Re: [Haskell-cafe] (no subject)

[Richard A. O'Keefe]

On 11/06/2013, at 1:58 AM, Alberto G. Corona wrote:

> I have ever wondered how a committee could have made Haskell.

A committee made Algol 60, described as "an improvement on most
of its successors".  A committee maintains Scheme.

On the other hand, an individual gave us Perl.
And an individual gave us JavaScript.
And let's face it, an individual gave C++ its big start.


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Re: [Haskell-cafe] (no subject)

[Alberto G. Corona]

I have ever wondered how a committee could have made Haskell.

My conclusion is the following:

For one side there were many mathematicians involved, the authors of the
most terse language(s) existent: the math notation.

For the other, the lemma "avoid success at all costs" which  kept the
committee away of pressures for doing it quick and dirty and also freed it
from deleterious individualities


2013/6/10 Tobias Dammers 

> On Mon, Jun 10, 2013 at 05:41:05PM +0530, Zed Becker wrote:
> >
> >  Haskell, is arguably the best example of a design-by-committee language.
>
> You do realize that "design-by-committee" is generally understood to
> refer to the antipattern where a committee discusses a design to death
> and delivers an inconsistent, mediocre spec, as opposed to a situation
> where a leader figure takes the loose ends, runs with them, and turns
> them into a coherent, inspiring whole?
>
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>



-- 
Alberto.
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Re: [Haskell-cafe] (no subject)

[Tom Ellis]

On Mon, Jun 10, 2013 at 05:44:26PM +0400, MigMit wrote:
> It really sounds rude, to demand promises from somebody who just gave you a 
> big present.

Without wishing to preempt Zed Becker, I interpreted his email as an
expression of delight at how well Haskell has been designed and of hope that
it may endure, rather than literally as a demand for the Haskell committee
to grant him promises.  I hope I haven't misunderstood.

Tom

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Re: [Haskell-cafe] (no subject)

[MigMit]

It really sounds rude, to demand promises from somebody who just gave you a big 
present.

Отправлено с iPhone

10.06.2013, в 16:11, Zed Becker  написал(а):

> Hi all,
> 
> Haskell, is arguably the best example of a design-by-committee language. The 
> syntax is clean and most importantly, consistent. The essence of a purely 
> functional programming is maintained, without disturbing its real world 
> capacity.
> 
> To all the people who revise the Haskell standard, and implement the language,
> Promise to me, and the rest of the community, that you will keep up the good 
> effort :)
> Promise to me, and the rest of the community, that Haskell will always 
> spiritually remain the same clean, consistent programming language as it is 
> now!
> 
> Regards,
> Zed Becker
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Re: [Haskell-cafe] (no subject)

[Jerzy Karczmarczuk]

Hm...

Haskell was /developed/ by teams, but we had BEFORE: hope, miranda, ML 
... The heritage is quite important.
And individuals (say, Mark Jones) contributed to Haskell constructs. So, 
the /design/ is not entirely "committe based"



1.

Promise to me, and the rest of the community, that
Haskell will always spiritually remain the same clean,
consistent programming language as it is now!


Yes.
Dear Mom, dear Dad! Promise me that you will never die...

I wish that for all of you.
Jerzy Karczmarczuk

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Re: [Haskell-cafe] (no subject)

[Flavio Villanustre]

Zed,

while I don't disagree regarding the clean and consistent syntax of
Haskell, do you realize that some people would argue that camels are horses
designed by committee too? :)

While designing by committee guarantees agreement across a large number of
people, it does not always ensure efficiency, as committees may lead to
poor compromises, sometimes.

However, Haskell may be an example of a good case of design-by-committee
computer language.

Flavio

Flavio Villanustre


On Mon, Jun 10, 2013 at 8:11 AM, Zed Becker  wrote:

>  Hi all,
>
>
>  Haskell, is arguably the best example of a design-by-committee language.
> The syntax is clean and most importantly, consistent. The essence of a
> purely functional programming is maintained, without disturbing its real
> world capacity.
>
>
>  To all the people who revise the Haskell standard, and implement the
> language,
>
>
>1.
>
>  Promise to me, and the rest of the community, that you will keep
>  up the good effort :)
>  2.
>
>  Promise to me, and the rest of the community, that Haskell will
>  always spiritually remain the same clean, consistent programming 
> language
>  as it is now!
>
>
>  Regards,
>
> Zed Becker
>
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Re: [Haskell-cafe] (no subject)

[Tom Ellis]

On Mon, Jun 10, 2013 at 05:41:05PM +0530, Zed Becker wrote:
>  Haskell, is arguably the best example of a design-by-committee language.
> The syntax is clean and most importantly, consistent. The essence of a
> purely functional programming is maintained, without disturbing its real
> world capacity.
> 
>  To all the people who revise the Haskell standard, and implement the
> language,
> 
>1.  Promise to me, and the rest of the community, that you will keep
>up the good effort :)
>2.  Promise to me, and the rest of the community, that Haskell will
>always spiritually remain the same clean, consistent programming
>language as it is now!

Hear hear!  Hopefully we, the Haskell community, will be able to support
this endevour with our time and efforts.

Tom

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Re: [Haskell-cafe] (no subject)

[Tobias Dammers]

On Mon, Jun 10, 2013 at 05:41:05PM +0530, Zed Becker wrote:
> 
>  Haskell, is arguably the best example of a design-by-committee language.

You do realize that "design-by-committee" is generally understood to
refer to the antipattern where a committee discusses a design to death
and delivers an inconsistent, mediocre spec, as opposed to a situation
where a leader figure takes the loose ends, runs with them, and turns
them into a coherent, inspiring whole?

