Re: [Haskell-cafe] IO help

[Adrian Neumann]

Have a look at the wikibook:

http://en.wikibooks.org/wiki/Haskell/Simple_input_and_output


Am 07.05.2009 um 11:46 schrieb applebiz89:



I havent done much IO at all in haskell, only within the function  
itself.
However I want to get the input from the interface for the function  
and

havent done this before.

In my main function, I want to ask the user what they would like to do
'become fan' for example, then with their choice initiate the  
function and
ask for the appropriate input for that function. This is the code  
below:


main :: IO()
do putStr Hi there! what is your name: 
fanName = getLine
do putStr 1 = Insert film, 2 = Become a Fan, 3 = The number of  
fans of a

film, 4 = Film released in a year: 
input = getLine
read input :: Int
(if input == 1 then main x = insertFilm [] else if input == 2 then  
main x =

becomeFan [] else if input == 3 then main x = numberOfFans [])

Say they choose film in a given year function function:

filmsInGivenYear :: Int - [Film] - [String]
filmsInGivenYear filmYear films = [ title | (Film title director  
year fans)

- films, year == filmYear]

I need to ask the user what filmYear they want to insert. But i  
need to do
this from the main function...I chose to do an if statement to  
choose what

function they want, but i dont know where to go from there?

any help will be greatly appreciated.

thanks

apple
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[Haskell-cafe] Unfold fusion

[Adrian Neumann]

Hello,

I'm trying to prove the unfold fusion law, as given in the chapter  
Origami Programming in The Fun of Programming. unfold is defined  
like this:


unfold p f g b = if p b then [] else (f b):unfold p f g (g b)

And the law states:

unfold p f g . h = unfold p' f' g'
with
p' = p.h
f' = f.h
h.g' = g.h

Foremost I don't really see why one would want to fuse h into the  
unfold. h is executed once, at the beginning and is never needed  
again. Can someone give me an example?


So, this is what I got so far:

unfold p f g.h = (\b - if p b then [] else (f b): unfold p f g (g b).h
= if p (h b) then [] else (f (h b)) : unfold p f g (g (h b))
= if p' b then [] else f' b: unfold p f g (h (g' b))

not very much. I kinda see why it works after I unfold some more,  
but I can't really prove it. I suspect I need some technique I  
haven't learned yet. I've heard about fixpoint induction, that looks  
promising, but Google knows very little about it.


I hope somebody can give me some hints.

Regards,

Adrian


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Re: [Haskell-cafe] Looking for the fastest Haskell primes algorithm

[Adrian Neumann]

I've just uploaded a package with some functions I had lying around.

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


Am 14.04.2009 um 14:40 schrieb Niemeijer, R.A.:

Today I happened to need a large list of prime numbers. Obviously  
this is a well-known problem, so I figured there would be something  
on Hackage that I could use. Surprisingly, there isn’t, or if there  
is it’s not easy to find. Searching for prime or primes on Hackage  
reveals nothing. Searching for primes on Hayoo gives  
Codec.Encryption.RSA.NumberTheory, but that uses the inefficient  
one-liner implementation. The HaskellWiki article on primes (http:// 
www.haskell.org/haskellwiki/Prime_numbers) has a number of  
implementations, but the faster they get, the longer and uglier  
they become.




Since it’s such a common problem I’d say it would be a good idea to  
add a package to Hackage that exports


primes :: [Integer]

and hides the ugly implementation details. Data.Numbers.Primes  
seems a logical choice for the namespace, but I’m open to suggestions.




The trick then is to find the most efficient implementation of  
primes. The Haskell wiki article mentions ONeillPrimes.hs as one of  
the fastest ones, but maybe there’s a faster version. So my  
question is: does anybody know what the fastest Haskell algorithm  
for generating primes is?


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[Haskell-cafe] Re: [Haskell-beginners] appropriateness of haskell for GUIs

[Adrian Neumann]

Am 21.03.2009 um 13:30 schrieb Michael Mossey:




Thomas Davie wrote:

On 21 Mar 2009, at 00:16, Michael P Mossey wrote:
Hello, I'm totally new to Haskell. I'm thinking of using it for a  
personal project, which is a gui-based musical score editor.
The rough situation of GUI programming on Haskell is that it works  
just as well as in any imperative programming language.  This is  
rather disappointing, simply because so many other things are  
massively easier in Haskell, and this isn't true of GUI  
programming (yet).


Hi Bob,

I can imagine that GUI programming is no easier (yet). It is  
inherently very stateful. GUI's have modes, such as which screens  
are displayed, which dialogs are displayed, which options within  
those dialogs are valid given the other state of the program, etc.  
When I write GUIs, I often diagram them as state machines to get a  
handle on what's going on.


So, I'm not familiar with GUI programming on Haskell, but would you  
say the statefulness of GUIs (in their typical implementations) is  
the reason they are no easier on Haskell?


I strongly prefer to use qtHaskell because I'm familiar with Qt,  
and Qt is extremely capable. For example, it can draw text and  
shapes with antialiasing, which will be great for a music score  
editor. Music scores have lots of small shapes to fit on the  
screen, and antialiasing will provide ease of reading. I don't know  
how much of Qt is implemented in qtHaskell, or whether the latest  
version of Qt (4.4) is implemented.


Thanks,
Mike


The main problem is, as far as I know, the complete lack of any  
usable GUI designer. You have to type everything yourself. That's  
very annoying. It's a lot easier in other languages because your  
tools take away the cumbersome twiddling with widgets.


However, I haven't googled. Maybe the situation has changed since I  
last looked.


Regards,

Adrian


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[Haskell-cafe] Re: [Haskell-beginners] laziness and optimization

[Adrian Neumann]

You should not rely on the compiler to spot such things. As far as I  
know GHC doesn't do automatic caching (in many cases that would hurt  
performance, I think). Have a look at http://haskell.org/haskellwiki/ 
Memoization perhaps.



Am 21.03.2009 um 14:02 schrieb Michael Mossey:

I understand a bit about the concept of lazy evaluation. I think  
of that as saying an imperative language would always make one  
evaluation, whereas Haskell might make 0 evaluations. I have  
another similar situation where an imperative language would make N  
evaluations of the same expression, and I would like Haskell to  
make only 1.


This is the situation: the graphical score editor displays  
LayoutItems. A LayoutItem can be a single displayed entity, like  
a round notehead, or it can be composed of several entities.


A common situation in my code is the need to determine the size and  
shape of a LayoutItem. For a fundamental item, this can be looked  
up in a table or read from the font properties. For a composite  
item, some computation is required: the code must determine the  
positions of each sub-item and compute the bounds of a shape  
containing all of them.


It's this latter computation, finding the bounds of a composite  
item, which might come up multiple times. Consider that I ask for  
the bounds of a composite-composite item (a composite item composed  
of composite items). It will run the computation associated with  
each composite sub-item, even though it is very likely I already  
make that computation when I first constructed and placed that sub- 
item.


In an imperative language, one might cache values for later lookup.  
This raises the problem of keeping the cache current to the current  
state.


So I'm wondering to what extent the haskell compiler recognizes  
computations it's done before. In a purely functional language this  
should be pretty easy, right? If it sees the same expression, it  
knows it will have the same value. That's my understanding, so far.


