2009/7/17 Andrew Coppin andrewcop...@btinternet.com:
I've been working hard this week, and I'm stumbled upon something which is
probably of absolutely no surprise to anybody but me.
Consider the following expression:
(foo True, foo 'x')
Is this expression well-typed?
Astonishingly, the
Is it possible to parallelize array-based functions such as in-place quicksort
in Haskell? If so, does anyone have a working implementation?
--
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e
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===
CALL FOR PAPERS
ACM SIGPLAN 2010 Workshop on
Partial Evaluation and Program Manipulation (PEPM'10)
Madrid, January 18-19, 2010
(Affiliated with POPL'10)
There is a C++ parser in C++, it may be of help :
http://42ndart.org/scalpel/
It's a quite advanced WIP.
On Fri, Jul 17, 2009 at 3:58 AM, Sterling Clover s.clo...@gmail.com wrote:
A parser for JavaScript (admittedly a much simpler beast) is part of
Brown's WebBits:
on haskell reddit today
powerSet = filterM (const [True, False])
is said to be beautiful / mind blowing. I just don't get it. I can
play with transformations until I get
powerSet [] = [[]]
powerSet (x:xs) =
let pxs = powerSet xs
in map (x:) pxs ++ pxs
which is understandable to me,
On Fri, Jul 17, 2009 at 1:35 AM, Thomas Hartman tphya...@gmail.com wrote:
on haskell reddit today
powerSet = filterM (const [True, False])
The M is the list, i.e. *nondeterminism* monad. For each element in the
list, there is one return value where it appears (True), and one where it
does
For each item, we ignore what the item actually is (hence `const`),
and say that we both want it (True) and don't want it (False) in the
output. Since we are using the list monad we are allowed to say this,
and the filter function gives us a list of lists. I think there's
probably a more intuitive
Thomas Hartman wrote:
on haskell reddit today
powerSet = filterM (const [True, False])
Does it help if we inline the 'const' function and rewrite [True, False]
in monadic notation as (return True `mplus` return False)?
powerSet = filterM (\x - return True `mplus` return False).
You can
Thank you all for your answers and sorry for the delay I'm writing
this message but before replying, I wanted to be sure to understand
your arguments!
Now, I'm starting to get into this tying the knot thing and
understand why the Haskell version of fib ties the knot while my
Python version does
On 17 Jul 2009, at 12:41, Cristiano Paris wrote:
Thank you all for your answers and sorry for the delay I'm writing
this message but before replying, I wanted to be sure to understand
your arguments!
Now, I'm starting to get into this tying the knot thing and
understand why the Haskell
On Jul 17, 2009 1:40pm, Thomas Hartman wrote:
my question to all 3 (so far) respondants is, how does your
explanation explain that the result is the power set?
I guess you forgot to reply to the cafe.
Well, to me the modified definition I posted looks like the essence of
powerset, the set
On Thu, Jul 16, 2009 at 9:57 PM, Ryan Ingramryani.s...@gmail.com wrote:
On Thu, Jul 16, 2009 at 8:22 PM, Thomas Hartmantphya...@gmail.com wrote:
Is this being worked on?
On Thu, Jul 16, 2009 at 12:35 PM, Bas van Dijkv.dijk@gmail.com wrote:
I have no idea.
Yes.
Bolingbroke,
Job Vranish wrote:
I was needing a way to zip generic data structures together today and
was very annoyed to find that there is no Zippable class, or variant
there of.
Notice that you can always do this if the LHS is traversable and the RHS
is Foldable (as a special case the RHS is the
On Fri, Jul 17, 2009 at 12:46 PM, Thomas Davietom.da...@gmail.com wrote:
Memoization is not a feature of lazyness. If you can do it in such a way
that you don't waste significant amount of RAM, then it may be a nice
optimisation, and an alternative evaluation strategy, but it would not be
2009/7/17 Gleb Alexeyev gleb.alex...@gmail.com:
On Jul 17, 2009 1:40pm, Thomas Hartman wrote:
my question to all 3 (so far) respondants is, how does your
explanation explain that the result is the power set?
Because powerset(s) = 2^s?
I was going to make some nice code but I ended up with
On Fri, Jul 17, 2009 at 12:41 PM, Cristiano Parisfr...@theshire.org wrote:
...
