Hi,
In the code here -
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28393#a28393
If I look at the type of modifiedImage, its simply ByteString - but isn't it
actually getting into and back out of the state monad? I am of the
understanding that once you into a monad, you cant get out of it? Is
You cannot break out of a monad if all you have available to use are
the monad typeclass functions, however there is nothing preventing an
instance from being created that allows escape. Many of these escape
methods come in the form of runX functions, but you can use
constructors to break out with
On Thu, Jul 29, 2010 at 11:48 PM, Lyndon Maydwell maydw...@gmail.comwrote:
You cannot break out of a monad if all you have available to use are
the monad typeclass functions, however there is nothing preventing an
instance from being created that allows escape. Many of these escape
methods
I'm trying to install SDL through Cabal -- I don't know another way to
install it. However, I'm getting this:
setup.exe: The package has a './configure' script. This requires a Unix
compatibility toolchain such as MinGW+MSYS or Cygwin.
cabal: Error: some packages failed to install:
SDL-0.5.10
Jason,
There is one case where you can break out of a monad without knowing which
monad it is. Well, kind of. It's cheating in a way because it does force
the use of the Identity monad. Even if it's cheating, it's still very clever
and interesting.
How is this cheating? Or better, how
a few weeks ago a question was posed on this list for examples how to
use couchDB. i post the 'hello world' for couchDB using the haskell
interface - perhaps it helps other to start!
a more extensive and complicate example is found at
www.maztravel.com/haskell/mySqlToCouchDB.html
I created with
C K Kashyap wrote:
In the code here -
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28393#a28393
If I look at the type of modifiedImage, its simply ByteString - but
isn't it actually getting into and back out of the state monad? I am of
the understanding that once you into a monad, you cant
The original poster states that the type of modifiedImage is simply
ByteString but given that it calls execState, is that possible?
Would it not be State ByteString?
Kevin
On Jul 30, 9:49 am, Anton van Straaten an...@appsolutions.com wrote:
C K Kashyap wrote:
In the code here -
Oops, I should have written
IO ByteString
as the State stuff is only *inside* execState.
But a monad none the less?
Kevin
On Jul 30, 9:59 am, Kevin Jardine kevinjard...@gmail.com wrote:
The original poster states that the type of modifiedImage is simply
ByteString but given that it calls
Or is it possible to call a function in a monad and return a pure
result? I think that is what the original poster was asking?
I know that unsafePerformIO can do this, but I thought that was a bit
of a hack.
I'm still trying to understand how monads interact with types so I am
interested in this
On Jul 30, 9:59 am, Kevin Jardine kevinjard...@gmail.com wrote:
The original poster states that the type of modifiedImage is simply
ByteString but given that it calls execState, is that possible?
Would it not be State ByteString?
Oops, I should have written
IO ByteString
as the State
I think that these are therefore the responses to the original
questions:
I am of the understanding that once you into a monad, you cant get out of it?
You can run monadic functions and get pure results. So it looks like
in that sense you can get out of it.
Is this breaking the monad scheme?
Kevin Jardine wrote:
I think that these are therefore the responses to the original
questions:
I am of the understanding that once you into a monad, you cant get out of it?
You can run monadic functions and get pure results.
Some clarifications:
First, many monads (including State) are
I think that we are having a terminology confusion here. For me, a
pure function is one that does not operate inside a monad. Eg. ++,
map, etc.
It was at one point my belief that although code in monads could call
pure functions, code in pure functions could not call functions that
operated
I don't understand why to call impure to the types instances of a class.
Monad is simply a class with their methods. Even the pure list is a monad.
The only difference between Monad and other classes is do notation, and only
affects notation.
The impure side is a type, not a class: IO.
Kevin Jardine kevinjard...@gmail.com writes:
I think that we are having a terminology confusion here. For me, a
pure function is one that does not operate inside a monad. Eg. ++,
map, etc.
No, a pure function is one without any side effects.
It was at one point my belief that although code
C K Kashyap wrote:
I am of the
understanding that once you into a monad, you cant get out of it?
That's not correct.
There are many monads, including Maybe, [], IO, ... All of these monads
provide operations (=), return and fail, and do notation implemented
in terms of these functions, as
Kevin Jardine wrote:
I think that we are having a terminology confusion here. For me, a
pure function is one that does not operate inside a monad. Eg. ++,
map, etc.