Thanks,
Mike
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Re: [Haskell-cafe] Fw: hw do i solve this problems:

[Adrian Neumann]

http://lmgtfy.com/?q=haskell+run+length+codingl=1

But you'll learn nothing if you just copy that

Am 19.03.2009 um 03:24 schrieb THANDO NTUANE:




- Forwarded Message 
From: THANDO NTUANE thandodar...@yahoo.com
To: Haskell-Cafe@haskell.org
Sent: Thursday, March 19, 2009 10:22:40 AM
Subject: hw do i solve this problems:

kindly help me..
im stack but managed to solve exercise 1 and 2.


task1.docx
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Re: [Haskell-cafe] Query on list comprehension

[Adrian Neumann]

Use a nested list comprehension, for example

Prelude [['a'| i - [0..n]]++\n|n - [1..3]]
[aa\n,aaa\n,\n]



Am 18.03.2009 um 07:47 schrieb Melanie_Green:



What are the limitations of list comprehension. I want to use
listcomprehension to output the pattern below. So a mixture of a's and
newline characters. The part im stuck at is creating arguments in the
listcomprehension to stop at some point then execute next loop. Is  
it even
possible to create the pattern below purely from list  
comprehension.Thankyou

in advance.

a
aa
aaa
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[Haskell-cafe] Re: [Haskell-beginners] folds -- help!

[Adrian Neumann]

That's why I said for appropriate g and f. But I see that my  
wording was misleading.

However you can say:

 foldl' oplus alpha bs =(foldr f id bs) alpha where
   f a g = \alpha - g (alpha `oplus` a)

 foldr' oplus alpha bs = (foldl f id bs) alpha where
f g a = \alpha - g (a `oplus` alpha)

And it works as long as oplus is strict in both arguments.

Am 10.03.2009 um 21:54 schrieb John Dorsey:


Adrian Neumann wrote:


Notice that there is no difference between

foldr g a
foldl f a

(for appropriate g and f) if g and f are strict in both arguments.


Be careful... as apfelmus noted elsewhere in this thread, that's  
not (in

general) true.

Prelude foldr (^) 2 [3,5]
847288609443
Prelude foldl (^) 2 [3,5]
32768

The reason?  Integer exponentiation (^) isn't associative and
commutative.  So the first is (3 ^ (5^2)) = 3^25, while the second is
((2 ^ 3) ^ 5) = 2^15.

Cheers,
John

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Re: [Haskell-cafe] How to catch error in array index when debugging

[Adrian Neumann]

You can use the ghci debugger

 http://www.haskell.org/ghc/docs/latest/html/users_guide/ghci- 
debugger.html


it can set breakpoints on exceptions.


Am 14.03.2009 um 09:39 schrieb Colin Paul Adams:


I'm getting a runtime failure Error in array index. This causes ghci
to exit.

Is there a way to get it to break instead, so I can find out which
function is failing?
--
Colin Adams
Preston Lancashire
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Re: [Haskell-cafe] A systematic method for deriving a defintion of foldl using foldr?

[Adrian Neumann]

Read this excellent paper:

http://www.cs.nott.ac.uk/~gmh/fold.pdf


Am 11.03.2009 um 19:24 schrieb R J:


foldl and foldr are defined as follows:

  foldr:: (a - b - b) - b - [a] - b
  foldr f e [] =  e
  foldr f e (x : xs)   =  f x (foldr f e xs)

  foldl:: (b - a - b) - b - [a] - b
  foldl f e [] =  e
  foldl f e (x : xs)   =  foldl f (f e x) xs

1.  I understand how these definitions work, and yet I'm unable to  
implement foldl in terms of foldr.  What's a systematic approach to  
identifying such an implementation, and what is the implementation?


2.  I believe that the reverse implementation--namely, implementing  
foldr in terms of foldl--is impossible.  What's the proof of that?


3.  Any advice on how, aside from tons of practice, to develop the  
intuition for rapidly seeing solutions to questions like these  
would be much appreciated.  The difficulty a newbie faces in  
answering seemingly simple questions like these is quite discouraging.


Express your personality in color! Preview and select themes for  
Hotmail®. See how.

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Re: [Haskell-cafe] Basic problem in Haskell language design?

[Adrian Neumann]

You could turn on -Wall to get a whole bunch of such warnings.

Am 01.03.2009 um 14:26 schrieb Nicu Ionita:


Hi,

Today I found the following problem when writing a simple function:


-- Whole info from a word8 list to moves
movesFromWord8s :: [Word8] - [Move]
movesFromWord8s (f:t:ws) = (f, t) : movesFromWord8s ws
moverFromWord8s _ = []


Here I made a small typo in the second equation, writing mover...  
instead
of moves... for the name of the declared function. Of course this  
is an

error, because I have non-exhaustive patterns in the function
movesFromWord8s. But the compiler (here GHC 6.8.2 on WinXP) has in  
principle

no chance to detect this mistake, I saw it only in QuickCheck (aka at
run-time).

I think this is a basic problem with the language design. In small  
programs
it's not so bad, but in large ones this could be. What do you think  
about

it? Are there possible solutions or workarounds?

Nicu Ionita


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Re: [Haskell-cafe] (Off-topic) CUDA

[Adrian Neumann]

Am 05.02.2009 um 09:10 schrieb Andrew Coppin:

And so, inspired by the marketing litrature, I just spent £££ on a  
very expensive new GPU that supports CUDA. The only problem is... I  
can't seem to get any software to use it.


Does anybody know how to make this stuff actually work?

(Also... Haskell on the GPU. It's been talked about for years, but  
will it ever actually happen?)


Have a look at Obsidian

 http://www.cse.chalmers.se/~joels/

and ask Mr Svenson if there is anything working.



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[Haskell-cafe] ANNOUNCE: DecisionTree 0.0

[Adrian Neumann]

Hello,

it is my pleasure to announce the DecisionTree package. It provides  
an implementation of the ID3 algorithm [1] and can be used to  
classify data with discrete valued attributes.


You can get it from

* hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 
DecisionTree
* my own webspace: http://page.mi.fu-berlin.de/aneumann/decisiontree/ 
DecisionTree-0.0.tar.gz


At this stage it is very likely buggy and slow, as I don't have a  
sufficiently large dataset to test with.


You are encouraged to have a look at the source, it is quite short  
and well commented. Please feel free to make suggestions, send in  
patches or point me to some dataset I could use for testing.


Regards,

Adrian


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Re: [Haskell-cafe] Elevator pitch for functional programming

[Adrian Neumann]

There was a thread about that:

 http://www.haskell.org/pipermail/haskell-cafe/2007-September/ 
031402.html


Am 20.01.2009 um 11:07 schrieb Jim Burton:


Hi, I will be a TA on a comparative PL course and I'm looking for
small examples (ammunition) which motivate the use of Haskell and
functional programming generally. The course is for 1st year Software
Engineers, none of whom are likely to have used a functional
language. They will all have experience programming Java and a little
C++, with a few of them knowing Python, Ruby, PHP etc etc too.

If anyone has code snippets which are the equivalent of an elevator
pitch for FP, I would be very grateful to see them. What I want
are some small concrete examples of idioms which are natural and
powerful in Haskell but difficult or impossible in, say, Java.

So I can produce examples of some of the things that make FP powerful,
elegant, expressive etc: higher order functions, polymorphism,
composition (ask them to write (.)  in Java :-)), partial application
and so on. I will point any interested souls to Hughes' great paper
[1]. But I have little time and it might be hard to put across why
they would want to do these things in the first place. I was looking
for something that speaks directly to the kind of problems they face
in languages like Java...

Types are a good example because Java programmers generally already
appreciate the help they get from compiler messages etc, so you can
sell a more flexible, enhanced form of this. Purity might appeal to
anyone who has longed to be able to reason about nastily complex code
with a lot of shared state. Laziness, streams? Hard to do in Java (I
presume) but also quite hard to sell the need.