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Re: [Haskell-cafe] (no subject)

[Antoine Latter]

On Thu, Jan 5, 2012 at 10:54 AM, Christoph Breitkopf
 wrote:
> Hello,
>
> I'm trying to figure out how to handle versioning of my IntervalMap
> package. I've just read the package versioning
> policy: http://www.haskell.org/haskellwiki/Package_versioning_policy
>
> I don't quite understand all the recommendations in the above document,
> though:
>
> a) You are not allowed to remove or change the types of existing stuff. Ok.
>
> b) You are allowed to add new functions. But that can break compilation
> because of name conflicts. Seems to be allowed on the grounds that this is
> easy to fix in the client code.

This will never break clients who are using qualified imports, or only
importing the symbols they use, which is strongly recommended
behavior.

>
> c) You are not allowed to add new instances. I don't get this - how is this
> any worse than b)?

Unlike adding functions, there is no way for a client of your library
to control which instances they import.

Antoine

>
> I do understand that it is not generally possible to prevent breaking code
> - for example if the client code depends on buggy behavior that gets fixed
> in a minor version update. That seems unavoidable - after all, bugfixes are
> _the_ reason for minor updates.
>
> Regards,
>
> Chris
>
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Re: [Haskell-cafe] (no subject)

[Erik Hesselink]

On Mon, Nov 28, 2011 at 23:55, Willem O  wrote:
> And I added this function:
> createPoint :: Int -> Point
> createPoint x = Point x
> When I loaded the file containing all this into ghci and executed 'Vector $
> map createPoint [1..5]' the result was '(1, 2, 3, 4, 5)' (without the
> quotes).

Note that you do not need this function. You can just use the 'Point'
constructor:

map Point [1..5]

Erik

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Re: [Haskell-cafe] (no subject)

[Willem O]

Yes, thank you. Here's my simple fix:
newtype Point = Point Int
instance Show Point where   show (Point a) = [chr $ a + 48]
data Vector = Vector [Point]
instance Show Vector where
   show (Vector ys) =
  let show' [z] = show z
   show' (x:xs)  = show x ++ ", " ++ show' xs
   show' []  = []
  in  "(" ++ show' ys ++ ")"
And I added this function: 
createPoint :: Int -> PointcreatePoint x = Point x
When I loaded the file containing all this into ghci and executed 'Vector $ map 
createPoint [1..5]' the result was '(1, 2, 3, 4, 5)' (without the quotes).This 
was actually more or less a test question as I'm new to haskell-cafe, but I 
hope people who will read this message will learn from my mistake.
Thank you.
> From: aslat...@gmail.com
> Date: Mon, 28 Nov 2011 16:20:54 -0600
> Subject: Re: [Haskell-cafe] (no subject)
> To: dub...@hotmail.com
> CC: haskell-cafe@haskell.org
> 
> On Mon, Nov 28, 2011 at 4:12 PM, Willem Obbens  wrote:
> > Hello,
> > I get this error when I try to derive an instance of the Show typeclass:
> > Abc.hs:21:60:
> > Couldn't match expected type `Vector' with actual type `[Point]'
> > In the first argument of `show'', namely `xs'
> > In the second argument of `(++)', namely `show' xs'
> > In the second argument of `(++)', namely `", " ++ show' xs'
> > Failed, modules loaded: none.
> > Here's the faulty code:
> > newtype Point = Point Int
> > instance Show Point where
> >show (Point a) = [chr $ a + 48]
> >
> > data Vector = Vector [Point]
> > instance Show Vector where
> >show (Vector ys) =
> >   let show' (Vector [z]) = show z
> >   show' (Vector (x:xs))  = show x ++ ", " ++ show' xs
> >   show' (Vector [])  = []
> >   in  "(" ++ show' ys ++ ")"
> 
> Here you're treating the value 'ys' as if its type was 'Vector', but
> its type is '[Point]'.
> 
> Does that help?
> 
> Antoine

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Re: [Haskell-cafe] (no subject)

[Brent Yorgey]

On Mon, Nov 28, 2011 at 04:20:54PM -0600, Antoine Latter wrote:
> On Mon, Nov 28, 2011 at 4:12 PM, Willem Obbens  wrote:
> > Hello,
> > I get this error when I try to derive an instance of the Show typeclass:
> > Abc.hs:21:60:
> >     Couldn't match expected type `Vector' with actual type `[Point]'
> >     In the first argument of `show'', namely `xs'
> >     In the second argument of `(++)', namely `show' xs'
> >     In the second argument of `(++)', namely `", " ++ show' xs'
> > Failed, modules loaded: none.
> > Here's the faulty code:
> > newtype Point = Point Int
> > instance Show Point where
> >    show (Point a) = [chr $ a + 48]
> >
> > data Vector = Vector [Point]
> > instance Show Vector where
> >    show (Vector ys) =
> >       let show' (Vector [z])     = show z
> >           show' (Vector (x:xs))  = show x ++ ", " ++ show' xs
> >           show' (Vector [])      = []
> >       in  "(" ++ show' ys ++ ")"

You've made  show' :: Vector -> String, but I'm guessing you actually
want to make it  show' :: [Point] -> String; i.e. get rid of the
Vector constructors in the show' patterns.