Now, to confirm my hypothesis, I wrote a slight different version of
fib, like follows:
fib' n = 1:1:(fib' n) `plus` (tail $ fib' n) where plus = zipWith (+)
i.e. I inserted a fictious argument n in the
Thomas Hartman wrote:
on haskell reddit today
powerSet = filterM (const [True, False])
is said to be beautiful / mind blowing.
Is this a uniquely haskell obfu, or is there a way of reading this
definition that makes sense?
To me, these are more obvious:
powerSet = map catMaybes . mapM
Hi All,
I am working on a diagraming utility in Haskell. I started with line drawing.
I am doing the basic stuff using the y = mx + c formula to draw a line between
(x1,y1) and (x2,y2)
Here's what I need to do -
if dx dy where dx = (x2 - x1) and dy = (y2 - y1) then I need to vary x
between
CK Kashyap wrote:
Hi All,
I am working on a diagraming utility in Haskell. I started with line
drawing.
I am doing the basic stuff using the y = mx + c formula to draw a line
between (x1,y1) and (x2,y2)
Hi,
Are you doing this to learn Haskell, learn about drawing lines, or to
just get it
Thanks Neil ...
Are you doing this to learn Haskell, learn about drawing lines, or to just
get it implemented? If either of the latter two, when drawing a straight
line you shouldn't need to do floating point operations such as this:
Actually, my reasons are first and third.
newY =
BTW, after a -O2 compilation, fib' is apparently as fast a fib.
The compiler is your friend. :o)
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Am Freitag, 10. Juli 2009 23:41 schrieben Sie:
On Jul 10, 2009, at 4:35 AM, Wolfgang Jeltsch wrote:
I fear that this instance doesn’t satisfy required laws. As far as
I know, the following equalities should hold:
(*) = ()
f * empty = empty
empty | g = g
This
Am Samstag, 11. Juli 2009 00:16 schrieben Sie:
On Friday 10 July 2009 4:35:15 am Wolfgang Jeltsch wrote:
I fear that this instance doesn’t satisfy required laws. As far as I
know, the following equalities should hold:
(*) = ()
f * empty = empty
IO already fails at this law,
Am Dienstag, 7. Juli 2009 14:42 schrieb Robin Green:
On Fri, 10 Jul 2009 10:44:51 +0200
Wolfgang Jeltsch g9ks1...@acme.softbase.org wrote:
PASCAL
uses “program”, not “programme”,
The word program (as in computer program) is spelled program in both
British and American English.
Probably
Am Mittwoch, 15. Juli 2009 05:27 schrieben Sie:
On Jul 10, 2009, at 8:44 PM, Wolfgang Jeltsch wrote:
Why do we use English for identifiers? Because English is the language of
computer science. What English should we use? It’s tempting to say, we
should use the original English, which is
Am Freitag, 10. Juli 2009 09:54 schrieb david48:
Hello all,
I made a small program for my factory and I wanted to try to document
it using haddock. The thing is, the comments are in French and the
resulting html pages are unreadable because the accentuated letters
are mangled.
It's not
On Fri, Jul 17, 2009 at 4:37 PM, Wolfgang
Jeltschg9ks1...@acme.softbase.org wrote:
To my knowledge, Haddock only supports ASCII as input encoding. If you want to
have characters outside ASCII, you have to escape them using something like
#xA0;.
Which would mean, while editing the code I'd have
On Fri, Jul 17, 2009 at 10:21 AM, Wolfgang
Jeltschg9ks1...@acme.softbase.org wrote:
Am Freitag, 10. Juli 2009 23:41 schrieben Sie:
Additionally, the second equality you provide is just wrong.
f * empty = empty is no more true than f * g = g,
I don’t understand this. The equation f * g = g
If one of you knows something about working at Jane Street, I'd be
happy to have exchange some mails. I am considering applying there.
Thanks!
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cabal -v3 update
will give you a more verbose version of what is going wrong.
cabal --help
regrettably, cabal --help doesn't tell you this but there is always
the man page I suppose.
2009/7/16 Tony Hannan tonyhann...@gmail.com:
Hello,
I'm on Ubuntu 8.10.
I installed ghc 6.10.4 (from binary
Thomas, if you did no know, where to look for `lazy-memory-hole', say
in your first example, how would you go about solving that puzzle
systematically with a Profiler (or other tools)?