The existence of an O'Reilly book will help, especially one that can
be sampled online, so I'll point them at RWH for extended concrete
examples. They will need to be already sold before they will bother
with that though.

Thanks,

Jim

[1] http://www.cs.chalmers.se/~rjmh/Papers/whyfp.html
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Re: [Haskell-cafe] definition of data

[Adrian Neumann]

You need some type constructor:

data Tree a = Leaf a | Branch (Tree a) (Tree a)

Am 01.01.2009 um 08:32 schrieb Max.cs:

hi all, I want to define a data type Tree a that can either be  a   
or Branch (Tree a) (Tree a)?


I tried

data Tree a = a | Branch (Tree a) (Tree a) deriving Show

but it seems not accpetable in haskell ?

any way I could achieve this ?

Thanks

max
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Re: [Haskell-cafe] instance Enum [Char] where ...

[Adrian Neumann]

Better would be

[] = 0
['a'] = 1
['b'] = 2
...
['z'] = 26
['a','a'] = 27
['a','b'] = 28

(asuming Char = ['a'..'z'])

Am 30.12.2008 um 04:25 schrieb JustinGoguen:

I am having difficulty making [Char] an instance of Enum. fromEnum  
is easy
enough: map fromEnum to each char in the string and take the sum.  
However,

toEnum has no way of knowing what the original string was.

For example, running fromEnum on the string d will result in 100.  
But when we
pass 100 to toEnum, it does not know if it should treat 100 as d  
or 22

(fromEnum '2' == 50).

Source so far:

instance Enum [Char] where
succ = undefined
pred = undefined
toEnum n = undefined -- what to do?
fromEnum xs = sum $ map fromEnum xs

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[Haskell-cafe] Data.List.sort, not so good?

[Adrian Neumann]

Hello Haskell-Cafe,

lately I've been playing around with sorting. I discovered that  
Data.List.sort is not as optimized I thought. At least not for random  
lists. I think we should consider adding a Data.List.Sort package to  
hackage (Similar to Data.List.Split) that offers a wide variety of  
sorting algorithms.


I don't consider myself to be a very advanced Haskell programmer, but  
I could come up with a Mergesort that beats List.sort, time- and  
spacewise.


 http://hpaste.org/13403

On my machine List.sort takes ~10 sec, mergeSort 7, qs 4 (compiled  
with -O2). List.sort eats too much ram to sort 20.000.000 ints, my  
algorithms don't.

QuickCheck says my implementations are correct.

I suppose List.sort does clever things for nonrandom lists, that I  
didn't consider? I had a look at the source, but I couldn't find  
anything obvious.



Adrian


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Re: [Haskell-cafe] Data.List.sort, not so good?

[Adrian Neumann]

Ah that's interesting. Now my Mergesort is exactly as fast as  
List.sort. I thought GHC was smart enough to do that kind of inter- 
module optimisation.


Still, Quicksort is twice as fast, so at least one argument for a  
Data.List.Sort package on hackage remains.



Am 29.12.2008 um 16:43 schrieb Bayley, Alistair:


From: haskell-cafe-boun...@haskell.org
[mailto:haskell-cafe-boun...@haskell.org] On Behalf Of Adrian Neumann

I don't consider myself to be a very advanced Haskell
programmer, but
I could come up with a Mergesort that beats List.sort, time- and
spacewise.


http://hpaste.org/13403


On my machine List.sort takes ~10 sec, mergeSort 7, qs 4 (compiled
with -O2). List.sort eats too much ram to sort 20.000.000 ints, my
algorithms don't.
QuickCheck says my implementations are correct.



Your single module might be benefiting from optimisation;  
specifically,

specialisation to Ints, which would allow the dictionary lookup to be
removed. Can you get the same performance if you move your mergeSort 
qs functions into a separate module, and only export mergeSort  qs?

Alistair
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Re: [Haskell-cafe] What are side effects in Haskell?

[Adrian Neumann]

Am 23.12.2008 um 15:16 schrieb Hans van Thiel:


Hello All,

I just saw somewhere that one of the purposes of monads is to capture
side effects. I understand what a side effect is in C, for example.  
Say
you want to switch the contents of two variables. Then you need a  
third

temporary variable to store an intermediate result. If this is global,
then it will be changed by the operation.


But the two variables have also changed. After all they have  
different values after the switch. You see, even locally changing a  
variable is a side-effect. It changes the state of the program. Pure  
Haskell programs on the other hand have no notion of state, there are  
no variables which can change their value. Every time you want to  
manipulate something you're actually generating an new copy. You  
mustn't think of a haskell program as a series of changes to some state.


However when you *do* want state you can simulate it with a monad.  
The IO Monad is a special case here, since its actions don't change  
your program, they change the world the program is running in  
(writing files etc.). getLine etc are functions when you think of  
them as taking a hidden parameter, the state of the world. So getChar  
would become


getChar :: World - (Char,World)

but the world stays hidden inside the IO Monad.

Regards,

Adrian



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[Haskell-cafe] Trouble with interact in ghci

[Adrian Neumann]

Hi,

I have a strange problem with interact on OS X (ghc 6.10.1). It  
seems to garble stdin.


I have some code here http://hpaste.org/13135#a2 , for testing purpose:

*Main main
1
 1.0
2
 1.5
3
 2.0
*Main setNonBlockingFD: invalid argument (Bad file descriptor)
11:40:45 ~/Desktop

(I hit ctrl-D)

*Main main
1
 1.0
2
 1.5
3
 2.0
^CInterrupted.
*Main main
*** Exception: stdin: hGetContents: illegal operation (handle is  
closed)

*Main

Someone on #haskell suspected my editline to be the culprit. Does  
anyone of you know what I can do about this problem?


Greetings,

Adrian


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Re: [Haskell-cafe] Compilers

[Adrian Neumann]

Am 27.11.2008 um 09:23 schrieb Don Stewart:


allbery:

On 2008 Nov 26, at 16:58, Matthias Kilian wrote:

On Wed, Nov 26, 2008 at 09:35:01PM +, Andrew Coppin wrote:
It is a fork of the JHC compiler, which should be easier to  
look up.

There is also Hugs, as you mentioned. In addition, you may want to
look at YHC and NHC.


Yeah, the implementations page on the Wiki basically says that
there's
GHC and Hugs, and there's also these things called YHC, NHC and
JHC. All
the documentation I've read makes these latter compilers sound  
highly

experimental and unusable.


I would't call nhc experimental; it's quite usable, at least for
standard Haskell-98 stuff (plus some language extensions).



On a related topic:  whatever happened to the compiler shootout?
(Aside from dons leaving unsw)


Malcolm continues the tradition,

http://code.haskell.org/nobench/


All result links are broken.



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Re: [Haskell-cafe] Counting beta reductions for a Haskell program...

[Adrian Neumann]

Hugs has, afaik, a output reduction count option somewhere. At  
least it had one the last time I used it.


- Adrian

Am 22.11.2008 um 06:22 schrieb kk08:



Thanks.
I heard that a Gofer compiler (a Haskell dialect) supports counting  
the Beta

reductions.
Hence I thought GHC/Hugs would have a similar facility.



Ryan Ingram wrote:


This doesn't make a whole lot of sense.  One of the reasons
GHC-compiled code is so fast is that it turns into straight-line code
whenever possible, via inlining, primitive optimizations, etc.


  - ryan

[1] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.3729

On Fri, Nov 21, 2008 at 8:46 PM, kk08 [EMAIL PROTECTED] wrote:


Does GHC supports/has a command for counting total beta  
reductions taken

by a
program?

Thanks.

