So, the Haskell98 solution to this is:
class StringableList a where
listToString :: [a] - String
-- now [a] is of the proper form; T = [], a is a type variable
instance StringableList a = Stringable [a] where
toString = listToString
-- now to make an instance for Stringable [Char]
--
leledumbo wrote:
module Main where
import Data.List
-- quicksort of any list
qsort [] = []
qsort (x:xs) = qsort(filter(x) xs) ++ [x] ++ qsort(filter(=x) xs)
-- optimized quicksort, uses middle element as pivot
qsortOpt [] = []
qsortOpt x = qsortOpt less ++ [pivot] ++ qsortOpt greater
Great! Thanks for putting the code out!
Manuel
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module Main where
import Data.List
-- quicksort of any list
qsort [] = []
qsort (x:xs) = qsort(filter(x) xs) ++ [x] ++ qsort(filter(=x) xs)
-- optimized quicksort, uses middle element as pivot
qsortOpt [] = []
qsortOpt x = qsortOpt less ++ [pivot] ++ qsortOpt greater
where
pivot = x
Janis Voigtlaender wrote:
Derek Elkins wrote:
Agreed. I'm extremely tired of the I haven't heard this term therefore
it must be 'scary' and complicated and beyond me attitude. Such people
need to stop acting like five year old children.
Not that it has much to do with the debate, but the
Simon Marlow wrote:
Don Stewart wrote:
* GHC.Prim was moved,
Nobody should be importing GHC.Prim, use GHC.Exts instead.
This is oft declared, but as things stand I don't think it's a tenable
position. If someone's bothering to dig as far as GHC.Exts then it's
very likely they're
Hello Albert,
Wednesday, October 15, 2008, 7:51:06 AM, you wrote:
Illegal instance declaration for `Stringable [Char]'
(All instance types must be of the form (T a1 ... an)
where a1 ... an are distinct type *variables*
Just in case: n=0 for instance Eq Blah, i.e., T a1 ...
Lennart Augustsson wrote:
I was just pointing out that the mechanism for doing the OO thing
exists in Haskell too, albeit looking a little different.
Indeed there is a mechanism for doing OO in Haskell -- several of
them. Most of them have nothing to do with Existentials. In the
OHaskell
Hi,
I'm wrapping a library where functions take
as parameters pointers to a few standard
structs (as, well, all C libraries). I
would like to ensure that only pointers of
correct structs are passed to those functions.
What is the Haskell way to do that? My
idea is to do something like this:
Hello Vivek,
Wednesday, October 15, 2008, 3:39:54 PM, you wrote:
i think that practical answer is suggestion to use `case` instead:
case () of
_ | x 5 - do abc
def
...
| x==5 - do ...
| otherwise - do ...
it's pretty common
Hello Mauricio,
Wednesday, October 15, 2008, 5:40:16 PM, you wrote:
newtype SomeStruct = SomeStruct ()
data SomeStruct = SomeStruct
looks even simpler. you don't need to shell around () since you anyway
will not use its value :)
--
Best regards,
Bulat
So in my quest to create bindings to BlueZ in Haskell, I've hit a bit
of a snag: sockets programming.
In C, you can use the standard sockets library and just pass around
addresses as arrays of 6 bytes instead of arrays of 4 bytes like you
normally would. The problem I'm having is that in
CALL FOR WORKSHOP PROPOSALS
ICFP 2009
14th ACM SIGPLAN International Conference on Functional Programming
31st August - 2nd September, 2009
Edinburgh, Scotland
On Thu, Oct 16, 2008 at 12:53 AM, Bulat Ziganshin
[EMAIL PROTECTED] wrote:
data SomeStruct = SomeStruct
You can even go one step further and do
data SomeStruct
which will prevent you from accidentally trying to the dummy
constructor. However, you'll need
{-# LANGUAGE EmptyDataDecls #-}
Does that help?
It helps me a lot. I never clearly understood that there are these two
different layout modes in my code (coddled by haskell-mode!) This will
cut down some more guesswork. Thanks!