-Brent

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Re: [Haskell-cafe] (no subject)

[Antoine Latter]

On Mon, Nov 28, 2011 at 4:12 PM, Willem Obbens  wrote:
> Hello,
> I get this error when I try to derive an instance of the Show typeclass:
> Abc.hs:21:60:
>     Couldn't match expected type `Vector' with actual type `[Point]'
>     In the first argument of `show'', namely `xs'
>     In the second argument of `(++)', namely `show' xs'
>     In the second argument of `(++)', namely `", " ++ show' xs'
> Failed, modules loaded: none.
> Here's the faulty code:
> newtype Point = Point Int
> instance Show Point where
>    show (Point a) = [chr $ a + 48]
>
> data Vector = Vector [Point]
> instance Show Vector where
>    show (Vector ys) =
>       let show' (Vector [z])     = show z
>           show' (Vector (x:xs))  = show x ++ ", " ++ show' xs
>           show' (Vector [])      = []
>       in  "(" ++ show' ys ++ ")"

Here you're treating the value 'ys' as if its type was 'Vector', but
its type is '[Point]'.

Does that help?

Antoine

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Re: [Haskell-cafe] (no subject)

[Chris Smith]

On Sat, 2011-07-30 at 15:07 -0700, KC wrote:
> A language that runs on the JVM or .NET has the advantage of Oracle &
> Microsoft making those layers more parallelizable.

On top of the answers you've got regarding whether this exists, let me
warn you against making assumptions like the above.  There are certainly
good reasons for wanting Haskell to run on the JVM or CLR, but
parallelism doesn't look like one of them.

The problem is that the cost models of things on the JVM or CLR are so
different that if you directly expose the threading and concurrency
stuff from the JVM or CLR, you're going to kill all the Haskell bits of
parallelism.  A huge contribution of Haskell is to have very
light-weight threads, which can be spawned cheaply and can number in the
tens of thousands, if not hundreds of thousands.  If you decide that
forkIO will just spawn a new Java or CLR thread, performance of some
applications will change by orders of magnitude, or they will just plain
crash and refuse to run.  Differences of that scope are game-changing.
So you risk, not augmenting Haskell concurrency support by that of the
JVM or CLR, but rather replacing it.  And that certainly would be a
losing proposition.

Maybe there's a creative way to combine advantages from both, but it
will require something besides the obvious one-to-one mapping of
execution contexts.

-- 
Chris


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Re: [Haskell-cafe] (no subject)

[Thomas DuBuisson]

Ian,
This requires dynamic typing using Data.Dynamic (for application) and
Data.Typeable (to do the typing).   Namely, you are asking for the
"dynApply" function:

 START CODE
import Data.Dynamic
import Data.Typeable
import Control.Monad

maybeApp :: (Typeable a, Typeable b, Typeable c) => a -> b -> Maybe c
maybeApp a = join . fmap fromDynamic . dynApply (toDyn a) . toDyn
 END CODE

In the above we obtain representations of your types in the form of
"Dynamic" data types using toDyn.  Then, using dynApply, we get a
value of type "Maybe Dynamic", which we convert back into a "c" type
with fromDynamic.  The "join" is just there to collapse the type from
a "Maybe (Maybe c)" into the desired type of "Maybe c".

Cheers,
Thomas

P.S.
If I totally misunderstood, and you want static typing then you just
need to realize you _don't_ want types "a" and "b" (fully polymorphic)
but rather types (b -> c) and b:

apply :: (b -> c) -> b -> c
apply a b = a b

But this seems rather silly, so I hope you were looking for my first answer.


On Wed, Jul 6, 2011 at 2:12 AM, Ian Childs  wrote:
> Suppose I have two terms s and t of type "a" and "b" respectively, and I
> want to write a function that returns s applied to t if "a" is an arrow type
> of form "b -> c", and nothing otherwise. How do i convince the compiler to
> accept the functional application only in the correct instance?
>
> Thanks,
> Ian
>
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Re: [Haskell-cafe] (no subject)

[Fernando Henrique Sanches]

I'm sorry, somehow my e-mail account got kidnapped. The link is a virus and
should NOT be opened. I apologise for any inconvenience.

Fernando Henrique Sanches


2011/6/13 Fernando Henrique Sanches 
>
>
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Re: [Haskell-cafe] (no subject)

[Brent Yorgey]

On Wed, May 19, 2010 at 01:37:49PM +, R J wrote:
> 
> This is another proof-layout question, this time from Bird 1.4.7.
> We're asked to define the functions curry2 and uncurry2 for currying and 
> uncurrying functions with two arguments.  Simple enough:
> curry2 :: ((a, b) -> c) -> (a -> (b -> c))curry2 f x y   =  f 
> (x, y)
> uncurry2   :: (a -> (b -> c)) -> ((a, b) -> c)uncurry2 f (x, y)  =  f 
> x y
> The following two assertions are obviously true theorems, but how are the 
> formal proofs laid out?

There are lots of variations, I wouldn't say there's one "right" way
to organize/lay out the proofs.  But here's how I might do it:

  curry2 (uncurry2 f) x y 
=  { def. of curry2 }
  uncurry2 f (x,y)
=  { def. of uncurry2 }
  f x y

I'll let you do the other one.

By the way, are you working through these problems just for
self-study, or is it homework for a class?

-Brent
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Re: [Haskell-cafe] (no subject)

[Robin Green]

At Thu, 15 Oct 2009 10:15:46 +0400,
Eugene Kirpichov wrote:
> but I don't know in what respect these two packages differ and why Don
> decided to create 'judy' despite the existence of HsJudy.

HsJudy doesn't compile against the latest judy library (as Don knew) -
presumably he had a good reason to start a new package instead of
patching the old one.

There should be a way to mark packages as deprecated on hackage, and
at the same time direct people to a more suitable alternative. Aside
from uploading a dummy new version (ugh!), I don't see a way to do
that currently.
-- 
Robin
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Re: [Haskell-cafe] (no subject)

[Eugene Kirpichov]

There are also the judy arrays
http://hackage.haskell.org/package/HsJudy
http://hackage.haskell.org/package/judy

dons recently advertised the latter as being 2x faster than IntMap,
but I don't know in what respect these two packages differ and why Don
decided to create 'judy' despite the existence of HsJudy.

