> > anything similar. Is it the assumption that this kind of constraint is
>>> > never really necessary, and thus no support for it is needed for ATS?
>>>
>>> I think that's right -- it's only needed for the Monad instance for
>>> WriterT.  But it is a convenience.  In any instance of MonadWriter, the
>>> w will be a monoid, as there'll be a WriterT in the stack somewhere,
>>> so the Monoid constraint just saves people writing general functions
>>> with MonadWriter from having to add it.
>>
>> Sure it's a convenience, and thinking about it some more it would seem
>> like that is always the case - it is never crucial to constrain a
>> parameter. But then again, the same applies to the Monad m constraint,
>> we could give the same argument there (all actual instances will be
>> monads, hence...). So my other question still stands, why not allow
>> constraints on associated types as well, as a convenience?
>
> Manuel (Chakravarty) and I agree that it should be possible to
> constrain associated type synonyms in the context of class
> definitions. Your example shows that this feature is actually
> needed. I will integrate it into phrac within the next few days.

I have now implemented this feature in PHRaC. You need to get PHRaC via darcs:

  http://www.cse.unsw.edu.au/~pls/repos/phrac/

An example is included in PHRaC's source code under
  
  tests/constrained-assoc-types/should_pass/000.phc. 

I haven't tested the extension extensively. Just tell me if you have
any problems.

Stefan

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Re: [Haskell-cafe] Infinite loop?

[Ralf Hinze]

Am Freitag 17 Februar 2006 21:12 schrieb rgo:
> Hi all,
> my program probably goes into infinite loop... But i cannot understand where 
> and why.
> 
> code:
> 
> import System.Directory
> 
> data MyFile = MyDir {
> dir_name :: String,
> dir_files :: [MyFile]
> }
> | MyFile {
> file_name :: String
> }
> 
> read_dir_entries :: [FilePath] -> IO [MyFile]
> read_dir_entries [] = return []
> read_dir_entries (name:names) = do
> isdir <- doesDirectoryExist name;
> entry <- if isdir
> then read_dir name
> else return (MyFile {file_name = name});
> entries <- read_dir_entries names;
> return (entry:entries)
> 
> 
> read_dir :: FilePath -> IO MyFile
> read_dir name = do 
> content <- getDirectoryContents name;
> files <- read_dir_entries content;
> return MyDir {
> dir_name = name,
> dir_files = files
> }
> 
> instance Show MyFile where
> show (MyDir {dir_name = name, dir_files = files}) = 
> "\n" ++ name ++ ":\n" ++ show files
> show (MyFile {file_name = name}) = "\t" ++ name ++ "\n"
> 
> main = do
> cwd <- getCurrentDirectory;
> dir <- read_dir cwd;
> print dir;

Maybe because the file system is a graph not a tree (. and ..)?
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Re: [Haskell-cafe] Infinite loop?

[Jon Fairbairn]

On 2006-02-17 at 09:22PST Jared Updike wrote:
> Yep. change one line to:
> 
>entry <- if isdir && name /= "." && name /= ".."
> 
> and it does in fact work.

Only if no-one has been tricky with symbolic links.

-- 
Jón Fairbairn  Jon.Fairbairn at cl.cam.ac.uk


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Re: [Haskell-cafe] Infinite loop?

[Jared Updike]

Yep. change one line to:

   entry <- if isdir && name /= "." && name /= ".."

and it does in fact work.

  Jared.

On 2/17/06, Jon Fairbairn <[EMAIL PROTECTED]> wrote:
> On 2006-02-17 at 20:12GMT rgo wrote:
> > Hi all,
> > my program probably goes into infinite loop... But i cannot understand 
> > where and why.
>
> getDirectoryContents will include "." and "..", so if you
> follow those, you're bound to loop.
>
> --
> Jón Fairbairn  Jon.Fairbairn at cl.cam.ac.uk
>
>
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--
http://www.updike.org/~jared/
reverse ")-:"
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Re: [Haskell-cafe] Infinite loop?

[Jon Fairbairn]

On 2006-02-17 at 20:12GMT rgo wrote:
> Hi all,
> my program probably goes into infinite loop... But i cannot understand where 
> and why.

getDirectoryContents will include "." and "..", so if you
follow those, you're bound to loop.