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Am Freitag, 17. Juli 2009 16:43 schrieben Sie:
On Fri, Jul 17, 2009 at 4:37 PM, Wolfgang
Jeltschg9ks1...@acme.softbase.org wrote:
To my knowledge, Haddock only supports ASCII as input encoding. If you
want to have characters outside ASCII, you have to escape them using
something like
I don't have a good answer to that, and I unable to reliably solve
this type of problem, which is one reason I am posting around on
haskell cafe hoping to accumulate wisdom.
Here for instance I think I did
t = last . take (10^6) $ repeat $ S.empty
which doesn't blow up, and by process of
On Fri, Jul 17, 2009 at 4:05 PM, Wolfgang
Jeltschg9ks1...@acme.softbase.org wrote:
Yes, it’s a pity. For me, it’s not such a big problem since I don’t write my
Haddock comments in my native language (German) but in English. I only
experience this problem because I use nice typography, i.e., “ ”
On Fri, Jul 17, 2009 at 4:15 PM, david48dav.vire+hask...@gmail.com wrote:
On Fri, Jul 17, 2009 at 4:05 PM, Wolfgang
GHC supports UTF-8 input, and Haddock uses GHC nowadays. So, in my opinion,
Haddock should also support UTF-8 input. Do you want to file a feature
request?
Sure. I'm
Hi,
How can I open and read colors of specific pixels of an image file in
Haskell? Which packages, functions do you recommend?
You can have a look at the quoted conversation for an idea on what I would
like to automate.
Best Regards,
Cetin Sert
Hi dear *^o^*,
Here are the points from
I tried using your original code and stuffing it into a profiler. But
all I get is a triangle of linearly increasing resource usage, and
then it breaks for lack of stack. I guess I am just to ignorant about
retainer profiling and such stuff.
___
Derek Elkins wrote:
The answer to your questions are on the back of this T-shirt.
http://www.cafepress.com/skicalc.6225368
Oh... dear God. o_O
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John Meacham wrote:
actually, the rules are pretty straightforward. It doesn't matter where
something is bound, just _how_ it is bound. Let-bound names (which
includes 'where' and top-level definitions) can be polymorphic.
lambda-bound or case-bound names (names bound as an argument to a
If you don't find anything more specific, I suggest taking a look at
Data.Binary and reading a simple format like BMP or so. (You can
convert your file to BMP with an external program before.)
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On Fri, Jul 17, 2009 at 06:31:20PM +0200, Cetin Sert wrote:
How can I open and read colors of specific pixels of an image file in
Haskell? Which packages, functions do you recommend?
You could try DevIL, SDL-image or Gtk, I guess.
--
Felipe.
___
Hi,
Can someone please send me a working example based on the contents posted in
the URL below?
http://yi-editor.blogspot.com/2008/12/prototypes-encoding-oo-style.html
Thanks,
Kashyap
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Kashyap,
Can someone please send me a working example based on the contents
posted in the URL below?
There's a small typo in the post: the definition of Proto should read
data Proto = Proto {a :: A, b :: B, c :: C}
The rest seems fine to me. (See below for an excerpt).
Cheers,
Stefan
Andrew,
That seems simple enough (although problematic to implement).
However, the Report seems to say that it matters whether or not the
bindings are muturally recursive [but I'm not sure precisely *how*
it matters...]
It means that functions can only be used monomorphically within
Wouldn't it be great if pattern variables could be used more than once
in a pattern? Like so:
foo [x,x,_,x] = The values are the same!
foo _ = They're not the same!
where this could be rewritten to:
foo [x,y,_,z] | x == y x == z = The values are the same!
foo _ =
Christopher,
Wouldn't it be great if pattern variables could be used more than once
in a pattern? Like so:
foo [x,x,_,x] = The values are the same!
foo _ = They're not the same!
These are called nonlinear patterns. I think Miranda (a precursor of
Haskell, sort of) used to have
Hi all,
I apologize that I didn't react to your posts, I was on a vacation. (BTW, if
you ever come to Slovakia, I strongly recommend visiting Mala (Lesser) Fatra
mountains. IMHO it's more beautiful than more-known Tatra mountains.)
Thanks for your interest and many intriguing ideas. Especially,
Why do Haskell programmers (and libraries) name their function like @ or
###?Why not use a more descriptive label for functions?
Daryoush
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On Jul 17, 2009, at 15:06 , Daryoush Mehrtash wrote:
Why do Haskell programmers (and libraries) name their function like
@ or ###?Why not use a more descriptive label for functions?