--
View this message in context:
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program...-tp20623025p20623025.html
Sent from the Haskell - Haskell-Cafe mailing list archive at  
Nabble.com.


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Re: [Haskell-cafe] [ANN] Haskell Cheatsheet v1.0

[Adrian Neumann]

Thank you for your work! I just glanced over it but I'll suggest it  
to be linked to from the homepage of my university's functional  
programming course.
However, thirteen pages can hardly be called cheatsheet. It's more  
like a quick reference.


You could add [100,99..] infinite liste of numbers from 100  
downwards to you numbers section, as it is an example where the  
range does go backward.


Adrian

Am 11.10.2008 um 01:08 schrieb Justin Bailey:


All,

I've created a cheat sheet for Haskell. It's a PDF that tries to
summarize Haskell 98's syntax, keywords and other language elements.
It's currently available on hackage[1]. Once downloaded, unpack the
archive and you'll see the PDF. A literate source file is also
included.

If you install with cabal install cheatsheet, run cheatsheet
afterwards and the program will tell you where the PDF is located.

The audience for this document is beginning to intermediate Haskell
programmers. I found it difficult to look up some of the less-used
syntax and other language stumbling blocks as I learned Haskell over
the last few years, so I hope this document can help others in the
future.

This is a beta release (which is why I've limited the audience by
using hackage) to get feedback before distributing the PDF to a wider
audience. With that in mind, I welcome your comments or patches[2].

Justin

[1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 
CheatSheet

[2] git://github.com/m4dc4p/cheatsheet.git
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[Haskell-cafe] Re: Health effects

[Adrian Neumann]

The Wikipedia says:

For a finite set of points in the plane, each colored red or  
blue,

there is a line that simultaneously bisects the red points and bisects
the blue points, that is, the number of red points on either side  
of the
line is equal and the number of blue points on either side of the  
line is equal.


Does this work with more than two colours? i.e. can I recursively  
subdivide the halves into quarters with another cut?




Am 01.10.2008 um 15:33 schrieb Dominic Steinitz:

Adrian Neumann aneumann at inf.fu-berlin.de writes:



I often wonder how many cuts you need to divide a steak in n pieces.
You can obviously get n pieces with (sqrt n) cuts by cutting a grid.
But I'm sure some smart mathematician thought of a (log n) way.



You might try the ham sandwich theorem
http://en.wikipedia.org/wiki/Ham_sandwich_theorem as an hors d'oeuvre.

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Re: [Haskell-cafe] Health effects

[Adrian Neumann]

I often wonder how many cuts you need to divide a steak in n pieces.  
You can obviously get n pieces with (sqrt n) cuts by cutting a grid.  
But I'm sure some smart mathematician thought of a (log n) way.


Adrian

Am 29.09.2008 um 21:43 schrieb Andrew Coppin:

The other day, I sat down to eat a 2 Kg block of chocolate - one of  
those ones that's divided into lots of little squares. I proceeded  
to recursively subdivide it into smaller and smaller blocks, and  
then eat the individual squares in depth-first order. It was only  
after getting through 16 of the things that I stopped to notice  
that the whole bar just happens to have an exact power of two  
squares on it.


And it was some time after *that* when I thought to myself  
...woah, maybe do too much Haskell? o_O


Seriously, who recursively subdivides their food? I think I have  
something wrong with me...


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Re: [Haskell-cafe] Looking for a more functional way to do this

[Adrian Neumann]

There is the State Monad which is build just for that kind of purpose, I 
believe:


http://www.haskell.org/all_about_monads/html/statemonad.html

That would safe you from passing around the State

Jefferson Heard schrieb:

Working with HOpenGL and GLUT, I find myself approaching a common
problem with a common solution that I don't really like all that much,
as it reeks of procedural programming.  Basically the problem is that
of complex program state, such that when the user provides input to
the program in the form of a mouse click or a typed string or
character, the program updates its internal state to reflect this,
whether that's changing the rotation, scale, or position of a screen
element, or deciding what data to have loaded from disc.

What I often do is something that looks like this:

data ProgramState  = ProgramState {
some_associative_data :: Map String String
  , position :: GL.Vector3 Float
  , look_at :: GL Vector3 Float
  , selectables :: Map GLuint NamedObject
  }

render :: IORef ProgramState - IO ()
render state = do
  st - readIORef state
  ...

handleMouseClicks :: IORef ProgramState - GLUT.KeyboardMouseHandler
handleMouseClicks state ... = do
  st - readIORef state
  ...

main = do
  ...
  let st = ProgramState { Map.empty ... }
  render' = render st
  mouse' = handleMouseClicks st

  GLUT.renderCallback $= render
  GLUT.keyboardMouseCallback $= Just mouse'

and so on and so forth.   Generally there are not fewer than 5 and not
more than about 32 variables that I have to track between mouse
clicks, and there's all this boilerplate code as well.  I'm searching
for a better way to do this, and I feel sure there is one.  I'm
considering using Template Haskell or possibly SYB to generate this
code, but it also seems like I also ought to be able to declare some
kind of state monad or continuation monad that can encapsulate
ProgramState without having to declare an explicit structure for it
everytime.

For one thing, I'd like to genericize this code and write something
akin to Processing for Haskell (http://www.processing.org).
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[Haskell-cafe] Exceptions

[Adrian Neumann]

Hello,

I think it'd be nice if the compiler could warn me if there are any  
exceptions which I'm not catching, similar to checked exceptions in  
Java. Does anyone know of a possibility to do that in Haskell?


Adrian


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Re: [Haskell-cafe] Help with optimization

[Adrian Neumann]

Maybe your image isn't strict enough and the computations are forced  
when the image gets written to disc?



Am 20.07.2008 um 14:13 schrieb Mitar:


Hi!

Profiling says that my program spends 18.4 % of time (that is around
three seconds) and 18.3 % of allocations in this function which is
saving the rendered image to a PPM file:

saveImageList :: String - Int - Int - [ViewportDotColor] - IO ()
saveImageList filename width height image = do
  B.writeFile filename file
where file = B.append header bytes
  header = C.pack $ P6\n ++ show width ++   ++ show height
++ \n255\n
  bytes = B.pack $ concatMap (color . dealpha .
(\(ViewportDotColor _ c) - c)) image
where color (VoxelColor red green blue _) = [floor $ red *
255, floor $ green * 255, floor $ blue * 255]
  dealpha c = addColor c (VoxelColor 1.0 1.0 1.0 1.0)
-- white background

For a 921615 bytes large file to save this is too much in my opinion.
And I think that it consumes to much allocations. Probably it should
not store intermediate lists?

Any suggestions?

Best regards


Mitar
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[Haskell-cafe] Fixed-Point Combinators

[Adrian Neumann]

Hello,

while studying for a exam I came across this little pearl:

Y = (L L L L L L L L L L L L L L L L L L L L L L L L L L L L)
where
L = λabcdefghijklmnopqstuvwxyzr. (r (t h i s i s a f i x e d p o i n  
t c o m b i n a t o r))


posted by Cale Gibbard to this list. Now I'm wondering how exactly  
does one finde such awesome λ expressions? Is there some mathemagical  
background that lets one conjure such beasts?


Adrian

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Re: [Haskell-cafe] Compiling large code with old machine

[Adrian Neumann]

I would assume -O0, that is, turning off all optimizations, should  
make compilation faster


Adrian

Am 17.06.2008 um 14:19 schrieb Samuel Silva:


Hello

I'm using GHC to compile around 700K of Haskell Code generated by  
HaXml.

How I compile this code.
My machine is Windows-XP(512MB RAM, 1.5GHz) running GHC-6.8.2.