-- 
Jón Fairbairn  Jon.Fairbairn at cl.cam.ac.uk


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[Haskell-cafe] Infinite loop?

[rgo]

Hi all,
my program probably goes into infinite loop... But i cannot understand where 
and why.

code:

import System.Directory

data MyFile = MyDir {
dir_name :: String,
dir_files :: [MyFile]
}
| MyFile {
file_name :: String
}

read_dir_entries :: [FilePath] -> IO [MyFile]
read_dir_entries [] = return []
read_dir_entries (name:names) = do
isdir <- doesDirectoryExist name;
entry <- if isdir
then read_dir name
else return (MyFile {file_name = name});
entries <- read_dir_entries names;
return (entry:entries)


read_dir :: FilePath -> IO MyFile
read_dir name = do 
content <- getDirectoryContents name;
files <- read_dir_entries content;
return MyDir {
dir_name = name,
dir_files = files
}

instance Show MyFile where
show (MyDir {dir_name = name, dir_files = files}) = 
"\n" ++ name ++ ":\n" ++ show files
show (MyFile {file_name = name}) = "\t" ++ name ++ "\n"

main = do
cwd <- getCurrentDirectory;
dir <- read_dir cwd;
print dir;
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Re: [Haskell-cafe] Badly designed Parsec combinators?

[Jan-Willem Maessen]

On Feb 16, 2006, at 7:32 PM, John Meacham wrote:


...

  Again that doesn't compile, because "when" requires a ()-returning
monad as its second parameter, but the "string" parser returns  
"String".
Same thing with if-then-else, when used to switch IO actions and  
such:
the IO actions must fully match in type, even if the returned  
value will

be discarded, and again that can be trivially resolved by adding the
"return ()".


This is a straight up bug in the definition of when I hope we fix. it
should have type

when :: Bool -> IO a -> IO ()
when = ...


Arguably this could be made true of *every* function which presently  
takes m () as an argument.  That is, we could systematically go  
through the libraries and convert every function of type:


f :: (Monad m) =>  -> m () -> ...

into

f :: (Monad m) =>  -> m otherwiseUnusedTypeVariable -> ...

This would basically eliminate the need for "ignore".  I can see  
taste arguments in either direction, but really the language ought to  
pick an alternative and use it everywhere (including for >>).


-Jan-Willem Maessen



John


--
John Meacham - ⑆repetae.net⑆john⑈
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Re: [Haskell-cafe] HaXml: ampersand in attribute value

[Malcolm Wallace]

Lennart Augustsson wrote:
> But speaking of HaXml bugs, I'm pretty sure HaXml doesn't handle
> % correctly.  It seem to treat % specially everywhere, but I think
> it is only special inside DTDs.  I have many XML files produced by
> other tools that the HaXml parser fails to process because of this.

I believe I fixed at least one bug to do with % characters around
version 1.14.  But that is the development branch in darcs, not formally
released yet.  Nevertheless, if you know of such bugs, do report them;
even better if you can send a small test case.

Regards,
Malcolm
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Re: [Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Udo Stenzel]

Peter wrote:
> So, How am I supposed to get the value of an IO Monad, such as "IO
> String", without returning an IO Monad?

You read correctly, this is impossible.  You already got some valid
answers, and here's another variant that preserves most of the nice
guarded expressions:


recv_headers' :: Socket.Socket -> String -> IO [[String]]
recv_headers' sock bulk = Socket.recv sock 1 >>= dispatch
  where dispatch ""= error "Connection died unexpectedly."
dispatch "\n" | endswith bulk "\r\n\r" = return [["foo", "bar"]]
dispatch _ = recv_headers' sock (bulk ++ received)

 
Udo.
-- 
"Gadgets are not necessarily an improvement, vide the succession:
Blackboard -> Overhead Projector -> PowerPoint"
-- E. W. Dijkstra


signature.asc
Description: Digital signature
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[Haskell-cafe] Re: HaXml: ampersand in attribute value

[Koen . Roelandt]

> But speaking of HaXml bugs, I'm pretty sure HaXml doesn't handle
> % correctly.  It seem to treat % specially everywhere, but I think
> it is only special inside DTDs.  I have many XML files produced by
> other tools that the HaXml parser fails to process because of this.

