Re: [Haskell-cafe] enforcing strictness on arbitrary subexpressions
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 http://users.aber.ac.uk/afc/stricthaskell.html. Udo. -- Today is the tomorrow you worried about yesterday. signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] descriptive symbols, was: monad combinator
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] HaXml: ampersand in attribute value
HaXml seems to choke on finding an ampersand in an attribute value. Is this normal? Is there any workaround? Cheers, Koen. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Code completion? (IDE)?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Code completion? (IDE)?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HaXml: ampersand in attribute value
[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. lt; or #20; 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Associated Type Synonyms question
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] (Newbie Question) How to get the value of an IO String Monad?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] haskell question
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (Newbie Question) How to get the value of an IO String Monad?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HaXml: ampersand in attribute value
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. lt; or #20; 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 ___ 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] (Newbie Question) How to get the value of an IO String Monad?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell question
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (Newbie Question) How to get the value of an IO String Monad?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] library sort
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/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (Newbie Question) How to get the value of an IO String Monad?
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] Re: Associated Type Synonyms question
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Associated Type Synonyms question
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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[Haskell-cafe] Re: HaXml: ampersand in attribute value
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. lt; or #20; 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 euml;. Regards, Koen. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (Newbie Question) How to get the value of an IO String Monad?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HaXml: ampersand in attribute value
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Badly designed Parsec combinators?
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⑈ ___ 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] Infinite loop?
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] Infinite loop?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Infinite loop?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- http://www.updike.org/~jared/ reverse )-: ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Infinite loop?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Problems instancing a class
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems instancing a class
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. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems instancing a class
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 /trac/haskell-prime/wiki/FlexibleInstances or UndecidableInstances /trac/haskell-prime/wiki/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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Infinite loop?
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] Unexpected results with simple IO
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 ___ 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] Problems instancing a class
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Unexpected results with simple IO
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 ___ 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] (Newbie Question) How to get the value of an IO String Monad?
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 ___ 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: (Newbie Question) How to get the value of an IO String Monad
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