Ivan Miljenovic has already given a good response, to which I'll only
add this:
I suspect that your idea of the meaning of
So far the comments here only increase my confusion (which as I say,
is not bad because it means that I am learning something!).
As a Haskell newbie, the first thing I learned about monads is that
they have a type signature that creates a kind of mud you can't wash
off.
eg.
f :: String -
Kevin == Kevin Jardine kevinjard...@gmail.com writes:
Kevin The more I learn about monads, however, the less I understand
Kevin them. I've seen plenty of comments suggesting that monads
Kevin are easy to understand, but for me they are not.
I used to have the same problem.
Then I
Anton van Straaten an...@appsolutions.com writes:
Ivan Miljenovic has already given a good response
Why thank you, kind sir!
/me bows
I suspect that your idea of the meaning of purity came from
over-generalization from the IO monad. IO actions may be impure, but
that's not true of all
Kevin Jardine kevinjard...@gmail.com writes:
The more I learn about monads, however, the less I understand them.
I've seen plenty of comments suggesting that monads are easy to
understand, but for me they are not.
How did you learn monads?
More and more people seem to be getting away from
Hi Eitan
You will probably need to do a manual install.
If you download and extract the SDL binding there is a file - WIN32 -
in the top level directory describing how to install on Windows.
Unfortunately this file might now be out-of-date. I haven't used the
Haskell SDL binding myself since
On Fri, Jul 30, 2010 at 03:46:09AM -0700, Kevin Jardine wrote:
When I plunged into Haskell earlier this year, I had no problem with
understanding static typing, higher level functions and even
separating pure functions from IO functions.
The more I learn about monads, however, the less I
On Jul 30, 1:11 pm, Brent Yorgey byor...@seas.upenn.edu wrote:
Monads are hard to understand. But they are *worth understanding*.
That's the most inspiring and encouraging statement I've seen all
week.
Thanks!
Kevin
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On 7/30/10 9:29, Stefan Holdermans wrote:
Jason,
There is one case where you can break out of a monad without knowing which
monad it is. Well, kind of. It's cheating in a way because it does force the
use of the Identity monad. Even if it's cheating, it's still very clever and
On 7/30/10 12:29, Tillmann Rendel wrote:
C K Kashyap wrote:
I am of the
understanding that once you into a monad, you cant get out of it?
That's not correct.
There are many monads, including Maybe, [], IO, ... All of these monads
provide operations (=), return and fail, and do notation
Martijn van Steenbergen mart...@van.steenbergen.nl writes:
On 7/30/10 12:29, Tillmann Rendel wrote:
C K Kashyap wrote:
I am of the
understanding that once you into a monad, you cant get out of it?
That's not correct.
There are many monads, including Maybe, [], IO, ... All of these monads
Hi,
I wrote:
There is nothing special about monads!
Kevin Jardine wrote:
I've seen plenty of comments suggesting that monads are easy to
understand, but for me they are not.
My point was that monads are not a language feature whith special
treatment of the compiler, but more like a design
Kevin Jardine wrote:
As a Haskell newbie, the first thing I learned about monads is that
they have a type signature that creates a kind of mud you can't wash
off.
There are places where you can't wash it off, and places where you can.
eg.
f :: String - MyMonad String
By mentioning the
Martijn,
In fact, I would argue that a monad which you cannot escape from is not very
useful at all. IO is the only exception I know of.
And that's only because, at least the runtime system allows for execution of a
computation inside the IO monad at top-level.
Cheers,
Stefan
You might try emailing the maintainer directly. The package appears to be
actively maintained.
- Job
On Fri, Jul 30, 2010 at 3:17 AM, Eitan Goldshtrom thesource...@gmail.comwrote:
I'm trying to install SDL through Cabal -- I don't know another way to
install it. However, I'm getting this:
I think you can just install the windows gtk dev libraries from here:
http://www.gtk.org/download.html
- Job
On Thu, Jul 29, 2010 at 5:20 PM, Alessandro Stamatto astama...@gmail.comwrote:
Installing Yi Editor i get the following error:
--
-BEGIN PGP SIGNED MESSAGE-
Hash: SHA1
On 7/30/10 03:31 , Andrew U. Frank wrote:
ps: if there is a better place to document examples? a wiki on
haskell.org would be nice and should be available for any project in
hackage.
haskell.org has (more precisely, *is*) a wiki; request an
Hello,
it's a bit hidden in Haskell, but a monad instance consists of three
functions:
fmap :: (a - b) - (m a - m b)
return :: a - m a
join :: m (m a) - m a
Nothing more is needed to define a monad. In Haskell, a monad is
expressed by 'return' and (=) instead, but this is equivalent.