I had a similar problem where the parser choked on % signs in attribute 
values in the XML file. I solved it by playing around in Lex.hs. I could 
check for the (ugly) solution...

> Yes, it is expected.  An ampersand indicates the start of a reference, 
> e.g. < or   If there is no semicolon to indicate the end of the 
> reference, then it is a parse error.  The XML specification is quite 
> clear that neither & nor < are valid standalone characters in an 
> attribute value.

Which is exactly the problem. The & is part of a reference, namely ë.

Regards,

Koen.
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Re: [Haskell-cafe] Re: Associated Type Synonyms question

[Ross Paterson]

On Fri, Feb 17, 2006 at 01:26:18PM +, Stefan Wehr wrote:
> Martin Sulzmann <[EMAIL PROTECTED]> wrote::
> > By possible you mean this extension won't break any
> > of the existing ATS inference results?
> 
> Yes, although we didn't go through all the proofs.
> 
> > You have to be very careful otherwise you'll loose decidability.

The paper doesn't claim a proof of decidability (or principal types),
but conjectures that it will go through.

Apropos of that, I tried translating the non-terminating FD example from
the FD-CHR paper (ex. 6) to associated type synonyms (simplified a bit):

data T a = K a;

class C a where {
type S a;
r :: a -> S a;
}

instance C a => C (T a) where {
type S (T a) = T (S a);
r (K x) = K (r x);
}

f b x = if b then r (K x) else x;

Phrac infers

f :: forall a . (S (T a) = a, C a) => Bool -> a -> T (S a)

The constraint is reducible (ad infinitum), but Phrac defers constraint
reduction until it is forced (as GHC does with ordinary instance
inference).  We can try to force it using the MR, by changing the
definition of f to

f = \ b x -> if b then r (K x) else x;

For this to be legal, the constraint must be provable.  In the
corresponding FD case, this sends GHC down the infinite chain of
reductions, but Phrac just gives up and complains about deferred
constraints being left over after type inference.  I don't think this
is right either, as in other cases the constraint will reduce away
to nothing.

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Re: [Haskell-cafe] Re: Associated Type Synonyms question

[Claus Reinke]

Something more controversial.
Why ATS at all? Why not encode them via FDs?


Funny you should say that, just when I've been thinking about 
the same thing. That doesn't mean that ATS aren't a nice way 
to describe some special cases of FDs, but my feeling is that
if ATS can't be encoded in FDs, then there is something 
wrong with _current_ FD versions that should be fixed.


I'd love to hear the experts' opinions about this claim!-)

The main argument for ATS is that the extra parameter for the
functionally dependend type disappears, but as you say, that
should be codeable in FDs. I say should be, because that does
not seem to be the case at the moment.

My approach for trying the encoding was slightly different from
your's, but also ran into trouble with implementations.

First, I think you need a per-class association, so your T a b
would be specific to C. Second, I'd use a superclass context
to model the necessity of providing an associated type, and
instance contexts to model the provision of such a type. No
big difference, but it seems closer to the intension of ATS:
associated types translate into type association constraints.

(a lot like calling an auxiliary function with empty accumulator,
to hide the extra parameter from the external interface)


Example

-- ATS
class C a where
 type T a
 m :: a->T a
instance C Int where
 type T Int = Int
 m _ = 1


-- alternative FD encoding attempt

class CT a b | a -> b
instance CT Int Int

class CT a b => C a where
   m :: a-> b

instance CT Int b => C Int where 
   m _ = 1::b



-- FD encoding
class T a b | a->b 
instance T Int Int


class C a where
 m :: T a b => a->b

instance C Int where
 m _ = 1

-- general recipe:
-- encode type functions T a via type relations T a b
-- replace T a via fresh b under the constraint C a b


referring to the associated type seems slightly awkward 
in these encodings, so the special syntax for ATS would 
still be useful, but I agree that an encoding into FDs should

be possible.