Because symbols can be used as infix functions directly, whereas
alphanumerics have to be wrapped in ``
Hi, I have probably a very simple question, but I wasn't able to figure it out
myself.
Consider a two-parameter data type:
data X a b = X a b
If I want to make it a functor in the last type variable (b), I can just define
instance Functor (X a) where
fmap f (X a b) = X a (f b)
But how do
dmehrtash:
Why do Haskell programmers (and libraries) name their function like @ or
#
##?Why not use a more descriptive label for functions?
Where are those functions defined??
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Petr,
If I want to make it a functor in the last type variable (b), I can
just define
instance Functor (X a) where
fmap f (X a b) = X a (f b)
But how do I write it if I want X to be a functor in its first type
variable?
Short answer: you can't. Easiest way to workaround is to define
System.Console.Curses? Sorry couldn't resist ...
On Fri, Jul 17, 2009 at 12:18 PM, Don Stewartd...@galois.com wrote:
dmehrtash:
Why do Haskell programmers (and libraries) name their function like @ or
#
##? Why not use a more descriptive label for functions?
Where are those functions
Petr,
Short answer: you can't. Easiest way to workaround is to define a
newtype wrapper around your original datatype:
newtype X' b a = X' {unX' :: X a b}
instance Functor (X' b) where
fmap g (X' (X a b)) = X' (X b (g a))
Err
fmap g (X' (X a b)) = X' (X (g a) b)
Cheers,
Stefan
data X a b = X a b instance Functor (X a) where
fmap f (X a b) = X a (f b)
Yeah, that works just fine.
On Fri, Jul 17, 2009 at 1:14 PM, Petr Pudlak d...@pudlak.name wrote:
Hi, I have probably a very simple question, but I wasn't able to figure it
out
myself.
Consider a two-parameter
Am Freitag, 17. Juli 2009 20:38 schrieb Stefan Holdermans:
Christopher,
Wouldn't it be great if pattern variables could be used more than once
in a pattern? Like so:
foo [x,x,_,x] = The values are the same!
foo _ = They're not the same!
These are called nonlinear
Am Freitag, 17. Juli 2009 21:06 schrieb Daryoush Mehrtash:
Why do Haskell programmers (and libraries) name their function like @
or ###?Why not use a more descriptive label for functions?
It’s for the same reason that mathematicians say 2 + 3 instead of plus(2,3):
it’s more readable at
In the last two days I was invigilator at the International Mathematic
Olympics 2009 in Bremen, Germany. There we got a problem different from
the official math problems. :-) Eventually I solved it using Haskell. Read
the detailed description at
Am Montag, 16. Februar 2009 15:27 schrieben Sie:
On Mon, 16 Feb 2009, Wolfgang Jeltsch wrote:
[redirecting to haskell-cafe]
Am Sonntag, 15. Februar 2009 00:25 schrieben Sie:
Hi Wolfgang,
I was wondering if I can use FLTK as GUI backend for Grapefruit?
This should be possible in
Hello Jeff,
it’s been some time that we had the conversation below and I have to tell you
the same thing I told Jamie in a haskell-cafe mail sent a few minutes ago:
The student who wrote the Qt binding generator never managed to send me a
final version of his code. At least, I was able to make
Leon Smith schrieb:
Two new packages have been uploaded to Hackage, one that implements bag
(multiset) and set operations on ordered lists, and another that offers
three different number theoretic sieves.
http://hackage.haskell.org/package/data-ordlist
Data.OrdList offers many of the
Wolfgang Jeltsch schrieb:
Am Mittwoch, 15. Juli 2009 05:27 schrieben Sie:
On Jul 10, 2009, at 8:44 PM, Wolfgang Jeltsch wrote:
Why do we use English for identifiers? Because English is the language of
computer science. What English should we use? It’s tempting to say, we
should use the
On Thu, 16 Jul 2009, Fernan Bolando wrote:
Hi all
I recently used 2 hours of work looking for a bug that was causing
Program error: Prelude.!!: index too large
A good way to avoid such problems is to avoid partial functions at all.
(!!) is also inefficient. Is it possible to define the
The code below is, I think, n log n, a few seconds on a million + element list.
I wonder if it's possible to get this down to O(N) by using a
hashtable implemementation, or other better data structure.
Further, is there a hashtable implementation for haskell that doesn't
live in IO? Maybe in ST
Felipe Lessa schrieb:
On Fri, Jul 17, 2009 at 06:31:20PM +0200, Cetin Sert wrote:
How can I open and read colors of specific pixels of an image file in
Haskell? Which packages, functions do you recommend?