2009/10/15 wren ng thornton :
> Jake McArthur wrote:
>>
>> staafmeister wrote:
>>>
>>> Yes I know but there are a lot of problems requiring O(1) array updates
>>> so then you are stuck with IO again
>>
>> Or use ST. Or use IntMap (which is O(log n), but n is going to max out on
>> the integer size for your architecture, so it's really just O(32) or O(64),
>> which is really just constant time).
>
> Actually, IntMap is O(min(n,W)) where W is the number of bits in an Int.
> Yes, IntMaps are linear time in the worst case (until they become
> constant-time). In practice this is competitive with all those O(log n)
> structures though.
>
> Whereas Data.Map is O(log n) for the usual balanced tree approach.
>
> --
> Live well,
> ~wren
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-- 
Eugene Kirpichov
Web IR developer, market.yandex.ru
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Re: [Haskell-cafe] (no subject)

[wren ng thornton]

Jake McArthur wrote:

staafmeister wrote:

Yes I know but there are a lot of problems requiring O(1) array updates
so then you are stuck with IO again


Or use ST. Or use IntMap (which is O(log n), but n is going to max out 
on the integer size for your architecture, so it's really just O(32) or 
O(64), which is really just constant time).


Actually, IntMap is O(min(n,W)) where W is the number of bits in an Int. 
Yes, IntMaps are linear time in the worst case (until they become 
constant-time). In practice this is competitive with all those O(log n) 
structures though.


Whereas Data.Map is O(log n) for the usual balanced tree approach.

--
Live well,
~wren
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Re: [Haskell-cafe] (no subject)

[Jake McArthur]

staafmeister wrote:

Yes I know but there are a lot of problems requiring O(1) array updates
so then you are stuck with IO again


Or use ST. Or use IntMap (which is O(log n), but n is going to max out 
on the integer size for your architecture, so it's really just O(32) or 
O(64), which is really just constant time).


And, realistically, very few problems actually require indexed access on 
a large scale like this.



[parsing stuff]


As far as parsing is concerned, maybe you should look at Parsec. I know 
it sounds like overkill, but it's easy enough to use that it's quite 
lightweight in practice. Your example scenario:


inputData :: Parser InputData
inputData = many1 digit *> newline *> many (testCase <* newline)
where testCase = many1 digit *> newline *> sepBy edge (char ' ')
  edge = liftA2 (,) (many nonspace <* char ' ')
(read <$> digits)

- Jake
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Re: [Haskell-cafe] (no subject)

[Sebastian Sylvan]

On Sat, Aug 22, 2009 at 3:20 PM, staafmeister  wrote:

>
>
> Thank you for the reply.
>
>
> Thomas ten Cate wrote:
> >
> > Although you most certainly can use a State monad, in most problems
> > this isn't necessary. Most algorithms that you need to solve
> > programming contest problems can be written in a purely functional
> > style, so you can limit monadic code to just a few helper functions.
> >
>
> Yes I know but there are a lot of problems requiring O(1) array updates
> so then you are stuck with IO again


Not necessarily. The ST monad will usually do just as well.


-- 
Sebastian Sylvan
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Re: [Haskell-cafe] (no subject)

[staafmeister]

Thank you for the reply.


Thomas ten Cate wrote:
> 
> Although you most certainly can use a State monad, in most problems
> this isn't necessary. Most algorithms that you need to solve
> programming contest problems can be written in a purely functional
> style, so you can limit monadic code to just a few helper functions.
> 

Yes I know but there are a lot of problems requiring O(1) array updates
so then you are stuck with IO again


Thomas ten Cate wrote:
> 
> For example, this reads input in the style you mention (assuming the
> strings don't contain whitespace):
> 
>> import Control.Monad
>>
>> answer = id
>>
>> parse [] = []
>> parse (s:p:r) = (s, (read p) :: Int) : parse r
>>
>> run = getLine >> getLine >>= putStrLn . show . answer . parse . words
>>
>> main = flip replicateM_ run =<< readLn
> 
> The answer function would be a pure function that computes the answer
> for a particular run. This main function is reusable for all problems
> with many runs.
> 
> Observe that the number of edges (e), provided as a convenience for
> memory allocation in many other languages, is not even necessary in
> Haskell :)
> 

Yes you're main is short. But how would you do it elegantly if 
instead of line breaks and spaces one would have only spaces.
Every thing on one big line. My C code would not mind one bit.


Thomas ten Cate wrote:
> 
> (If anyone knows a better way than explicit recursion to map over a
> list, two elements at a time, or zip its even elements with its odd
> elements, I'd love to hear! I can imagine a convoluted fold with a
> boolean in its state, but it's ugly.)
> 

Yes I missed such a function in a couple of problems I wanted to solve.
I would expect a generic function
groupN::Int -> [a] -> [[a]]
that groups a list into groups of N

Best,
Gerben
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Re: [Haskell-cafe] (no subject)

[Thomas ten Cate]

On Sat, Aug 22, 2009 at 01:23, staafmeister wrote:
> But in general you have a structure like
>
> first line -- integer specifying the number of testcases (n)
> Then for each testcase
> a line with an integer specifying the number of edges (e)
> a line with e pairs of string s and int p where p is the number asociated
> with string s, etc.
>
> Such a structure cannot be parsed by map read.lines
> What I used is "words" to tokenize and put the list in a State monad with
> readInt, readString, etc. functions, to mimic
> C code. This seems to be a lot of overkill, so there must be an simpler
> way

Although you most certainly can use a State monad, in most problems
this isn't necessary. Most algorithms that you need to solve
programming contest problems can be written in a purely functional
style, so you can limit monadic code to just a few helper functions.
For example, this reads input in the style you mention (assuming the
strings don't contain whitespace):

> import Control.Monad
>
> answer = id
>
> parse [] = []
> parse (s:p:r) = (s, (read p) :: Int) : parse r
>
> run = getLine >> getLine >>= putStrLn . show . answer . parse . words
>
> main = flip replicateM_ run =<< readLn

The answer function would be a pure function that computes the answer
for a particular run. This main function is reusable for all problems
with many runs.