The FD program won't type check under ghc but this
doesn't mean that it's not a legal FD program.


glad to hear you say that. but is there a consistent version
of FDs that allows these things - and if not, is that for lack
of trying or because it is known not to work?

Cheers,
Claus


It's wrong to derive certain conclusions
about a language feature by observing the behavior
of a particular implementation!

Martin


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Re: [Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Daniel Fischer]

Hi,

Am Freitag, 17. Februar 2006 14:42 schrieb Peter:
> Hello,
>
> For the purpose of familiarizing myself with Haskell, and also because I
> love Haskell :),

very good!

> I am trying to re-make a script that I made in Python that sends a
> request to a server and extracts the list of email addresses from a
> Mailman web-page by using an XML Parser on the page's HTML that has been
> converted to XHTML by "HTML Tidy".
>
> However, I cannot seem to figure out a way to get the state of a Monad;
> Specifically I cannot get the value of an "IO String" Monad.
>
> I have read some tutorials on Monads but I guess I must have missed
> something.
>
> I have read that the >>= operator is the only way to extract the state
> of an action as a string, and pipe it to a function. So far so good.
> But, That does not seem to work, because as I understand the >>=
> operator, it expects the function on the right hand side to return an IO
> Monad, which completely defeats the purpose here.

I find do-notation often more readable, then you write val <- action and 
val is an ordinary value, you can use afterwards (within the same do-block, of 
course)

>
> So, How am I supposed to get the value of an IO Monad, such as "IO
> String", without returning an IO Monad?
>
> If this is of any help, here is the function I am stuck on:
> recv_headers' :: Socket.Socket -> String -> IO [[String]]
> recv_headers' sock bulk
>
> | received == ""= error "Connection died unexpectedly."
> | received == "\n"
>
>   && endswith bulk "\r\n\r" = return [["foo", "bar"]]
>
> | otherwise = recv_headers' sock (bulk ++ received)
>
> where received = (Socket.recv sock 1)
> --- End code ---

This should do it:

recv_headers' :: Socket.Socket -> String -> IO [[String]]
recv_headers' sock bulk
 = do
 { received <- Socket.recv sock 1
 ; case received of
 "" -> error "Connection died unexpectedly."
 "\n" | endswith bulk "\r\n\r" -> return [["foo", "bar"]]
 _  -> recv_headers' sock (bulk ++ received)
 }

at least, it compiles.
>
> And here is the (expected) error I get from trying to compare "IO
> String" to "String":
> MemberBackup.hs:29:18:
> Couldn't match `IO String' against `[Char]'
>   Expected type: IO String
>   Inferred type: [Char]
> In the second argument of `(==)', namely `""'
> In a pattern guard for
>the definition of `recv_headers'':
> received == ""
> Failed, modules loaded: none.
> --- End error ---
>
> Thanks for the help,
> Peter
>

Cheers,
Daniel
-- 

"In My Egotistical Opinion, most people's C programs should be
indented six feet downward and covered with dirt."
-- Blair P. Houghton

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Re: [Haskell-cafe] library sort

[Radu Grigore]

On 2/16/06, Jared Updike <[EMAIL PROTECTED]> wrote:
> If you need an easier way to search the Haskell APIs, use Hoogle:

Hoogle is very nice. Thanks to everyone who answered my question about
finding a sort library function.

--
regards,
  radu
http://rgrig.blogspot.com/
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Re: [Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Lemmih]

On 2/17/06, Peter <[EMAIL PROTECTED]> wrote:
> Hello,
>
> For the purpose of familiarizing myself with Haskell, and also because I
> love Haskell :),
> I am trying to re-make a script that I made in Python that sends a
> request to a server and extracts the list of email addresses from a
> Mailman web-page by using an XML Parser on the page's HTML that has been
> converted to XHTML by "HTML Tidy".
>
> However, I cannot seem to figure out a way to get the state of a Monad;
> Specifically I cannot get the value of an "IO String" Monad.
>
> I have read some tutorials on Monads but I guess I must have missed
> something.
>
> I have read that the >>= operator is the only way to extract the state
> of an action as a string, and pipe it to a function. So far so good.
> But, That does not seem to work, because as I understand the >>=
> operator, it expects the function on the right hand side to return an IO
> Monad, which completely defeats the purpose here.
>
> So, How am I supposed to get the value of an IO Monad, such as "IO
> String", without returning an IO Monad?
>
> If this is of any help, here is the function I am stuck on:
> recv_headers' :: Socket.Socket -> String -> IO [[String]]
> recv_headers' sock bulk
> | received == ""= error "Connection died unexpectedly."
> | received == "\n"
>   && endswith bulk "\r\n\r" = return [["foo", "bar"]]
> | otherwise = recv_headers' sock (bulk ++ received)
> where received = (Socket.recv sock 1)