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I'd like to thank everyone who replied to my OP, and also perhaps
clarify one point. I wasn't trying to be argumentative or negative
about any work people have done to make Haskell approachable for OO
programmers (or any other programmers, for that matter). I simply
wanted to know what others
On Wed, Oct 15, 2008 at 8:08 AM, John Lato [EMAIL PROTECTED] wrote:
I'd like to thank everyone who replied to my OP, and also perhaps
clarify one point. I wasn't trying to be argumentative or negative
about any work people have done to make Haskell approachable for OO
programmers (or any
Simon Michael wrote:
Does that help?
It helps me a lot. I never clearly understood that there are these two
different layout modes in my code (coddled by haskell-mode!) This will
cut down some more guesswork. Thanks!
There is a new indentation module which does much better at the
The (=) operation for the state monad can be implemented with no
understanding at all.
Just watch djinn make the code for it. And djinn doesn't understand
the state monad, I promise. :)
-- Lennart
2008/10/15 David Leimbach [EMAIL PROTECTED]:
On Wed, Oct 15, 2008 at 8:08 AM, John Lato
Hello David,
Wednesday, October 15, 2008, 7:16:09 PM, you wrote:
I've read a lot of the Monad tutorials, and I feel like I only get
most of it to be 100% honest. The State Monad still boggles my
mind a little bit. I understand what it's supposed to do and I get
the idea about how it works.
Hi
I didn't understand Monads until I read this:
http://www.haskell.org/haskellwiki/Monads_as_Containers
It took me quite a long time to get them too, but slowly over time it
will sink in.
Thanks
Neil
From: [EMAIL PROTECTED]
[mailto:[EMAIL
Mauricio wrote:
What I actually want to use that way are build
time configs. For instance, 'isThisLibraryThreadSafe'
or 'maximumNumberOfBigObjects'. Actually, I don't
know why people allow build time options at all.
We always use the best set of options, and the
alternatives are there just to
John Van Enk wrote:
Could you, perhaps, outline a little more of what you're trying to do?
I'm having a hard time seeing what exactly you're doing, and why you
can't use the package provided by your distribution.
We'd love to help you, but you're not being very clear with what your
problem
Am Mittwoch, 15. Oktober 2008 13:39 schrieb Ramaswamy, Vivek:
Hello All~
I have just started with Haskell, and I must confess; I am in love with
it.
However one area that I am really confused about is indentation.
Lets take a look at if-else if- else block.
The way I understand it:
Ramaswamy, Vivek wrote:
Hello All~
I have just started with Haskell, and I must confess; I am in love with it.
However one area that I am really confused about is indentation.
Lets take a look at if-else if- else block.
Important point 1.
There are two contexts in haskell programs. Layout
On Wed, Oct 15, 2008 at 6:05 AM, [EMAIL PROTECTED] wrote:
Regarding existentials, the web page
http://okmij.org/ftp/Computation/Existentials.html
demonstrates how to systematically eliminate existentials. In fact,
the object encoding via existentials can be easily transformed into
Thanks to all that replied (Derek Ryan for the major understanding,
Albert Luke also)*. I guess I was getting confused with the error
message:
(All instance types must be of the form (T a1 ... an)
I was interpreting this with Stringable/Enum as T and [Char]/Blah as
a1.
Now I have clarity! I
Albet Lai wrote:
John Lato wrote:
Are you advocating introducing existential types to beginning
Haskellers? I think something with the scary name existential
quantification would greatly increase the head'splodin' on the
learnin' slope.
OOP(*) advocates introducing existential types to
wren ng thornton wrote:
Simon Marlow wrote:
Don Stewart wrote:
* GHC.Prim was moved,
Nobody should be importing GHC.Prim, use GHC.Exts instead.
This is oft declared, but as things stand I don't think it's a tenable
position. If someone's bothering to dig as far as GHC.Exts then it's
What do you mean by need? From a theoretical or practical perspective?
We don't need them from a theoretical perspective, but in practice I'd
rather use existentials than encodinging them in some tricky way.