Observe that the number of edges (e), provided as a convenience for
memory allocation in many other languages, is not even necessary in
Haskell :)

(If anyone knows a better way than explicit recursion to map over a
list, two elements at a time, or zip its even elements with its odd
elements, I'd love to hear! I can imagine a convoluted fold with a
boolean in its state, but it's ugly.)

Hope that helps,

Thomas
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Re: [Haskell-cafe] (no subject)

[Luke Palmer]

On Fri, Aug 21, 2009 at 7:03 PM, Sebastian
Sylvan wrote:
>> I think that there must be standard function that can do this. What do
>> experienced Haskellers use?
>
> I usually just whip up a quick parser using Text.ParserCombinators.Parsec

I usually prefer ReadP for quick stuff, for an unknown reason.  I
guess it feels like there is less infrastructure to penetrate, it
gives me the primitives and I structure the parser according to my
needs.

But yeah, I think parser combinators are the way to go.  It's really
not much work at all once you get the hang of it.
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Re: [Haskell-cafe] (no subject)

[Sebastian Sylvan]

On Fri, Aug 21, 2009 at 11:42 PM, Stavenga, G.C.  wrote:

>
>
> Hi, I'm just started to learn Haskell. Coming from a programming contest
> background (where it is important to be able to solve problems in a small
> amount of code) I'm wondering what the best way is for simple IO.
>
> A typical input file (in a programming contest) is just a bunch of numbers
> which you want to read one by one (sometimes interspersed with strings). In
> C/C++ this is easily done with either scanf or cin which reads data
> separated by spaces. In Haskell I have not found an equally satisfactionary
> method. The methods I know of
>
> 1) Stay in the IO monad and write your own readInt readString functions. A
> lot
> of code for something easy.
>
> 2) Use interact together with words and put the list of lexemes in a State
> monad and define getInt where at least you can use read.
>
> 3) Use ByteString.Char8 which has readInt (but I couldn't find a
> readString). But one has to put it also in a State monad.
>
> I think that there must be standard function that can do this. What do
> experienced Haskellers use?
>

I usually just whip up a quick parser using
Text.ParserCombinators.Parsec

-- 
Sebastian Sylvan
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Re: [Haskell-cafe] (no subject)

[staafmeister]

Don Stewart-2 wrote:
> 
> G.C.Stavenga:
>> 
>> 
>> Hi, I'm just started to learn Haskell. Coming from a programming contest
>> background (where it is important to be able to solve problems in a small
>> amount of code) I'm wondering what the best way is for simple IO.
>> 
>> A typical input file (in a programming contest) is just a bunch of
>> numbers
>> which you want to read one by one (sometimes interspersed with strings).
>> In
>> C/C++ this is easily done with either scanf or cin which reads data
>> separated by spaces. In Haskell I have not found an equally
>> satisfactionary
>> method. The methods I know of
>> 
>> 1) Stay in the IO monad and write your own readInt readString functions.
>> A lot
>> of code for something easy.
>> 
>> 2) Use interact together with words and put the list of lexemes in a
>> State
>> monad and define getInt where at least you can use read.
>> 
>> 3) Use ByteString.Char8 which has readInt (but I couldn't find a
>> readString). But one has to put it also in a State monad.
>> 
>> I think that there must be standard function that can do this. What do
>> experienced Haskellers use?
> 
> 
> map read . lines
> 
> Thank you for the reply. But this only works for if you read only integers
> all on different lines.
> But in general you have a structure like
> 
> first line -- integer specifying the number of testcases (n)
> Then for each testcase 
> a line with an integer specifying the number of edges (e)
> a line with e pairs of string s and int p where p is the number asociated
> with string s, etc.
> 
> Such a structure cannot be parsed by map read.lines
> What I used is "words" to tokenize and put the list in a State monad with
> readInt, readString, etc. functions, to mimic
> C code. This seems to be a lot of overkill, so there must be an simpler
> way
> ___
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
> 
> 

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Re: [Haskell-cafe] (no subject)

[Don Stewart]

G.C.Stavenga:
> 
> 
> Hi, I'm just started to learn Haskell. Coming from a programming contest
> background (where it is important to be able to solve problems in a small
> amount of code) I'm wondering what the best way is for simple IO.
> 
> A typical input file (in a programming contest) is just a bunch of numbers
> which you want to read one by one (sometimes interspersed with strings). In
> C/C++ this is easily done with either scanf or cin which reads data
> separated by spaces. In Haskell I have not found an equally satisfactionary
> method. The methods I know of
> 
> 1) Stay in the IO monad and write your own readInt readString functions. A lot
> of code for something easy.
> 
> 2) Use interact together with words and put the list of lexemes in a State
> monad and define getInt where at least you can use read.
> 
> 3) Use ByteString.Char8 which has readInt (but I couldn't find a
> readString). But one has to put it also in a State monad.
> 
> I think that there must be standard function that can do this. What do
> experienced Haskellers use?


map read . lines
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Re: [Haskell-cafe] (no subject)

[Claus Reinke]

--
type F a = Int

class A a where
 foo :: A b => a (F b)
--

GHC - OK
Hugs - Illegal type "F b" in constructor application


This time, I'd say Hugs is wrong (though eliminating that initial
complaint leads back to an ambiguous and unusable method 'foo').