Try:

recv_headers' sock bulk
  = do received <- Socket.recv sock 1
   case received of
 "" -> error ...
 "\n" | endswith bulk "\r\n\r" -> ...
 _ -> recv_headers' sock (bulk ++ received)

--
Friendly,
  Lemmih
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Re: [Haskell-cafe] haskell question

[Ketil Malde]

"IBRAHIM MOSHAYA" <[EMAIL PROTECTED]> writes:

> I need to write a function to return a given number

Homework assignment?

Perhaps if you get stuck, you can post your best current effort, and
people will be able to nudge you in the right direction?

-k
-- 
If I haven't seen further, it is by standing in the footprints of giants

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Re: [Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Ketil Malde]

Peter <[EMAIL PROTECTED]> writes:

> So, How am I supposed to get the value of an IO Monad, such as "IO
> String", without returning an IO Monad?

Short answer: you don't.  IO is a one way street.  

Build your application top down in the IO monad (starting with
'main'), and bottom up with pure code, and hope you can make them meet
somewhere in the middle. :-)

-k
-- 
If I haven't seen further, it is by standing in the footprints of giants

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Re: [Haskell-cafe] HaXml: ampersand in attribute value

[Lennart Augustsson]

But speaking of HaXml bugs, I'm pretty sure HaXml doesn't handle
% correctly.  It seem to treat % specially everywhere, but I think
it is only special inside DTDs.  I have many XML files produced by
other tools that the HaXml parser fails to process because of this.

-- Lennart

Malcolm Wallace wrote:

[EMAIL PROTECTED] wrote:
HaXml seems to choke on finding an ampersand in an attribute value. Is 
this normal? Is there any workaround?


Yes, it is expected.  An ampersand indicates the start of a reference, 
e.g. < or   If there is no semicolon to indicate the end of the 
reference, then it is a parse error.  The XML specification is quite 
clear that neither & nor < are valid standalone characters in an 
attribute value.


Regards,
Malcolm
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Re: [Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Marc Weber]

You can access IO values only from within do blocks (see any tutorial,
previous posts or google).
It looks like this then:
do=
  myvalue <- functionwhichreturnsIOValue
  dosomethingwith myvalue

Due to monads you don't have to leave the IO monad this way.
Oh. Have to go now.

do is translated into >>= syntax, see do expansion.

Hope this did help else wait for another longer answer ;-)

Marc
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[Haskell-cafe] haskell question

[IBRAHIM MOSHAYA]

please can you help me with this function:

Significant Figures
I need to write a function to return a given number (1st argument) to a 
given number of significant figures
(2nd argument). The function should be called sigFig and have a type 
signature of the form:

sigFig :: (???) => a -> Int -> a
For instance:
*Main> sigFig 123.456 2
120.0
*Main> sigFig 123.456 4
123.5
*Main> sigFig 0.987 2
0.99
*Main> sigFig 0.432 2
0.43
The type variable a will need to be constrained, but you should aim for the 
most general implementation you
can (replacing ??? above with the appropriate constraint(s)). You may make 
free use of Prelude functions.
Referring to [Jon03] you may wish to consider the use of functions / 
operators including:

/, div, mod, ^, ^^, **, round, fromIntegral, realToFrac

thanks alot


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[Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Peter]

Hello,

For the purpose of familiarizing myself with Haskell, and also because I
love Haskell :),
I am trying to re-make a script that I made in Python that sends a
request to a server and extracts the list of email addresses from a
Mailman web-page by using an XML Parser on the page's HTML that has been
converted to XHTML by "HTML Tidy".