Ertugrul Soeylemez e...@ertes.de writes:
Hello,
it's a bit hidden in Haskell, but a monad instance consists of three
functions:
fmap :: (a - b) - (m a - m b)
You don't even need fmap defined for it to be a monad, since fmap f m =
liftM f m = m = (return . f)
--
Ivan Lazar Miljenovic
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Hash: SHA1
On 7/30/10 06:06 , Kevin Jardine wrote:
I think that we are having a terminology confusion here. For me, a
pure function is one that does not operate inside a monad. Eg. ++,
map, etc.
It was at one point my belief that although code in monads
Here is my understanding with respect to the question.
In the general case, you cannot come out of a monad, because the monad
typeclass does not include any functions without of the form (m a - a).
Also, as a category theoretic construct, a monad does not have to have an
exit function. (caveat:
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On 7/30/10 11:48 , Ivan Lazar Miljenovic wrote:
Ertugrul Soeylemez e...@ertes.de writes:
it's a bit hidden in Haskell, but a monad instance consists of three
functions:
fmap :: (a - b) - (m a - m b)
You don't even need fmap defined for it
Vo Minh Thu schrieb:
There other possibilities to deal with raster graphics:
- Use gtk; i.e. something like
http://hackage.haskell.org/package/AC-EasyRaster-GTK
- Output the data in some image format (if you want to do it yourself,
the most simple is PPM)
- Use X11 directly (if you're on
On Jul 30, 11:17 am, Anton van Straaten wrote:
Prelude :m Control.Monad.State
Prelude Control.Monad.State let addToState :: Int - State Int ();
addToState x = do s - get; put (s+x)
Prelude Control.Monad.State let mAdd4 = addToState 4
Prelude Control.Monad.State :t mAdd4
m :: State Int ()
On Fri, Jul 30, 2010 at 12:29 AM, Stefan Holdermans
ste...@vectorfabrics.com wrote:
Jason,
There is one case where you can break out of a monad without knowing
which monad it is. Well, kind of. It's cheating in a way because it does
force the use of the Identity monad. Even if it's
On Fri, 30 Jul 2010, Eitan Goldshtrom wrote:
HGL actually looks like EXACTLY what I need. I only need to set pixels, which
looks like
just what HGL would be good at. Only problem is that I can't find a single
tutorial for
HGL. Does anyone know or any, or where I could find one?
I found the
HGL actually looks like EXACTLY what I need. I only need to set pixels,
which looks like just what HGL would be good at. Only problem is that I
can't find a single tutorial for HGL. Does anyone know or any, or where
I could find one?
-Eitan
On 7/30/2010 12:22 PM, Henning Thielemann wrote:
From: Data.Maybe
Description
The Maybe type, and associated operations.
From: Data.List
Description
Operations on lists.
One description has the type and associated operations, the other only has the
operations.
Where can I find the type definition for List, and why isn't it in Data.List?
Excerpts from michael rice's message of Fri Jul 30 14:41:44 -0400 2010:
One description has the type and associated operations, the other only has
the operations.
Where can I find the type definition for List, and why isn't it in Data.List?
Hello Michael,
This is because the List datatype
List are actually built in to the language, but they are roughly
equivalent to the following definition:
data List a =
[]
| a:List a
The reason why this definition never actually appears is because it
defines the constructors using operators rather than names, which is not
allowed in
Excerpts from Edward Z. Yang's message of Fri Jul 30 14:48:34 -0400 2010:
Const x xs is x:xs (constructor)
That should be a Cons, not Const. :o)
Cheers,
Edward
___
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On Fri, Jul 30, 2010 at 7:50 PM, Gregory Crosswhite
gcr...@phys.washington.edu wrote:
The reason why this definition never actually appears is because it defines
the constructors using operators rather than names, which is not allowed in
vanilla Haskell. (There is an extension,
Thanks all,
Now that I have a (very) rudimentary understanding of Haskell, I figured I'd
back up and have a closer (conceptual) look at type definitions to see what
they have in common, and just happen to pick Maybe and List.