How much time it spend to compile this file?
I spent more than 1 hour and it doesn't finished.

What flags make compiling fast?
I try with -H500m but dont't expect.

It is possible GHC running endless cycle at compilation stage?

Thanks.

--
Don't hug that, Hugs ME!
Samuel Silva
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[Haskell-cafe] (no subject)

[Adrian Neumann]

{- I downloaded the source and put my file in the same directory
   You may need to adjust the imports -}
module Main where
import Picture
import Draw -- change xWin to 1000 and yWin to 700 for this to work
import EnableGUI -- I use a Mac
import SOE hiding (Region)
import qualified SOE as G (Region)
import Data.List
import Random

-- lines are not Shapes unfortunately
linie = ((Shape $ Polygon [(-0.1,-0.01),(-0.1,0.01),(0.1,0.01), 
(0.1,-0.01)]), (-0.1,0), (0.1,0))


main = enableGUI  do
w - openWindow Lindenmayer System (xWin, yWin)
newStdGen
g - getStdGen
drawPic w (aufgabe2 g)
k - getKey w
if (k=='q') then do
closeWindow w
return () else do
clearWindow w
main

-- one big ugly line of code, not that interesting though
aufgabe2 g= dasBild where
r = rotateRegion (pi/2) $ Translate (-2.5,0) $ renderLSystem  
linie (lSystem 20 g)
dasBild = Region White r `Over` Region Black ( Translate  
(0,-1.8) $ Scale (1,0.3)$ Translate (0,-2.6) $ rotateRegion (pi/2+pi/ 
3) $ Translate (0,2.6) $ r) `Over` Region Green (Shape $ Polygon  
[(-5,-3.5),(-5,-1.5),(5,-1.5),(5,-3.5)]) `Over` Region Yellow  
(Translate (4,1.5) (Shape $ circle (0.5))) `Over`

Region Blue (Shape $ Rectangle 14 7)

-- start of the interesting part:
-- A - Axiom, the base shape we use for rendering later
--F - Forward
--Branch - what it says

data LSys = A LSys | F LSys | Branch StdGen [LSys] LSys | Done  
deriving Show


-- a Axiom is a region with two connector points
type Axiom = (Region, Vertex, Vertex)

-- this seems not to be used anymore?

scaleAxiom :: Float - Axiom - Axiom
scaleAxiom f (r,u,v) = (Scale (f,f) r, f .*. u, f .*. v)

-- just for testing purposes
testLSys = A (Branch (mkStdGen 5) [A (F ((Branch (mkStdGen 5) [A  
(Branch (mkStdGen 5) [A (F ((Branch (mkStdGen 5) [A (F Done), A (F  
Done)] Done))), A (F Done)] Done), A (F Done)] Done))), A (F Done)]  
Done)


-- a 2D rotation matrix
drehM :: Float - (Float, Float, Float, Float)
drehM w = (cos w, -sin w, sin w, cos w)

-- matrix vector multiplication
(.**.) :: (Float, Float, Float, Float) - Vertex - Vertex
(.**.) (a,b,c,d) (px,py) = (a*px+b*py, c* px+d*py)

-- other vector stuff
(.-.) (a,b) (c,d) = (a-c,b-d)
(.+.) (a,b) (c,d) = (a+c,b+d)
(.*.) l (c,d) = (c*l,d*l)
abs' (a,b) = (abs a, abs b)
betr (a,b) = sqrt (a*a+b*b)

-- SOE doesn't come with a way to rotate Regions, so I wrote my own
rotateRegion :: Float - Region - Region
rotateRegion f (Shape s) = Shape (rotateS f s)
rotateRegion f (Translate v r) = Translate ((drehM f).**.v)  
(rotateRegion f r)


-- the scaling part is not right I think. Everything seems to break  
if I try to incorporate scaling

-- into the rendering

rotateRegion f (Scale v r) = Scale ((betr v/ betr nv) .*. nv)  
(rotateRegion f r) where

x = ((drehM f).**. (fst v,0))
y = ((drehM f) .**. (0,snd v))
nv = (abs' x) .+. (abs' y)
rotateRegion f (Complement r) =Complement (rotateRegion f r)
rotateRegion f (Union r1 r2) = Union (rotateRegion f r1)  
(rotateRegion f r2)
rotateRegion f (Intersect r1 r2) = Intersect (rotateRegion f r1)  
(rotateRegion f r2)

rotateRegion f (Xor r1 r2) = Xor (rotateRegion f r1) (rotateRegion f r2)
rotateRegion _ s=s

rotateS f (Polygon pts) = Polygon (map ((drehM f) .**.) pts)
rotateS f x = x

-- nondeterministically generate a word in our LSys language
-- lots of copypaste here, any way to do this better?

lSystem :: Int - StdGen - LSys
lSystem n g = f n g (A undefined) where
f :: Int - StdGen - LSys - LSys
f 0 _ _ = Done
f (n+1) g (A _)
| choose = 1 = A (f n ng (F undefined))
| choose == 0 = A (f n ng (Branch ng [f n ng' (A undefined),  
f n ng'' (A undefined)] undefined)) where

(choose, ng) = randomR (0::Int,3::Int) g
(ng', ng'') = split ng
f (n+1) g (F _)
| choose = 1 = F (f n ng (F undefined))
| choose == 0 = F (f n ng (Branch ng [f n ng' (A undefined),  
f n ng'' (A undefined)] undefined)) where

(choose, ng) = randomR (0::Int,3::Int) g
(ng', ng'') = split ng
f (n+1) g (Branch h lSys _)
| choose = 1 = Branch h lSys  (f n ng (F undefined))
| choose == 0 = Branch h lSys (f n ng (Branch ng [f n ng' (A  
undefined), f n ng'' (A undefined)] undefined)) where

(choose, ng) = randomR (0::Int,5::Int) g
(ng', ng'') = split ng

-- recursivly render a LSys
renderLSystem :: Axiom - LSys - Region
renderLSystem _ Done = Empty
renderLSystem (r,u,v) (A lSys) = r `Union` renderLSystem (r,u,v) lSys
renderLSystem (r,u,v) (F lSys) = r'' `Union` renderLSystem (r'', u . 
+. o , v .+.o) lSys where

r'' =  Translate o   $  r
o = (v .-. u)
renderLSystem (r,u,v) (Branch g lSys rest) =
theBranches `Union` renderLSystem (r,u,v) rest where
theBranches = Translate o $ foldr Union Empty $
-- we need to rotate around the u-Connector, not around (0,0)
-- thus translation
map 

Re: [Haskell-cafe] Lindenmayer Systems, WAS: (no subject)

[Adrian Neumann]

I screwed up the email, sorry about that. What I wanted to say was:

Hello,

as homework I was assigned to design and draw an image using the  
SOE Graphics library [1]. In order to impress my classmates I decided  
to draw a bush-like thingy using a Lindenmayer-System. It turns out  
quite nice [2], and so I thought I might share my code with you. Of  
course criticism is very welcome.


