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Today's Topics:
1. find problem (Roelof Wobben)
2. Re: find problem (aditya siram)
3. Re: problem exercise 3 page 60 Programming in Haskell
(David Place)
4. Re: very impure [global] counter (Thomas)
5. Re: very impure [global] counter (Davi Santos)
6. a problem with maps (Dennis Raddle)
7. Re: a problem with maps (Dennis Raddle)
8. Re: a problem with maps (David Place)
----------------------------------------------------------------------
Message: 1
Date: Fri, 22 Jul 2011 19:14:53 +0000
From: Roelof Wobben <rwob...@hotmail.com>
Subject: [Haskell-beginners] find problem
To: <beginners@haskell.org>
Message-ID: <snt118-w38c0683e5bbc527cecaf61ae...@phx.gbl>
Content-Type: text/plain; charset="iso-8859-1"
Hello,
Sorry for asking so much but I really want to understand this.
I have the function find k t [v, (k',v') <- t,k = v]
Let's say k = false and t = [false, true, false, true]
Then It will be find "false" ["false", "true", "false", "true]
But where on earth are t,k = v and (k', v') come from ?
Roelof
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Message: 2
Date: Fri, 22 Jul 2011 15:06:06 -0500
From: aditya siram <aditya.si...@gmail.com>
Subject: Re: [Haskell-beginners] find problem
To: Roelof Wobben <rwob...@hotmail.com>
Cc: beginners@haskell.org
Message-ID:
<cajrreygyop73tjw3jru3pu8f3ax4yu4endv+ar1nzh6kc8w...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hi Roelof,
The function itself will not work. Do you have a working Haskell
environment? If so, please try the function out first and let us know
what errors you see. I have a feeling from this and previous questions
that you're trying to understand the code just by trying to reading
it. You really need to execute it. Since you're posting here I presume
you have access to a computer, or at least the Web. Have you used
http://tryhaskell.org/?
That said I'll try and guess what "find" is supposed to do. It seems
to take a key "k" and output the values associated with the key. It
also appears that "t" is a list of key-value pairs eg. [("a",
"aardvard"), ("a", "armadillo"), ("b", bull")]. From your definition
of "find" it is different from a standard lookup table in that it will
will return all values associated with a key instead of just the
first. Eg. find "a" [("a", "aardvark"), ("a", "armadillo"), ("b",
bull")] == ["aardvark", "armadillo"].
This definition of "find":
find :: (Eq a) => a -> [(a,b)] -> [b]
find k t = [v' | (k', v') <- t , k == k']
seems to be what you want.
Firstly you're chunking the list comprehension incorrectly, the above
is the same as:
find k t = [v' | (k',v') <- t,
k == k']
The first part of the comprehension "(k',v') <- t" gets a tuple out of
the list and assigns first part to "k'" and the second to "v'". The
second part "k == k'" places a constraint on the list so that only
tuples whose first element is equal to the key passed in "k" are
allowed. The result of the comprehension, the v' before the "|"
character indicates that the second element of the tuple should be
output.
Taking the above definition try to trace by hand the execution of
"find". It'll give you a better idea of how result is generated.
-deech
On Fri, Jul 22, 2011 at 2:14 PM, Roelof Wobben <rwob...@hotmail.com> wrote:
> Hello,
>
> Sorry for asking so much but I really want to understand this.
>
> I have the function find? k t [v, (k',v') <- t,k = v]
>
> Let's say k = false and t = [false, true, false, true]
>
> Then It will be? find? "false" ["false", "true", "false", "true]
> But where on earth are t,k = v and (k', v') come from ?
>
> Roelof
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>
------------------------------
Message: 3
Date: Fri, 22 Jul 2011 16:11:59 -0400
From: David Place <d...@vidplace.com>
Subject: Re: [Haskell-beginners] problem exercise 3 page 60
Programming in Haskell
To: Roelof Wobben <rwob...@hotmail.com>
Cc: beginners@haskell.org
Message-ID: <3363e864-d1a5-4267-81a2-d3f287350...@vidplace.com>
Content-Type: text/plain; charset="iso-8859-1"
On Jul 22, 2011, at 2:29 PM, Roelof Wobben wrote:
> But the exercise says you have to use one generator and I see still two.