However, I cannot seem to figure out a way to get the state of a Monad;
Specifically I cannot get the value of an "IO String" Monad.

I have read some tutorials on Monads but I guess I must have missed
something.

I have read that the >>= operator is the only way to extract the state
of an action as a string, and pipe it to a function. So far so good.
But, That does not seem to work, because as I understand the >>=
operator, it expects the function on the right hand side to return an IO
Monad, which completely defeats the purpose here.

So, How am I supposed to get the value of an IO Monad, such as "IO
String", without returning an IO Monad?

If this is of any help, here is the function I am stuck on:
recv_headers' :: Socket.Socket -> String -> IO [[String]]
recv_headers' sock bulk
| received == ""= error "Connection died unexpectedly."
| received == "\n"
  && endswith bulk "\r\n\r" = return [["foo", "bar"]]
| otherwise = recv_headers' sock (bulk ++ received)
where received = (Socket.recv sock 1)
--- End code ---

And here is the (expected) error I get from trying to compare "IO
String" to "String":
MemberBackup.hs:29:18:
Couldn't match `IO String' against `[Char]'
  Expected type: IO String
  Inferred type: [Char]
In the second argument of `(==)', namely `""'
In a pattern guard for
   the definition of `recv_headers'':
received == ""
Failed, modules loaded: none.
--- End error ---

Thanks for the help,
Peter

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[Haskell-cafe] Re: Associated Type Synonyms question

[Stefan Wehr]

Martin Sulzmann <[EMAIL PROTECTED]> wrote::

> Stefan Wehr writes:
> > [...]
> > Manuel (Chakravarty) and I agree that it should be possible to
> > constrain associated type synonyms in the context of class
> > definitions. Your example shows that this feature is actually
> > needed. I will integrate it into phrac within the next few days.
> > 
>
> By possible you mean this extension won't break any
> of the existing ATS inference results?

Yes, although we didn't go through all the proofs.

> You have to be very careful otherwise you'll loose decidability.

Do you have something concrete in mind or is this a more general
advice?

Stefan

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Re: [Haskell-cafe] HaXml: ampersand in attribute value

[Malcolm Wallace]

[EMAIL PROTECTED] wrote:
HaXml seems to choke on finding an ampersand in an attribute value. Is 
this normal? Is there any workaround?


Yes, it is expected.  An ampersand indicates the start of a reference, 
e.g. < or   If there is no semicolon to indicate the end of the 
reference, then it is a parse error.  The XML specification is quite 
clear that neither & nor < are valid standalone characters in an 
attribute value.


Regards,
Malcolm
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Re: [Haskell-cafe] Code completion? (IDE)?

[Ketil Malde]

Marc Weber <[EMAIL PROTECTED]> writes:

> I tried downloading, eclipse and the latest EcplipseFP zip file.
> I couldn't use the standard installer (Help-> Add Features ..) because
> it didn't find anything..

Strange, that worked for me.

>From my quick look, Eclipse looks like a workable candidate.  Since I
have the Emacs key combinations pretty much hard-wired in my hands, I
was pleased to see there was an Emacs mode, and pleasantly surprised
that unlike some Emacs modes, it actually seemed to be fairly faithful
and complete.

However, I got a bit lost in the various menus, and having no previous
experience with it, I don't know whether any problems are due to bugs
or incompletenesses in EclipseFP, or my lack of experience with
Eclipse itself.  So at the moment, I'm still using Emacs.

-k
-- 
If I haven't seen further, it is by standing in the footprints of giants

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Re: [Haskell-cafe] Code completion? (IDE)?

[Marc Weber]

On Thu, Feb 16, 2006 at 09:50:51AM -0300, Thiago Arrais wrote:
> On 2/16/06, Thiago Arrais <[EMAIL PROTECTED]> wrote:
> > Just take a look at the latest integration build that you are able to find 
> > at
> >
> > http://eclipsefp.sourceforge.net/download
> 
> There is also a screenshot at
> 
> http://eclipsefp.sourceforge.net/images/first-content-assist.png

Is this content aware?
Doing this kind of complition is easy to achieve in vim, too.