On Wed, Oct 15, 2008 at 11:05 AM, [EMAIL PROTECTED] wrote:
The web page
begs a
On Wed, Oct 15, 2008 at 12:26 PM, Bryan O'Sullivan [EMAIL PROTECTED] wrote:
On Wed, Oct 15, 2008 at 6:54 AM, Creighton Hogg [EMAIL PROTECTED] wrote:
Is there a way around this that I just haven't seen, or should I write
a patch to Network to add an extra constructor to SockAddr and code to
Hello All~
I have just started with Haskell, and I must confess; I am in love with
it.
However one area that I am really confused about is indentation.
Lets take a look at if-else if- else block.
The way I understand it:
{--}
if something
then do
something 1
something2
else if nothing
Don Stewart wrote:
mjm2002:
On 10/13/08, Andrew Coppin wrote:
Cool. Is there any progress on getting GHC to *not* freak out when you
ask it to compile a CAF containing several hundred KB of string literal? :-}
Yes and no. There's dons' compiled-constants pkg which has a solution:
John Lato wrote:
Are you advocating introducing existential types to beginning
Haskellers? I think something with the scary name existential
quantification would greatly increase the head'splodin' on the
learnin' slope. Certainly there's a place for them, but I wouldn't
want to see new Haskell
Thanks for the portable solution.
I'd also like to know how is the following different from
-XFlexibleInstances with [Char]? Stronger, weaker, same thing?
{-# OPTIONS -XTypeSynonymInstances #-}
class Stringable a where
toString :: a - String
instance Stringable String where
On Wed, Oct 15, 2008 at 6:54 AM, Creighton Hogg [EMAIL PROTECTED] wrote:
Is there a way around this that I just haven't seen, or should I write
a patch to Network to add an extra constructor to SockAddr and code to
handle it?
Linux and Windows support Bluetooth sockets, but they have
Derek Elkins wrote:
In general, to encode OO...
turns out all you needed was recursive bounded
existential quantification.
Do you have a reference for that?
Thanks,
Yitz
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The instance selection for an interface is done at run-time and this is
inherently necessary. The instance (in a different sense) selection for
type classes is almost always resolvable statically. In Haskell 98
In both cases, the dispatch is inherently dynamic, and in both cases,
most
leaders and pioneers: if you know one language, picking up others
should be easy, they just differ in syntax.
I have heard it, and I used to believe it. Now I think it's only true
provided the one language you know is suitably advanced (and currently
non-existent, I think).
It all depends
The equivalent won't compile in Haskell, because the actual return
type does matter, and *is determined by the calling code*. Our
fictional GetListOfData can't return a List or a Mylist depending on
some conditional, in fact it can't explicitly return either one at
all, because the actual
On Wed, 2008-10-15 at 11:56 -0700, Daryoush Mehrtash wrote:
The equivalent won't compile in Haskell, because the actual
return
type does matter, and *is determined by the calling code*.
Our
fictional GetListOfData can't return a List or a Mylist
I like Parsec. I use it for everything. But it does have one irritating
problem.
Consider the following parser:
expressions = many1 expression
Suppose this is the top-level parser for my language. Now suppose the
user supplies an expression with a syntax error half way through it.
What I
On Wed, 15 Oct 2008, Andrew Coppin wrote:
Suppose this is the top-level parser for my language.
snip
Does anybody know how to fix this irratiting quirk? I can see why it happens,
but not how to fix it.
One of:
expressions = many1 (try expression | myFail)
where myFail = {- eat your way
On Wed, 2008-10-15 at 20:22 +0100, Andrew Coppin wrote:
I like Parsec. I use it for everything. But it does have one irritating
problem.
Consider the following parser:
expressions = many1 expression
Suppose this is the top-level parser for my language.
I always wrap my top-level
On Wed, 15 Oct 2008, Andrew Coppin wrote:
Suppose this is the top-level parser for my language. Now suppose the user
supplies an expression with a syntax error half way through it. What I *want*
to happen is for an error to be raised. What *actually* happens is that Parsec
just ignores all
On Wed, Oct 15, 2008 at 2:22 PM, Andrew Coppin
[EMAIL PROTECTED] wrote:
So if + is not a valid token, but the user writes
x = 1; y = 2; z = 3 + z; w = 4;
then what my program receives back is x = 1; y = 2; z = 3
You said you expect one or more 'expression'. It looks as if your
expression can
Philippa Cowderoy wrote:
expressions = do es - many1 expression
eof
return es
Ah - so eof fails if it isn't the end of the input?