Ok, here we go:



{- I downloaded the source and put my file in the same directory
   You may need to adjust the imports -}
module Main where
import Picture
import Draw -- change xWin to 1000 and yWin to 700 for this to work
import EnableGUI -- I use a Mac
import SOE hiding (Region)
import qualified SOE as G (Region)
import Data.List
import Random

-- lines are not Shapes unfortunately
linie = ((Shape $ Polygon [(-0.1,-0.01),(-0.1,0.01),(0.1,0.01), 
(0.1,-0.01)]), (-0.1,0), (0.1,0))


main = enableGUI  do
w - openWindow Lindenmayer System (xWin, yWin)
newStdGen
g - getStdGen
drawPic w (aufgabe2 g)
k - getKey w
if (k=='q') then do
closeWindow w
return () else do
clearWindow w
main

-- one big ugly line of code, not that interesting though
aufgabe2 g= dasBild where
r = rotateRegion (pi/2) $ Translate (-2.5,0) $ renderLSystem  
linie (lSystem 20 g)
dasBild = Region White r `Over` Region Black ( Translate  
(0,-1.8) $ Scale (1,0.3)$ Translate (0,-2.6) $ rotateRegion (pi/2 
+pi/3) $ Translate (0,2.6) $ r) `Over` Region Green (Shape $  
Polygon [(-5,-3.5),(-5,-1.5),(5,-1.5),(5,-3.5)]) `Over` Region  
Yellow (Translate (4,1.5) (Shape $ circle (0.5))) `Over`

Region Blue (Shape $ Rectangle 14 7)

-- start of the interesting part:
-- A - Axiom, the base shape we use for rendering later
--F - Forward
--Branch - what it says

data LSys = A LSys | F LSys | Branch StdGen [LSys] LSys | Done  
deriving Show


-- a Axiom is a region with two connector points
type Axiom = (Region, Vertex, Vertex)

-- this seems not to be used anymore?

scaleAxiom :: Float - Axiom - Axiom
scaleAxiom f (r,u,v) = (Scale (f,f) r, f .*. u, f .*. v)

-- just for testing purposes
testLSys = A (Branch (mkStdGen 5) [A (F ((Branch (mkStdGen 5) [A  
(Branch (mkStdGen 5) [A (F ((Branch (mkStdGen 5) [A (F Done), A (F  
Done)] Done))), A (F Done)] Done), A (F Done)] Done))), A (F Done)]  
Done)


-- a 2D rotation matrix
drehM :: Float - (Float, Float, Float, Float)
drehM w = (cos w, -sin w, sin w, cos w)

-- matrix vector multiplication
(.**.) :: (Float, Float, Float, Float) - Vertex - Vertex
(.**.) (a,b,c,d) (px,py) = (a*px+b*py, c* px+d*py)

-- other vector stuff
(.-.) (a,b) (c,d) = (a-c,b-d)
(.+.) (a,b) (c,d) = (a+c,b+d)
(.*.) l (c,d) = (c*l,d*l)
abs' (a,b) = (abs a, abs b)
betr (a,b) = sqrt (a*a+b*b)

-- SOE doesn't come with a way to rotate Regions, so I wrote my own
rotateRegion :: Float - Region - Region
rotateRegion f (Shape s) = Shape (rotateS f s)
rotateRegion f (Translate v r) = Translate ((drehM f).**.v)  
(rotateRegion f r)


-- the scaling part is not right I think. Everything seems to break  
if I try to incorporate scaling

-- into the rendering

rotateRegion f (Scale v r) = Scale ((betr v/ betr nv) .*. nv)  
(rotateRegion f r) where

x = ((drehM f).**. (fst v,0))
y = ((drehM f) .**. (0,snd v))
nv = (abs' x) .+. (abs' y)
rotateRegion f (Complement r) =Complement (rotateRegion f r)
rotateRegion f (Union r1 r2) = Union (rotateRegion f r1)  
(rotateRegion f r2)
rotateRegion f (Intersect r1 r2) = Intersect (rotateRegion f r1)  
(rotateRegion f r2)
rotateRegion f (Xor r1 r2) = Xor (rotateRegion f r1) (rotateRegion  
f r2)

rotateRegion _ s=s

rotateS f (Polygon pts) = Polygon (map ((drehM f) .**.) pts)
rotateS f x = x

-- nondeterministically generate a word in our LSys language
-- lots of copypaste here, any way to do this better?

lSystem :: Int - StdGen - LSys
lSystem n g = f n g (A undefined) where
f :: Int - StdGen - LSys - LSys
f 0 _ _ = Done
f (n+1) g (A _)
| choose = 1 = A (f n ng (F undefined))
| choose == 0 = A (f n ng (Branch ng [f n ng' (A  
undefined), f n ng'' (A undefined)] undefined)) where

(choose, ng) = randomR (0::Int,3::Int) g
(ng', ng'') = split ng
f (n+1) g (F _)
| choose = 1 = F (f n ng (F undefined))
| choose == 0 = F (f n ng (Branch ng [f n ng' (A  
undefined), f n ng'' (A undefined)] undefined)) where

(choose, ng) = randomR (0::Int,3::Int) g
(ng', ng'') = split ng
f (n+1) g (Branch h lSys _)
| choose = 1 = Branch h lSys  (f n ng (F undefined))
| choose == 0 = Branch h lSys (f n ng (Branch ng [f n  
ng' (A undefined), f n ng'' (A undefined)] undefined)) where

(choose, ng) = randomR (0::Int,5::Int) g
(ng', ng'') = split ng

-- recursivly render a LSys
renderLSystem :: Axiom - LSys - Region
renderLSystem _ Done = Empty
renderLSystem (r,u,v) (A lSys) = r `Union` renderLSystem (r,u,v) lSys

Re: [Haskell-cafe] cgi liftM liftIO

[Adrian Neumann]

I think you need to put liftIO in front of the IO actions you want to  
do inside the CGI Monad. Like in this example


 http://www.haskell.org/haskellwiki/ 
Practical_web_programming_in_Haskell#File_uploads


(Why did I need to use google to find that? The wiki search in awful.  
Searching for CGI returns nothing, whereas with google the above is  
the first hit.)


Am 13.06.2008 um 15:41 schrieb Cetin Sert:


Hi,

Could someone please care to explain what I am doing wrong below in  
cgiMain2 and how can I fix it?



./Main.hs:25:15:
No instance for (MonadCGI IO)
  arising from a use of `output' at ./Main.hs:25:15-20
Possible fix: add an instance declaration for (MonadCGI IO)
In the first argument of `($)', namely `output'
In the expression: output $ renderHtml $ page import fileForm
In the definition of `upload':
upload = output $ renderHtml $ page import fileForm

./Main.hs:57:29:
Couldn't match expected type `CGI CGIResult'
   against inferred type `IO CGIResult'
In the first argument of `handleErrors', namely `cgiMain2'
In the second argument of `($)', namely `handleErrors cgiMain2'
In the expression: runCGI $ handleErrors cgiMain2


import IO
import Network.CGI
import Text.XHtml

import qualified Data.ByteString.Lazy as BS

import Control.Monad (liftM)
import Data.Maybe (fromJust)

import Interact

fileForm = form ! [method post, enctype multipart/form-data] 
 [afile file, submit  Upload]

page t b = header  thetitle  t +++ body  b

cgiMain1 = do
  getInputFPS file ↠ λms → maybe upload contents ms ↠ return
  where
upload   = output $ renderHtml $ page import fileForm
contents = outputFPS

cgiMain2 = do
  getInputFPS file ↠ λms → maybe upload contents ms ↠ return
  where
upload   = output $ renderHtml $ page import fileForm
contents = λs → do
  (i,o,h,_) ← runUnzip
  BS.hPutStr i s
  c ← BS.hGetContents o
  outputFPS c


{-
  (i,o,h,_) ← runUnzip
  BS.hPutStr i s
  BS.hGetContents o ↠ outputFPS
-}



{-
liftM :: (Monad m) = (a1 - r) - m a1 - m r
liftIO :: (MonadIO m) = IO a - m a

saveFile n =
do cont - liftM fromJust $ getInputFPS file
   let f = uploadDir ++ / ++ basename n
   liftIO $ BS.writeFile f cont
   return $ paragraph  (Saved as  +++ anchor ! [href f]   
f +++ .)