I tried downloading, eclipse and the latest EcplipseFP zip file.
I couldn't use the standard installer (Help-> Add Features ..) because
it didn't find anything..

But now I don't know how to switch to "haskell" mode or wether it's
properly installed.

But I think I can't use it because Eclipse is slow compared to vim and
at the moment I'm running Windows and colinux on the same system (using
colinux) with 512 RAM.. When using Eclipse swapping starts.. but if you
get this working for jEdit.. you may make me using it ..

I think my way to go is: using already existing haskell parsers and add
haskell script support to vim.. Using hs-plugins it shouldn't be that
hard.. But I have to learn that stuff first.

Marc
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[Haskell-cafe] HaXml: ampersand in attribute value

[Koen . Roelandt]

HaXml seems to choke on finding an ampersand in an attribute value. Is 
this normal? Is there any workaround?

Cheers,

Koen.
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[Haskell-cafe] descriptive symbols, was: monad combinator

[Christian Maeder]

Tomasz Zielonka wrote:

On Thu, Feb 16, 2006 at 04:36:06PM +0100, Christian Maeder wrote:

Udo Stenzel wrote:

(*>) :: Monad m => m a -> m b -> m a
m *> n = do a <- m ; n ; return a
Right, that one is really useful. I named it (<<), though, conforming to 
 (>>=) versus (=<<).


But =<< first executes the second argument...


... so much to striking symbolic identifiers. (In fact, I've never used 
=<<.)


Christian
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Re: [Haskell-cafe] Decidable type systems? (WAS: Associated TypeSynonyms question)

[Tomasz Zielonka]

On Thu, Feb 16, 2006 at 12:27:54PM +, Robin Green wrote:
> But are there any decidable type checking algorithms that have been
> seriously proposed or used which would take far too long to terminate
> for real code? If not, then decidability is the only thing that matters.

Perhaps they weren't proposed because decidability was not the only
thing that mattered?

Best regards
Tomasz

-- 
I am searching for programmers who are good at least in
(Haskell || ML) && (Linux || FreeBSD || math)
for work in Warsaw, Poland
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Re: [Haskell-cafe] enforcing strictness on arbitrary subexpressions

[Matthias Fischmann]

Thanks Udo, that helped a lot.

I added the link to haskell.org.  I couldn't find a proper place to
put it, so I added a page 'By topic' (still quite empty) linked from
the main page.  (I hope that wasn't against any rules I missed; if
anybody objects please feel free to remove it.)

cheers,
matthias



On Thu, Feb 16, 2006 at 01:10:14PM +0100, Udo Stenzel wrote:
> To: Matthias Fischmann <[EMAIL PROTECTED]>
> Cc: haskell-cafe@haskell.org
> From: Udo Stenzel <[EMAIL PROTECTED]>
> Date: Thu, 16 Feb 2006 13:10:14 +0100
> Subject: Re: [Haskell-cafe] enforcing strictness on arbitrary subexpressions
> 
> Matthias Fischmann wrote:
> > I want to force evaluation on an arbitrary expression.
> > [...]
> 
> > main :: IO ()
> > main = do
> >hPutStr stdout veryLongString  -- lazy
> >veryLongString `seq` hPutStr stdout veryLongString  -- still lazy?
> >(StrictThingy veryLongString) `seq` hPutStr stdout veryLongString  
> > -- strict (or at least i hope it is)
> 
> The last line is actually equivalent to the second.  Strict data
> constructors are defined in term of seq, and seq only forces the
> evaluation of the outermost constructor.  So after seq'ing a string, it
> is either empty or has at least one element, but that element and the
> whole tail are still unevaluated.  You have to recurse into the string
> to evaluate everything:
> 
> > hPutStr stdout $ foldr seq veryLongString veryLongString
> 
> There is no primitive to do this for arbitrary data types, but the
> DeepSeq type class comes close.  You can find DeepSeq and some more
> hints on strict evaluation at
> .
> 
> 
> Udo.
> -- 
> Today is the tomorrow you worried about yesterday.


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