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On Wed, 15 Oct 2008, Andrew Coppin wrote:
Philippa Cowderoy wrote:
expressions = do es - many1 expression
eof
return es
Ah - so eof fails if it isn't the end of the input?
eof = notFollowedBy anyChar
(assuming I've got the identifiers right,
Philippa Cowderoy wrote:
On Wed, 15 Oct 2008, Andrew Coppin wrote:
Philippa Cowderoy wrote:
expressions = do es - many1 expression
eof
return es
Ah - so eof fails if it isn't the end of the input?
eof = notFollowedBy anyChar
Would you please explain this a bit more:
the various
unfortunate consequences of type erasure in Java are avoided by the fact
that Haskell types lack constructors, so the user never expects to be
able to conjure up a value of an unknown type.
Thanks,
daryoush
On Wed, Oct 15, 2008 at 12:04
Hello Daryoush,
Wednesday, October 15, 2008, 10:56:39 PM, you wrote:
If you notice java generics has all sort of gotchas (e.g.
http://www.ibm.com/developerworks/java/library/j-jtp01255.html). I
large prob;em of OOP languages with generics is interaction between
those two types of
Maybe I'm doing something wrong. I've created several dialog boxes in
Glade, and I'm calling Gtk.runDialog on them when a user clicks the
mouse in my main window. On Linux, they work mostly right, but the
insertion point never shows in the Gtk.Entry areas and the dialog
itself comes up without
I am having hard time understanding this statement:
Haskell types lack constructors, so the user never expects to be
able to conjure up a value of an unknown type.
I am not sure how say in a Java language a constructor can conjure up a
value of an unknown type.
daryoush
On Wed, Oct 15, 2008
I'm trying that one now. Thanks for the tip!
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Hi,
I'm writing a Cabal package, using main=defaultMain
in Setup.hs. It has a library, and I want to also
build a few executables so I can test the library.
How am I supposed to do that? My attempt was to
create 'executable' sections for those tests. However,
I don't know how to include the main
Jason Dagit wrote:
On Mon, Oct 13, 2008 at 8:32 PM, Tommy M. McGuire [EMAIL PROTECTED]
mailto:[EMAIL PROTECTED] wrote:
Java (and presumably C#) generics are very much like a weakened
version of normal parametric polymorphism.
I'm curious, in what way are they weakened?
That's a good
I'm writing a Cabal package, using main=defaultMain
in Setup.hs. It has a library, and I want to also
build a few executables so I can test the library.
How am I supposed to do that? My attempt was to
create 'executable' sections for those tests. However,
I don't know how to include the main
Ryan Ingram schrieb:
On Mon, Oct 13, 2008 at 2:04 AM, J. Garrett Morris
[EMAIL PROTECTED] wrote:
Indeed - MTL seems to have been rewritten at some point in the past to
prefer exhaustive enumeration to overlap.
Indeed, and I actually think this is a weakness of the current
implementation.
Yitzchak Gale wrote:
Derek Elkins wrote:
In general, to encode OO...
turns out all you needed was recursive bounded
existential quantification.
Do you have a reference for that?
I'm not sure if this is precisely what Derek had in mind, but Bruce,
Cardelli, and Pierce did a comparison of
I suspect that more has been done since 1997. Isn't that pre-Oleg?
Karl Mazurak wrote:
Yitzchak Gale wrote:
Derek Elkins wrote:
In general, to encode OO...
turns out all you needed was recursive bounded
existential quantification.
Do you have a reference for that?