-}

runUnzip = runInteractiveCommand unzip -l /dev/stdin

main = runCGI $ handleErrors cgiMain2

Best Regards,
Cetin Sert

P/s: what are lifts o_O?
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Re: [Haskell-cafe] Copying Arrays

[Adrian Neumann]

Isn't fast IO what ByteStrings where invented for?

Adrian

Tom Harper schrieb:

I'm trying to implement some file I/O where I can read in a file to an
array, but do so without having to know how much space I will need.
(Before you suggest it, lists are too slow/space consuming.)  I was
thinking that one possible solution is to use a strategy used in
dynamic arrays, where everytime you run out of space you double the
capacity of the array.  Trouble is, most arrays in Haskell would
require you to traverse the entire array and copy each individual
cell, which would make it worthless.

Are there any low-level array types (MutableByteArray#, for example)
that support a memcpy-like function where I could copy the entire
array into the new one instead of copying it one value at a time?  Is
there another solution that I'm missing?







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[Haskell-cafe] appending an element to a list

[Adrian Neumann]

Hello,

I was wondering how expensive appending something to a list really is. 
Say I write


I'd say longList ++ [5] stays unevaluated until I consumed the whole 
list and then appending should go in O(1). Similarly when concatenating 
two lists.


Is that true, or am I missing something?

Adrian



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Re: [Haskell-cafe] HTTP and file upload

[Adrian Neumann]

Yes

http://hpaste.org/6990

Am 14.04.2008 um 19:07 schrieb Adam Smyczek:

Is form based file upload supported in HTTP module (HTTP-3001.0.4)?

Adam


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[Haskell-cafe] Windows CE, Xscale Compiler?

[Adrian Neumann]

Is there any Haskell compiler for Windows CE on a XScale platform? I 
googled around a bit and found references to a Hugs implementation but 
couldn't find any place to download it.


I have the opportunity to program a teledrive[1] and would very much 
like to avoid something disfunctional like C.


[1] http://www.iavproducts.com/de/produkte/teledrive/teledrive.php



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[Haskell-cafe] HDBC, character encoding

[Adrian Neumann]

Hi,

I wrote a CGI program to access a Postgres database using HDBC. The  
database stores books and I want to display those from a certain  
author. Everything works fine, unless I search for someone with an  
umlaut in his name. Böll, for example. I have a function like this


 bookByAuthor :: Connection - AutorName - IO [[String]]
 bookByAuthor c aName = do
writeFile ./err.log ((show aName)++ ++(show $ toSql aName))
	rows - quickQuery c SELECT * FROM buecher WHERE lower 
(autor_name) LIKE ? ORDER BY autor_name, buch_name [toSql $ map  
toLower $ '%':aName++%]

return $ map (map fromSql) rows

It returns me a SqlError. However, doing the same in ghci works  
perfectly. I can't understand why. err.log contains


 b\195\182ll SqlString b\195\182ll

which is ok I think. Since

 quickQuery c SELECT * FROM buecher WHERE lower(autor_name) LIKE ?  
ORDER BY autor_name, buch_name [toSql %b\195\182%]


works in ghci. I have tried b\246ll, but that doesn't even work in  
ghci, although the database-encoding is utf-8. This all is really  
annoying...


Adrian

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[Haskell-cafe] Performance Issues

[Adrian Neumann]

Hello Haskell-Cafe!

I've read about Control.Parallel and wanted to give it a try. Here's  
what I wrote:


---
import Control.Parallel
import Data.List

splitList :: [Integer] - [[Integer]]
splitList = unfoldr f where
f [] = Nothing
f ~x = Just (splitAt 3 x)

map' :: (a-b) - [a] - [b]
map' f (x:xs) =fx `par` (seq mfxs (fx:mfxs)) where
fx = f x
mfxs = map' f xs
map' f [] = []

product' :: [Integer] - Integer
product' = foldr1 (\x y - seq x (x*y))

fak :: Integer - Integer
fak n = product' $ map' product' (splitList [1..n])

main = putStrLn $ seq (fak 100) done
---

This computes 100!. This version takes 8m29.189s to execute.  
Replace foldr1 with foldr and that goes down to 7m4.315s. Replace  
product' with the Prelude product and it takes only 6m17.685s. Why is  
that so? I'm using ghc 6.8.1 on Mac OS X.


Regards,

Adrian


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[Haskell-cafe] type trickery

[Adrian Neumann]

Hello haskell-cafe!

After making data Number = Zero | Succ Number an instance of  
Integral, I wondered how I could do the same with galois fields. So  
starting with Z mod p, I figured I'd need something like this


data GF = GF Integer Integer

so that each element of the finite field would remember p. However I  
can't think of a way to use the typesystem to ensure that p is always  
the same. I think that would need an infinite number of different  
types, but the type hackers here probably know something better.


Adrian


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[Haskell-cafe] Trees

[Adrian Neumann]

Good morning,

as an exercise for my Algorithms and Programming course I have to  
program a couple of simple functions over trees. Until now everything  
we did in Java could be done in Haskell (usually much nicer too)  
using the naive


 data Tree a = Leaf a | Node a [Tree a]

But now the assignments require more than a simple top-down  
traversal. For example: given a tree t and two nodes u,v, find the  
first common ancestor.
In Java this is really simple, because each node has a parent  
reference. That way I only need to follow those references until I  
find the first common ancestor. This should take something like O(log  
n) in the average case.


In Haskell however the best way I've come up with so far is doing a  
BFS and looking for the last common node in the paths to u and v.  
This is neither fast, nor particularly elegant.
So how would you smart guys do it? With a Zipper? It would be nice if  
there was an elegant solution without monads.


--Adrian


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Re: [Haskell-cafe] Monad.Reader 8: Haskell, the new C++

[Adrian Neumann]

-BEGIN PGP SIGNED MESSAGE-
Hash: RIPEMD160

I heard only rumors, but isn't Lisp supposed to be just that? A
programmable programming language?

Peter Verswyvelen schrieb:
 This is all very cool stuff, but sometimes I wander if it isn't possible
 to drop the special languages for fiddling with types, and introduce
 just a single language which has no types, only raw data from which you
 can built your own types (as in the old days when we used macro
 assemblers ;-), but the language has two special keywords: static and
 dynamic, where code annotated with static runs in the compiler domain,
 and code annotated with dynamic runs in application domain. Of course,
 I don't know much about this, so this idea might be totally insane ;-)
 Probably this is impossible because of the halting problem or something...
 
 Pete
 
 Don Stewart wrote:
 Better here means better -- a functional language on the type 
 system,
 to type a functional language on the value level.

 -- Don
   
 For a taste, see Instant Insanity transliterated in this functional 
 language:

 http://hpaste.org/2689

 NB: it took me 5 minutes, and that was my first piece of coding ever 
 with Type families
 

 Wow. Great work!
 The new age of type hackery has dawned.

 -- Don
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[Haskell-cafe] Data.Binary Endianness

[Adrian Neumann]

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I try to write a Windows Bitmap File using Data.Binary, but I have some
trouble.

For example, the internet states, that the magic number, that puts 'BM'
in the first two bytes of the file is 19778. But when I

 put (19778::Word16)

I get 'MB' instead. I read on the german Wikipedia, that bmp uses little
endian encoding, but Data.Binary uses big endian. Obviously this can't work.

Is there a way to switch endianness? Or at least a function that
converts between the two?

I looked at the Data.Binary Haddock documentation, but found nothing
like that.