I only just recognized the horrible error message from the first
example.. what Hugs is trying to tell us about is a kind error!

The kind of 'a' in 'F' defaults to '*', but in 'A', 'F' is applied to
'b', which, via 'A b' is constrained to '*->*'. So Hugs is quite
right (I should have known!-).

The error message can be improved drastically, btw:

   :set +k
   ERROR file:.\hugs-vs-ghc.hs:19 - Kind error in constructor application
   *** expression : F b
   *** constructor : b
   *** kind : a -> b
   *** does not match : *

See http://cvs.haskell.org/Hugs/pages/hugsman/started.html and
search for '+k' - highly recommended if you're investigating kinds.

Claus


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Re: [Haskell-cafe] (no subject)

[Claus Reinke]

--
type F a = Int

class A a where
 foo :: A b => a (F b)
--

GHC - OK
Hugs - Illegal type "F b" in constructor application


This time, I'd say Hugs is wrong (though eliminating that initial
complaint leads back to an ambiguous and unusable method 'foo').

4.2.2 Type Synonym Declarations, lists only class instances as
exceptions for type synonyms, and 'Int' isn't illegal there.


--
type F a = Int

class A a where
 foo :: F a

instance A Bool where
 foo = 1

instance A Char where
 foo = 2

xs = [foo :: F Bool, foo :: F Char]
--

GHC:

M.hs:14:6:
   Ambiguous type variable `a' in the constraint:
 `A a' arising from a use of `foo' at M.hs:14:6-8
   Probable fix: add a type signature that fixes these type variable(s)

M.hs:14:21:
   Ambiguous type variable `a1' in the constraint:
 `A a1' arising from a use of `foo' at M.hs:14:21-23
   Probable fix: add a type signature that fixes these type variable(s)

Hugs: [1,2]


Neither seems correct? 4.3.1 Class Declarations, says:

   The type of the top-level class method vi is: 
   vi :: forall u,w. (C u, cxi) =>ti 
   The ti must mention u; ..


'foo's type, after synonym expansion, does not mention 'a'.

Claus


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Re: [Haskell-cafe] (no subject)

[Richard O'Keefe]

On 5 Mar 2009, at 4:02 am, R J wrote:


Could someone provide an elegant solution to Bird problem 4.2.13?


This is the classic Lisp "SAMEFRINGE" problem in disguise.

You say that the method of converting CatLists to lists and then
comparing those is a "hack", but I take leave to doubt that.
It's easy to get right, and it works.

== and < are, in general, O(n) operations on lists,
so the O(n) cost of converting trees to lists isn't
unreasonable.  In fact given ((Wrap 1) ++ ..) ++ ..) )
it can take O(n) time to reach the very first element.
Best of all, the fact that Haskell is lazy means that
converting trees to lists and comparing the lists are
interleaved; if comparison stops early the rest of the
trees won't be converted.

One way to proceed in a strict language is to work with a
(pure) state involving
- the current focus of "list" 1
- the current focus of "list" 2
- the rest of "list" 1 (as a list of parts)
- the rest of "list" 2 (as a list of parts).

I thought I had demonstrated this when one last check showed
a serious bug in my code.  In any case, this relies on lists
to implement the stacks we use for "the rest of the tree".
Your "unwrap" approach is much easier to get right.


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Re: [Haskell-cafe] (no subject)

[Dougal Stanton]

2008/11/25 apostolos flessas <[EMAIL PROTECTED]>:
> hi,
>
> i am looking for someone to help me with an assignment!
> can anyone help me?

Hi Tolis!

Have a look at the homework help policy, so you know what people will
and will not answer.

http://www.haskell.org/haskellwiki/Homework_help


Then let us know what you're trying to do, and what your difficulty has been.


Cheers,


D
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Re: [Haskell-cafe] (no subject)

[Daniel Fischer]

Am Donnerstag, 8. Mai 2008 15:36 schrieb [EMAIL PROTECTED]:
> Hi I have a bit of a dilemma.I have a list of lists, eg,
> [[1,2,3],[4,5,6],[7,8,9]]. Imagine they represent a grid with 0-2 on the x
> axis and 0-2 on the y axis, eg, (0,0) is 1, (1,0) is 2, (2,1) is 6, etc and
> (2,3) is 9. I want to be able to put in the list of lists, and the (x,y)
> coordinate, and return the value. 
>
> Also, I need to be able to replace a value in the list. Eg, if I wanted to
> replace (2,3) with 100, then the output of the expression would be
> [[1,2,3],[4,5,6],[7,8,100]].
>
> Any help would be great!

To get the value at a position, look up (!!)
To replace a value, you could use zipWith

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Re: [Haskell-cafe] (no subject)

[Bulat Ziganshin]

Hello Thomas,

Thursday, August 9, 2007, 8:12:27 PM, you wrote:

> In the following code which uses template haskell, how can I get
> back the macro-expanded code generated from

citating http://www.haskell.org/bz/thdoc.htm :

In order to make debugging Template Haskell programs easier, compiler
supports flag -ddump-splices, which shows the expansion of all
top-level splices as they happen.