I'm not sure if this is
George Pollard schrieb:
I'm a little confused. Why is this allowed:
data Blah = Blah
instance Eq Blah where
x == y = True
But not this:
class Stringable a where
toString :: a - String
instance Stringable [Char] where
toString = id
(Resulting in:)
Here's what I have in one file:
-- | Parse the text of an event with the given parser @[EMAIL PROTECTED]
parse :: (Monad m) = P.CharParser () a - String - Derive.DeriveT m a
parse p text = do
(val, rest) - case P.parse (p_rest p) text of
Left err - Derive.throw $
parse
David Roundy schrieb:
Why not look for a heuristic that gets the common cases right, rather
than going with an elegant wrong solution? After all, these
enumerations are most often used by people who neither care nor know
how they're implemented, but who most likely would prefer if haskell
Don Stewart schrieb:
numeric-prelude-0.0.4
Easy: Lanuage pragma
My question was still not answered: I used the non-existing pragma
LANGUAGE_HOW_CAN_WE_ENABLE - I hoped it would be ignored, but it was
parsed and made GHC fail. Why? Bug or feature?
On Wed, Oct 15, 2008 at 5:44 PM, leledumbo [EMAIL PROTECTED] wrote:
module Main where
import Data.List
-- quicksort of any list
qsort [] = []
qsort (x:xs) = qsort(filter(x) xs) ++ [x] ++ qsort(filter(=x) xs)
-- optimized quicksort, uses middle element as pivot
qsortOpt [] = []
Google median order statistic.
E.g. this is an interesting (and colorful) discussion:
http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-046JFall-2005/60D030CD-081D-4192-9FB5-C220116E280D/0/lec6.pdf
Toby Hutton wrote:
On Wed, Oct 15, 2008 at 5:44 PM, leledumbo
On Wed, Oct 15, 2008 at 11:25:57PM +0200, Henning Thielemann wrote:
David Roundy schrieb:
Why not look for a heuristic that gets the common cases right, rather
than going with an elegant wrong solution? After all, these
enumerations are most often used by people who neither care nor know
schlepptop:
Don Stewart schrieb:
numeric-prelude-0.0.4
Easy: Lanuage pragma
My question was still not answered: I used the non-existing pragma
LANGUAGE_HOW_CAN_WE_ENABLE - I hoped it would be ignored, but it was
parsed and made GHC fail. Why? Bug or feature?
Feature.
Daryoush Mehrtash wrote:
I have had an unresolved issue on my stack of Haskell vs Java that I wonder
if your observation explains.
If you notice java generics has all sort of gotchas (e.g.
http://www.ibm.com/developerworks/java/library/j-jtp01255.html). I somehow
don't see this discussion in
I'm supposed to write a function isPrime that checks whether or not a given
integer is a prime number or not. The function has to use recursion. The
only advice I was given, was to use a helper function.
I still have no clue how to do it :confused:
I'm new to Haskell by the way..please help..
On Thu, Oct 16, 2008 at 9:01 AM, Dan Weston [EMAIL PROTECTED] wrote:
Google median order statistic.
E.g. this is an interesting (and colorful) discussion:
Hello Kalashnikov,
Thursday, October 16, 2008, 2:41:05 AM, you wrote:
I'm supposed to write a function isPrime that checks whether or not a given
integer is a prime number or not. The function has to use recursion. The
only advice I was given, was to use a helper function.
seems that russian
Henning Thielemann wrote:
I long thought that it is unnecessary use of type system extensions to
require multi-parameter type classes for simple monads and its
transformer versions. I thought it would be enough to have atomar monads
like ST, IO and Identity, and monads like State, Reader,
On Wed, 2008-10-15 at 13:01 -0700, Daryoush Mehrtash wrote:
I am having hard time understanding this statement:
Haskell types lack constructors, so the user never expects to
be
able to conjure up a value of an unknown type.
I am not sure how say in a Java language
On Wednesday 15 October 2008 05:21:04 John Dorsey wrote:
Should all floating point numerals be in the IO Monad?
I'm deviating from the thread's topic, but I tend to agree with this one.
Maybe not IO directly, but some kind of STM-style monad, at least (that is,
FP operations are composable but
Section 8.13.2 of the GHC manual[1] states:
GHC keeps trying to apply the rules as it optimises the program. For
example, consider:
let s = map f
t = map g
in
s (t xs)
The expression s (t xs) does not match the rule map/map, but GHC
will substitute for s and t, giving an
I'm pleased to announce the first release of hledger, a command-line
accounting tool similar to John Wiegley's c++ ledger. hledger generates
simple ledger-compatible transaction account balance reports from a
plain text ledger file. It's simple to use, at least for techies.