Thanks in advance,

Adrian
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[Haskell-cafe] IO inside CGI

[Adrian Neumann]

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I'm toying around with web programming in Haskell. I'm trying to write a
script which GETs an id and returns a couple of random numbers.
Something like this:

cgiMain :: CGI CGIResult
cgiMain = do
inp - getInput id
let gen = parse inp
output $ take 10 (randoms gen)

parse :: Maybe String- StdGen
parse (Just x) = read x
parse Nothing = undefined

Now I'd like to get a new StdGen, in case no id was supplied to the script.

cgiMain :: CGI CGIResult
cgiMain = do
inp - getInput id
gen - parse inp
output $ take 10 (randoms gen)

parse :: Maybe String- IO StdGen
parse (Just x) = return $ read x
parse Nothing = getStdGen

Obviously this doesn't work because I'm trying to do IO inside CGI
(right?). Is there some incantation I can perform to make this possible?
Like

gen - arcaneMagic parse inp

As you probably have noticed I don't know very much about monads, all I
did until now was reading or writing some files.

Thanks in advance
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[Haskell-cafe] Sudoku Solver

[Adrian Neumann]

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Hi,

I wrote a brute-force sudoku solver. It takes a [[Int]] and spits out
the first solution it finds.
Why is it, that

[0,0,0,7,0,6,9,0,0]
[9,0,0,0,0,4,7,0,0]
[7,0,0,0,0,0,4,0,0]
[0,2,7,0,3,5,8,0,0]
[6,9,5,8,2,0,0,0,0]
[0,8,0,0,0,0,5,0,0]
[0,3,0,0,0,0,0,0,0]
[8,7,0,9,0,0,0,0,0]
[0,0,0,0,0,0,0,0,0]

is solved instantly, but when I remove just one number my program takes
forever?

===

import Array
import List
import System

type Field = Array Int (Array Int Int)

isEmpty :: Int - Bool
isEmpty = (==) 0

done :: Field - Bool
done a = isFull a  checkField a

isFull::Field - Bool
isFull a = notElem (0) $ (concat.(map elems).elems) a

readField :: [[Int]]-Field
readField = (listArray (1,9).map (listArray (1,9)))

checkField :: Field - Bool
checkField a =  check (rows a)   check (columns a)  check (squares  a)
where
check b = and $ ((map ((==1).length)).concat.(map group).clean) 
b
clean =  map (dropWhile isEmpty.sort)

columns::Field - [[Int]]
columns a = [[a!i!j|i-[1..9]]|j-[1..9]]

rows :: Field - [[Int]]
rows a = [[a!i!j|j-[1..9]]|i-[1..9]]

squares :: Field - [[Int]]
squares a = [[a!(i+n)!(j+m)|i-[1..3],j-[1..3]] |n-[0,3,6],m-[0,3,6]]


- -- creates a list of all allowed entries
genMoves :: Field - [Field]
genMoves a = concat [update a i j|i-[1..9],j-[1..9],isEmpty (a!i!j)]
where update b i j= [b //[(i,b!i //[(j,x)])]|x-[1..9], checkField (b
//[(i,b!i //[(j,x)])])]

play :: [Field] - Maybe Field
play (f:a)
| done f= Just f
| isFull f= play a
| otherwise = play ((genMoves f)++a)
play [] = Nothing

main :: IO ()
main = do
  x - getArgs
  let field = read (x!!0) :: [[Int]]
  let erg=play [readField field]
  case erg of
Just a - putStrLn $ concat $ map ((++\n).show) (rows 
a)
Nothing - putStrLn Es gibt keine Loesung
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[Haskell-cafe] Cannot compile Network.CGI programs

[Adrian Neumann]

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Hi,

I installed the Network.CGI package and tried to compile the Hello World
example on my Ubuntu machine.

ghc cgi.hs -o cgi

gives me

 cgi.o: In function `r1hk_info':
 (.text+0x56): undefined reference to 
 `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure'
 cgi.o: In function `r1hm_info':
 (.text+0xb6): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGIziMonad_zdf4_closure'
 cgi.o: In function `r1ho_info':
 (.text+0x11a): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGI_runCGI_closure'
 cgi.o: In function `r1ho_info':
 (.text+0x121): undefined reference to 
 `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure'
 cgi.o: In function `r1hq_info':
 (.text+0x17e): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGI_output_closure'
 cgi.o: In function `s1hE_info':
 (.text+0x2aa): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGI_handleErrors_closure'
 cgi.o: In function `r1hk_srt':
 (.rodata+0x0): undefined reference to 
 `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure'
 cgi.o: In function `r1hm_srt':
 (.rodata+0x4): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGIziMonad_zdf4_closure'
 cgi.o: In function `r1ho_srt':
 (.rodata+0xc): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGI_runCGI_closure'
 cgi.o: In function `r1ho_srt':
 (.rodata+0x10): undefined reference to 
 `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure'
 cgi.o: In function `r1hq_srt':
 (.rodata+0x14): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGI_output_closure'
 cgi.o: In function `s1hE_srt':
 (.rodata+0x38): undefined reference to 
 `cgizm2006zi9zi6_NetworkziCGI_handleErrors_closure'
 collect2: ld gab 1 als Ende-Status zurück

However runghc cgi.hs produces a Hello World

 Content-type: text/html; charset=ISO-8859-1
 
 Hello World!

Why is that so? What can I do to compile CGI-programs?

(GHC 6.6, libghc6-cgi-dev 2006.9.6-3)
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[Haskell-cafe] Intermediate Haskell Books?

[Adrian Neumann]

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Are there any good books about intermediate to advanced Haskell? The
descriptions here http://haskell.org/haskellwiki/Books_and_tutorials
aren't very helpful.

Adrian
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[Haskell-cafe] The Trivial Monad

[Adrian Neumann]

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I've read this blogpost about the trivial monad
http://sigfpe.blogspot.com/2007/04/trivial-monad.html, because I still
don't understand what this monad thingy is all about.

The author defines three functions:

data W a = W a deriving Show

return :: a - W a
return x = W x

fmap :: (a - b) - (W a - W b)
fmap f (W x) = W (f x)

bind :: (a - W b) - (W a - W b)
bind f (W x) = f x

and asks the reader to prove the tree monad laws for them. However I
don't understand the type signatures for bind and fmap. I'd say (and
ghci's type inference agrees) that bind and fmap have the type

bind:: (a-W b) - W a - W b
fmap:: (a-b) - W a - W b

They take a function f and something and return what f does to that. I
don't see why they should return a function.

This of course makes it hard for me to prove the monad laws. The first
however works nonetheless:

1) bind f (return a)= f a

= bind f (return a)= bind f (W a) = f a

Can someone explain bind and fmap (and possible law 2 and 3)?

Thanks,

Adrian
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[Haskell-cafe] (newbie) instance Enum MyType where, smarter way?

[Adrian Neumann]

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Hello,

I defined an enumeration datatype like this

data MyType = One | Two | Four | Eight

and want to make it an instance of the class Enum. deriving Enum won't
do what I want, as it labels the items 0,1,2,3. Is there a better way to
do this than

instance Enum MyType where
fromEnum One = 1
fromEnum Two = 2
...
toEnum 8 = Eight

Something like

instance Enum MyType where
fromEnum One = 1
fromEnum x = 2*pred x
toEnum 1 = One
toEnum x = succ (toEnum x `div` 2)

Which doesn't work because succ and pred are not (properly?) defined. Is
there a way to let deriving Enum do *some* of work (i.e. defining succ
and pred) while manually defining the other functions?
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