-- 
Best regards,
 Bulatmailto:[EMAIL PROTECTED]

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Re: [Haskell-cafe] (no subject)

[Tillmann Rendel]

Hello,

Ketil Malde wrote:

Makes me wonder whether one should have binary be the default?  I'm a
stranger in Windows-land, but are there cases where you want reading
of a file to be terminated on ^Z?  Seems pretty awful to me.


The ghc docs state about openBinaryFile:


Like openFile, but open the file in binary mode. On Windows, reading
a file in text mode (which is the default) will translate CRLF to LF,
and writing will translate LF to CRLF. [...] text mode treats control-Z as EOF


The CRLF-to-LF translation is the more important part. It allows '\n' to 
stand for the end of a line on windows, too, even if lines are 
terminated by two characters in windows text files.


  Tillmann
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Re: Re: [Haskell-cafe] (no subject)

[L.Guo]

Very thanks for your example, I have not notice that there is a group of
hGetxxx functions in ByteString. In other words, I was using hGetxxx which
implemented in IO module. So it always failed.



--   
L.Guo
2007-05-24

-
From: Donald Bruce Stewart
At: 2007-05-24 17:31:02
Subject: Re: Re: [Haskell-cafe] (no subject)

I mean, what problem are you trying to solve? Ptrs aren't the usual way
to manipulate files in Haskell.

...

-- Don

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Re: Re: [Haskell-cafe] (no subject)

[Ketil Malde]

On Thu, 2007-05-24 at 17:01 +0800, L.Guo wrote:

> Tring openBinaryFile, 

Well, did you get it to work?

> I can not locate which module including readBinaryFile.

This is what I find in System.IO (ghci> :b System.IO):

  openBinaryFile :: FilePath -> IOMode -> IO Handle
  openBinaryTempFile :: FilePath -> String -> IO (FilePath, Handle)
  hSetBinaryMode :: Handle -> Bool -> IO ()

so you have the option of either using openBinaryFile or openFile and
using hSetBinaryMode to true. I guess - I've never had to use them.  

I can't find a readBinaryFile either, but writing one might be a good
excercise?

Makes me wonder whether one should have binary be the default?  I'm a
stranger in Windows-land, but are there cases where you want reading of
a file to be terminated on ^Z?  Seems pretty awful to me.

Concerning mutable buffers, it is of course possible, but hardly
idiomatic Haskell.  Why do you need mutability?

-k



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Re: Re: [Haskell-cafe] (no subject)

[Donald Bruce Stewart]

leaveye.guo:
> To read the handle openBinaryFile returns, both the hGetBuf and
> hGetBufNonBlocking needs one parameter _buf_ of type Ptr a.
> I can not get one data of that type.
> 
> In the doc, there is only nullPtr, and also some type cast functions.
> I failed to find some other buffer-maker function.
> 
> What should I do ?

I mean, what problem are you trying to solve? Ptrs aren't the usual way
to manipulate files in Haskell.

Here, for example, is a small program to print the first byte of a
binary file:

import System.IO
import qualified Data.ByteString as B

main = do
h <- openBinaryFile "a.out" ReadMode
s <- B.hGetContents h
print (B.head s)

When run:

$ ./a.out 
127

Note there's no mallocs or pointers involved.

-- Don
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Re: Re: [Haskell-cafe] (no subject)

[L.Guo]

To read the handle openBinaryFile returns, both the hGetBuf and
hGetBufNonBlocking needs one parameter _buf_ of type Ptr a.
I can not get one data of that type.

In the doc, there is only nullPtr, and also some type cast functions.
I failed to find some other buffer-maker function.

What should I do ?

--   
L.Guo
2007-05-24

-
From: Donald Bruce Stewart
At: 2007-05-24 17:03:55
Subject: Re: Re: [Haskell-cafe] (no subject)

What are you trying to do?

-- Don

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Re: Re: [Haskell-cafe] (no subject)

[Donald Bruce Stewart]

leaveye.guo:
> to Ketil :
> 
> Tring openBinaryFile, I notice that I cannot make one usable buffer,
> just because I can not find one function to "malloc" a memory or just
> get one "change-able" buffer.
> 
> :-$

No 'malloc' here in Haskell land: that's done automatically.  Recall
that 'getContents' will read your opened file into a [Char]. (or use
Data.ByteString to get a stream of Word8).

What are you trying to do?

-- Don
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Re: Re: [Haskell-cafe] (no subject)

[L.Guo]

to Ketil :

Tring openBinaryFile, I notice that I cannot make one usable buffer,
just because I can not find one function to "malloc" a memory or just
get one "change-able" buffer.

:-$


to Marc:

I can not locate which module including readBinaryFile.
And I use hoogle search engine.



Could you give me some more hints ?

--   
L.Guo
2007-05-24

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Re: [Haskell-cafe] (no subject)

[Donald Bruce Stewart]

marco-oweber:
> On Thu, May 24, 2007 at 02:38:05PM +0800, L.Guo wrote:
> > Thanks for your suggestion, and sorry for the subject.
> > 
> > I have read the introduction of Data.ByteString, it is helpful.
> > 
> > And also, there is one problem left. When i read a binary file, data is 
> > truncated at the charactor EOF.
> 
> You have to use readBinaryFile instead of readFile.
> I had the same trouble as well.
> 
> I finally implemented accessing single characters in C and did use ffi
> because I didn't know haw to do this i haskell properly. ( using
> peek/poke functions 4 bytes got written (wihch is annotateted somewhere
> ) If you are interested I can sent you the modified ByteString package.
> 
> If someone can tell me which haskell function to use to set a random
> char in a memory buffer I would be pleased ..