I also noticed Maybe has a list of Instances
Monad Maybe
Functor
Hello,
I'd like to download 1,000 web pages with up to 6 six concurrent
downloads at a time.
How can I express such a thread limit within the orc EDSL?
Günther
___
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On Fri, Jul 30, 2010 at 8:54 PM, michael rice nowg...@yahoo.com wrote:
Thanks all,
Now that I have a (very) rudimentary understanding of Haskell, I figured I'd
back up and have a closer (conceptual) look at type definitions to see what
they have in common, and just happen to pick Maybe and
On Friday 30 July 2010 21:54:20, michael rice wrote:
Thanks all,
Now that I have a (very) rudimentary understanding of Haskell, I figured
I'd back up and have a closer (conceptual) look at type definitions to
see what they have in common, and just happen to pick Maybe and List.
I also
Hi,
I am having trouble getting a small program to compile. The helpful folks at
#haskell created a version of the program that does compile -
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28406#a28408 but it is not
very clear to them (and to me) why the original program wouldn't type
compile in
Hey everyone,
I am pleased to announce the release of the type-level-natural-number
package, which implements natural numbers at the type level. Although
there are many packages on Hackage that implement type level natural
numbers, this package is distinguished by its simplicity: it does not
Hi Daniel,
OK, now I'm getting somewhere. I was looking here:
http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Data-Maybe.html
instead of in the source.
Seem to be on the right track. I'm going to do some more poking around.
Thanks a lot, guys.
Michael
--- On Fri, 7/30/10,
I was one of the people on #haskell discussing this with Anupam.
Note that that when you remove the signature of d, the result complies and
ghci will state the inferred type of d is exactly the signature that you
are not allowed to write.
In my opinion, this is a bug in the Haskell 98 report
On Fri, 30 Jul 2010 13:28:19 -0700
Gregory Crosswhite gcr...@phys.washington.edu wrote:
Hey everyone,
I am pleased to announce the release of the type-level-natural-number
package, which implements natural numbers at the type level. Although
there are many packages on Hackage that
On Fri, 30 Jul 2010 09:29:59 +0200
Stefan Holdermans ste...@vectorfabrics.com wrote:
Jason,
There is one case where you can break out of a monad without knowing which
monad it is. Well, kind of. It's cheating in a way because it does force
the use of the Identity monad. Even if it's
Excerpts from Günther Schmidt's message of Fri Jul 30 16:16:38 -0400 2010:
I'd like to download 1,000 web pages with up to 6 six concurrent
downloads at a time.
How can I express such a thread limit within the orc EDSL?
One solution that comes to mind is place all 1000 web pages in an MVar
Hello,
In the latest ACM CACM is a paper on Singularity. Here also is an
overview: http://lambda-the-ultimate.org/node/1081. I haven't finished the
CACM paper yet but I only mention of languages like C# and F#. Singularity
is predicated around providing a safe
environment. IMO Haskell is even
On Sat, Jul 31, 2010 at 01:32:50AM +0400, Alexey Khudyakov wrote:
The envisioned use case is for annotating types with natural numbers, so
that extra functionality such as addition and subtraction of natural
numbers is unnecessary. Nothing is stopping someone from implementing
this
On Sat, Jul 31, 2010 at 01:49:43AM +0400, Alexey Khudyakov wrote:
No I think here we breaking out from _arbitrary_ monad. If monadic
function works for every monad then it must work for identity monad
too. Here is simplest form of purify function:
purify2 :: (forall m . Monad m = m a) - a
On Fri, Jul 30, 2010 at 11:57:00AM -0500, Jason Catena wrote:
By this example State doesn't seem to give you anything more than a
closure would, since it doesn't act like much of an accumulator (by,
for example, storing 6 as its new internal value).
Could you use State for something like
Yeah I recently ran into this myself. (
http://osdir.com/ml/haskell-cafe@haskell.org/2010-07/msg00020.html). It's
not a bug, just a limitation of haskell's inference algorithm for mutually
recursive groups of functions.