You could try DevIL, SDL-image or Gtk, I guess.
Perhaps
Friends
Is closing a class this easy?
--
module Moo
( Public(..)
) where
class Private x = Public x where
blah :: ...
class Private x where
instance Private A where
instance Public A where
blah = ...
instance Private B where
instance Public B
When we printf doubles from C (like when using hsc2hs to bind to a
constant) we can get something that's not valid Haskell. See these
2 examples:
3.40282347e+38F
inf
Do you know some way to printf a double using printf (or any other
standard function) that's always going to give me valid
Record punning is not all that useful with qualified module names. If
I write '(M.Record { M.rec_x })' it says Qualified variable in
pattern and if I write '(M.Record { rec_x })' it says 'Not in scope:
`rec_x''. Could it be this extension be further extended slightly so
that 'f (M.Record {
You probably want something like printf(%.10Lg,d);. Here's a shot C
example and its output:
#include stdio.h
int main(int argc, char * argv[])
{
long double d = 0.123456789;
printf(%.30Lf\n, d);
printf(%.20Lg\n, d);
printf(%.20Le\n, d);
}
/*
0.1234567887336054491370
I've used a similar approach for a while, for instance in
http://comonad.com/haskell/type-int/src/Data/Type/Boolean.hs
http://comonad.com/haskell/type-int/src/Data/Type/Boolean.hs
But I think your approach is cleaner than mine, because it doesn't need my
seemingly superfluous closure term or
There is a Zip class in category-extras 's Control.Functor.Zip on hackage
that covers this use-case.
http://hackage.haskell.org/packages/archive/category-extras/latest/doc/html/Control-Functor-Zip.html
It can basically be viewed as the ap of an Applicative functor chosen to be
the left inverse
Can anyone explain the theoretical reason for this limitation, ie other than
it is a syntactical restriction, what would it take to lift this restriction
?
- Original Message -
From: Stefan Holdermans ste...@cs.uu.nl
To: Petr Pudlak d...@pudlak.name
Cc: haskell-cafe@haskell.org
Haskell hash tables are a notorious performance pig, mostly due to the fact
that when we deal with big arrays, if the mutable array changes at all the
garbage collector will have to retraverse the entire thing during the next
collection. Guess the most common scenario for imperative hash tables
That helps, but:
#include stdio.h
#include math.h
int main ()
{
long double d = HUGE_VAL;
printf(%.30Lf\n, d);
}
still prints just (as it should, I think):
inf
Is there maybe some way to check if a double or
long double do have a proper value?
You probably want something like
2009/7/18 Edward Kmett ekm...@gmail.com:
I wrote a short blog post on this:
http://comonad.com/reader/2008/zipping-and-unzipping-functors/
and one on the less powerful dual operations (less powerful because while
every Haskell Functor is strong, much fewer are costrong):
On Jul 17, 2009, at 22:27 , Maurí cio wrote:
Is there maybe some way to check if a double or
long double do have a proper value?
isNaN :: a - Bool
True if the argument is an IEEE not-a-number (NaN) value
isInfinite :: a - Bool
True if the argument is an IEEE infinity or negative infinity
[Ack, missed the reply-all button...]
This was part of my motivation behind the 'removing polymorphism from Functor'
thing... to select a different parameter we'd essentially need type-level
lambdas... I'll use 'Λ' (capital lambda) for it:
instance Functor (Λ a. X a b) where
fmap f (X a
On Jul 18, 2009, at 2:27 AM, Wolfgang Jeltsch wrote:
Probably just because British English took it from American English.
It’s
similar to the “German” word “Computer”. It’s not native.
The spelling program goes back to 1633 at least;
it cannot then have referred to computers, and is
not
On Jul 18, 2009, at 2:35 AM, Wolfgang Jeltsch wrote:
So I should upload a package with German identifiers to Hackage?
Sure, why not? The fact that I can't read it is my loss,
not your fault, and there will be plenty of other German-
reading Haskellers to benefit from it. I've happily worked
On Jul 18, 2009, at 6:35 AM, Christopher Done wrote:
[non-linear patterns]
This kind of matching is provided in Prolog and Erlang.
Neither of them lets the user define equality.
We find the same issue with
n+k patterns (e.g., n+1 as a pattern)
l++r patterns
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