'poke'

or else use unboxed Word8 arrays

Check the src for Data.ByteString for examples.
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Re: [Haskell-cafe] (no subject)

[Marc Weber]

On Thu, May 24, 2007 at 02:38:05PM +0800, L.Guo wrote:
> Thanks for your suggestion, and sorry for the subject.
> 
> I have read the introduction of Data.ByteString, it is helpful.
> 
> And also, there is one problem left. When i read a binary file, data is 
> truncated at the charactor EOF.

You have to use readBinaryFile instead of readFile.
I had the same trouble as well.

I finally implemented accessing single characters in C and did use ffi
because I didn't know haw to do this i haskell properly. ( using
peek/poke functions 4 bytes got written (wihch is annotateted somewhere
) If you are interested I can sent you the modified ByteString package.

If someone can tell me which haskell function to use to set a random
char in a memory buffer I would be pleased ..

Marc
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Re: [Haskell-cafe] (no subject)

[L.Guo]

Sorry for not familiar to the email client.

My system is WinXP, and using GHC 6.6.
And is read from file.
Data is truncated at the ^Z char.

I just wrote one simple test code.

> import IO
> 
> writeTest fn = do
>   h <- openFile fn WriteMode
>   mapM_ (\p -> hPutChar h (toEnum p::Char)) $ [0..255] ++ [0..255]
>   hClose h
> 
> accessTest fn = do
>   h <- openFile fn ReadMode
>   s <- hGetContents h
>   putStrLn . show . map fromEnum $ s
>   hClose h
> 
> main = do
>   writeTest "ttt"
>   accessTest "ttt"


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Re: [Haskell-cafe] (no subject)

[Ketil Malde]

> And also, there is one problem left. When i read a binary file, data is 
> truncated at the charactor EOF.

Which character is this: ^D or ^Z?  Which operating system - Windows,
perhaps?  And you are reading from a file, not from stdin?

-k

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Re: [Haskell-cafe] (no subject)

[Donald Bruce Stewart]

leaveye.guo:
> Thanks for your suggestion, and sorry for the subject.
> 
> I have read the introduction of Data.ByteString, it is helpful.
> 
> And also, there is one problem left. When i read a binary file, data
> is truncated at the charactor EOF.
> 
> Which function could do this work correctly ?

Hmm. Do you have an example?

-- Don
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Re: [Haskell-cafe] (no subject)

[L.Guo]

Thanks for your suggestion, and sorry for the subject.

I have read the introduction of Data.ByteString, it is helpful.

And also, there is one problem left. When i read a binary file, data is 
truncated at the charactor EOF.

Which function could do this work correctly ?

--   
L.Guo
2007-05-24

-
发件人：Donald Bruce Stewart
发送日期：2007-05-24 14:03:27
收件人：L.Guo
抄送：MailList Haskell-Cafe
主题：Re: [Haskell-cafe] (no subject)

leaveye.guo:
> Hi MailList Haskell-Cafe:
> 
> Till now, which module / package / lib can i use to access binary
> file ? And is this easy to use in GHC ?

Data.Binary? Or perhaps just Data.ByteString, available on hackage,

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-0.3

or in base.

-- Don


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Re: [Haskell-cafe] (no subject)

[Donald Bruce Stewart]

leaveye.guo:
> Hi MailList Haskell-Cafe:
> 
> Till now, which module / package / lib can i use to access binary
> file ? And is this easy to use in GHC ?

Data.Binary? Or perhaps just Data.ByteString, available on hackage,

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-0.3

or in base.

-- Don


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Re: [Haskell-cafe] (no subject)

[Bulat Ziganshin]

Hello José,

Wednesday, March 15, 2006, 5:54:49 PM, you wrote:

JMV> #ifdef __WIN32__

i use the following:

#if defined(mingw32_HOST_OS) || defined(__MINGW32__) || defined(_MSC_VER)


-- 
Best regards,
 Bulatmailto:[EMAIL PROTECTED]

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Re: [Haskell-cafe] (no subject)

[Paul Hudak]

To add briefly to what John wrote, there is a webpage for Yampa:
www.haskell.org/yampa
which includes all of our publications on FRP/Yampa as well as a decent 
release of our latest implementation of Yampa (based on arrows).  The 
release has ample examples of how to use Yampa for graphics, animation, 
and basic control systems such as used in robotics.

Also, although most of the developers have dispersed, I believe that 
most of them are still interested in the ideas, and the 
[EMAIL PROTECTED] mailing list would probably be responsive if 
anyone bothered to use it.

  -Paul Hudak
John C. Peterson wrote:
From: John Peterson <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Subject: Re: [Haskell-cafe] Functional Reactive Programming
Functional Reactive Programming is alive but in need of some new
students to push the effort a bit.  A lot of us have taken teaching
or industrial positions so the old FRP team is a bit depleted.
I don't think anyone is working on Yampa directly at the moment.
Although it's stable and working well it lacks a critical mass of nice
libraries to make it attractive.
I'm still plugging on a wxHaskell port to Yampa (the wxFruit stuff).
I've made some semantic changes to Yampa so I probably shouldn't say
it's real Yampa but pretty close.  I should have something to release
later this fall. 

Aside from that, we have a student working in the hybrid modeling
area.  That's good stuff but not likely to produce software of
interest to Joe Haskell.  Another student is keeping the robotics side
of things alive but it's in the context of a very specialized robotic
hardware environment.
So there you go!
   John
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Professor Paul Hudak
Chair, Dept of Computer Science   Office: (203) 432-1235
Yale University   FAX:(203) 432-0593
P.O. Box 208285   email:  [EMAIL PROTECTED]
New Haven, CT 06520-8285  WWW:www.cs.yale.edu/~hudak
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