The problem is that haskell infers groups (the strongly connected
components)
Heh. I was just thinking I needed type level naturals last night at the
pub. I wanted to support gcc's vector type extension in jhc
http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html
which allow diretly expressing vector operations that use the SIMD
features of modern CPUS, I didn't want
Tillmann Rendel wrote:
C K Kashyap wrote:
I am of the
understanding that once you into a monad, you cant get out of it?
That's not correct.
Indeed. The correct formulation of the statement is that it's not safe
to leave a monad. Where safe has the same connotation as in all the
unsafeFoo
Ivan Lazar Miljenovic wrote:
More and more people seem to be getting away from trying to say that
monads are containers/burritos/etc. and just teaching them by way of the
definition, either = and return or just join
You always need return. The choice of primitives is:
return, (=)
or:
dear traversable geniuses --
i am looking for better implementations of
unzipMap :: M.Map a (b, c) - (M.Map a b, M.Map a c)
unzipMap m = (M.map fst m, M.map snd m)
unliftMap :: (Ord a) = M.Map a (b - c) - M.Map a b - M.Map a c
unliftMap mf ma = M.mapWithKey (\k v - mf M.! k $ v) ma
the first
Martijn van Steenbergen wrote:
In fact, I would argue that a monad which you cannot escape from is not
very useful at all. IO is the only exception I know of.
You can escape IO just fine. Just compile your program, and then run it
in the real life monad. Results aren't guaranteed to be the
On Fri, Jul 30, 2010 at 08:13:43PM -0700, Ben wrote:
unliftMap :: (Ord a) = M.Map a (b - c) - M.Map a b - M.Map a c
unliftMap mf ma = M.mapWithKey (\k v - mf M.! k $ v) ma
I always thought a useful map primitive would be
unionWithJoin
:: (a - b - c) -- combine values that appear in both
Alex Rozenshteyn wrote:
I also found myself thinking about list as a monad in terms of this
discussion. I think it's an interesting case: it's pure, but it doesn't
really make sense to come out of it. Head, indexing, and last all break
out of it, but none of them can be the default, and all
Brandon S Allbery KF8NH wrote:
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On 7/30/10 11:48 , Ivan Lazar Miljenovic wrote:
Ertugrul Soeylemez e...@ertes.de writes:
it's a bit hidden in Haskell, but a monad instance consists of three
functions:
fmap :: (a - b) - (m a - m b)
You don't
Jason Catena wrote:
By this example State doesn't seem to give you anything more than a
closure would, since it doesn't act like much of an accumulator (by,
for example, storing 6 as its new internal value).
Reader r = (r-)
Writer m = Monoid m = (m,)
State s = (s-) . (s,)
So, yes, the state
Ben Millwood wrote:
On Fri, Jul 30, 2010 at 7:50 PM, Gregory Crosswhite
gcr...@phys.washington.edu wrote:
The reason why this definition never actually appears is because it defines the
constructors using operators rather than names, which is not allowed in vanilla
Haskell. (There is an
Probably a more poignant question would be a comparison of Haskell's type
system and Sing#'s (http://en.wikipedia.org/wiki/Sing_sharp).
Vasili
On Fri, Jul 30, 2010 at 5:19 PM, Vasili I. Galchin vigalc...@gmail.comwrote:
Hello,
In the latest ACM CACM is a paper on Singularity. Here also
Ben wrote:
unliftMap :: (Ord a) = M.Map a (b - c) - M.Map a b - M.Map a c
unliftMap mf ma = M.mapWithKey (\k v - mf M.! k $ v) ma
the first is obviously inefficient as it traverses the map twice. the
second just seems like it is some kind of fmap.
It's the (*) operator of Applicative, which
Ben wrote:
dear traversable geniuses --
i am looking for better implementations of
unzipMap :: M.Map a (b, c) - (M.Map a b, M.Map a c)
unzipMap m = (M.map fst m, M.map snd m)
I don't think you can give a more efficient implementation using the
public interface of Data.Map. You need to have
Alexey,
There is one case where you can break out of a monad without knowing which
monad it is. Well, kind of. It's cheating in a way because it does force
the use of the Identity monad. Even if it's cheating, it's still very
clever and interesting.
How is this cheating? Or better, how
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