This has been my
I was just reminded of one of the joke definitions of recursion:
recursion: see recursion.
Perhaps there is a similar one for fixed points?
To learn about fixed points find the fixed point of the process:
Given somebody learn about fixed points from them.
Course, my understanding of fixed points
simon:
I'm pleased to announce the first release of hledger, a command-line
accounting tool similar to John Wiegley's c++ ledger. hledger generates
simple ledger-compatible transaction account balance reports from a
plain text ledger file. It's simple to use, at least for techies.
On Wed, Oct 15, 2008 at 5:01 PM, Simon Michael [EMAIL PROTECTED] wrote:
I'm pleased to announce the first release of hledger, a command-line
accounting tool similar to John Wiegley's c++ ledger. hledger generates
simple ledger-compatible transaction account balance reports from a plain
text
On Wed, Oct 15, 2008 at 5:55 PM, wren ng thornton [EMAIL PROTECTED] wrote:
Doing it that way removes the polymorphism that MonadState, MonadReader, etc
offer to clients. For example, the backwards-state monad[1] is a MonadState
but not a StateT (without extra plumbing). There are other
On Wed, 2008-10-15 at 14:45 -0400, Stefan Monnier wrote:
The instance selection for an interface is done at run-time and this is
inherently necessary. The instance (in a different sense) selection for
type classes is almost always resolvable statically. In Haskell 98
In both cases, the
On Wed, 2008-10-15 at 08:16 -0700, David Leimbach wrote:
On Wed, Oct 15, 2008 at 8:08 AM, John Lato [EMAIL PROTECTED] wrote:
I'd like to thank everyone who replied to my OP, and also
perhaps
clarify one point. I wasn't trying to be argumentative or
negative
On 16 Oct 2008, at 9:01 am, Daryoush Mehrtash wrote:
I am not sure how say in a Java language a constructor can conjure
up a value of an unknown type.
... Class anUnknownClass;
Object anInstance;
anInstance = anUnknownClass.getConstructor().newInstance();
If you know that the
On 16 Oct 2008, at 12:09 pm, Jonathan Cast wrote:
I am not sure how say in a Java language a constructor can conjure
up
a value of an unknown type.
Well, that's the point. It can't, in Haskell or in Java. If you
understand that --- that you can't call the default constructor of a
class
Thanks Jason! Glad you liked it.
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Would you please explain this a bit more:
the various unfortunate consequences of type erasure in Java are
avoided by the fact that Haskell types lack constructors, so the user
never expects to be able to conjure up a value of an unknown type.
Even if Haskell had Java-style constructors, it
The compiler doesn't know what kind of list you are trying to read,
sort, and print.
So, the type must be specific? Then why it's possible to call the sorting
function with any list?
I'm curious as to why taking the pivot from the middle is an
'optimized' version.
Consider if it's used in a
At the risk of doing someone's homework...
A naive solution is to do trial division by all integers from 2 up to sqrt
n.
{-
isPrime :: Integer - BoolisPrime n
| n 2 = False
| otherwise = f 2 n
where f k n
= if k isqrt
then True
else undefined -- exercise for the reader
-}
On Thu, 2008-10-16 at 01:24 +0200, Ariel J. Birnbaum wrote:
On Wednesday 15 October 2008 05:21:04 John Dorsey wrote:
Should all floating point numerals be in the IO Monad?
I'm deviating from the thread's topic, but I tend to agree with this one.
Maybe not IO directly, but some kind of
On 2008 Oct 16, at 0:53, leledumbo wrote:
The compiler doesn't know what kind of list you are trying to read,
sort, and print.
So, the type must be specific? Then why it's possible to call the
sorting
function with any list?
A function may have a polymorphic type; this allows its actual
re: the importance of existential-cleansing
On the one hand, it's easy to concur that existentials are simpler
than the alternatives, the tortuous elimination of CC Shan's
translucent existential being a case in point.
And it's also easy to dismiss such caprice as a penchant for Houdinian
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