You misunderstand the exercise. It says to restate the list comprehension as
two comprehensions each with a single generator.
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Message: 4
Date: Sat, 23 Jul 2011 01:50:43 +0200
From: Thomas <hask...@phirho.com>
Subject: Re: [Haskell-beginners] very impure [global] counter
To: beginners@haskell.org
Message-ID: <4e2a0cd3.90...@phirho.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hello David,
since your first approach was via the filesystem I assume you only need
to call a more or less limited number of methods in Java. If this is the
case then I would try to embed Java into C and call C from Haskell via
the FFI.
I have done both individually, it's not hard and it worked pretty well.
I'm not sure if there are any lurking problems when "double embedding".
But even if that should fail: before using the filesystem as
communication mechanism I'd probably use some sort of IPC (probably
network).
If you really want a virtual filesystem for the communication, you can
always set one up in your host OS:
http://www.vanemery.com/Linux/Ramdisk/ramdisk.html
Or search the web for "ramdisk <os of choice>" if you're not using Linux.
I don't think Haskell provides this functionality as a library. I may be
wrong, though.
HTH,
Thomas
On 22.07.2011 19:23, Davi Santos wrote:
> Aditya,
> as I could search, If there is a C version of Weka, it appears to be very
> outdated by now.
> The library I use is http://www.cs.waikato.ac.nz/ml/weka/.
>
> Davi
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 5
Date: Fri, 22 Jul 2011 21:42:58 -0300
From: Davi Santos <dps....@gmail.com>
Subject: Re: [Haskell-beginners] very impure [global] counter
To: Thomas <hask...@phirho.com>
Cc: beginners@haskell.org
Message-ID:
<CANWsST_i2JyLs5Bq=bo9kfjxpupmr1pov90boz3n0eo709a...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Thomas,
the Weka Java library has a myriad of methods I want to call.
To be clear, actually, it comes in three flavours: library, GUI and program.
Unfortunately, as a library, it is unfeasible to map all of them to C or to
a message passing system.
When used as a program (such as I've done in Haskell),
it arranges the methods in a way I can do almost everything just via command
line options.
The downside is that data can be exchanged only by files.
So, I manipulate data in Haskell and send it ready to a file every time I
need Weka to be called. Thousand times, BTW.
Anyway, the perfect and impossible world would be to instantiate java
classes directly in Haskell and use all Weka features.
In time: when I run the program, I create a ramdisk this way:
sudo mount -t tmpfs -o size=1024M tmpfs /tmp/ram/
it works, but makes the program to depend on external settings.
May be I should do that via a Haskell system call also.
I was hoping somebody would point out a secret Jaskell-like solution. :)
Ah, to make myself even clearer, all of this is to avoid programming in
Java, or better, to avoid hunting bugs all day in my own code.
Davi[d]
[the names are the same by here, no problem T. Holubar!]
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Message: 6
Date: Fri, 22 Jul 2011 18:40:20 -0700
From: Dennis Raddle <dennis.rad...@gmail.com>
Subject: [Haskell-beginners] a problem with maps
To: Beginners@haskell.org
Message-ID:
<cakxlvopl3xjbv_nps2m8nh-3nuhpojanxxkxi2u1uysycoz...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
In my application I find an issue coming up frequently. I have a map
Ord k => Map k [a]
That is, a list of things may be stored at each key. I sometimes want to
convert it to
[(k,a)]
where each value is paired with its key, and some keys may repeat in that
form of the list. No particular order is needed.
so I have the following code. Would appreciate hearing if there are any
tricks I'm missing
import qualified Map as M
listOutMap :: Ord k => Map k [a] -> [(k,a)]
listOutMap m = concatMap (\(k,a) -> zip (repeat k) a) (M.toList m)
mapOutList :: Ord k => [(k,a)] -> Mpa k [a]
mapOutList list = M.fromList $ map (second (: [])) list
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Message: 7
Date: Fri, 22 Jul 2011 18:41:57 -0700
From: Dennis Raddle <dennis.rad...@gmail.com>
Subject: Re: [Haskell-beginners] a problem with maps
To: Beginners@haskell.org
Message-ID:
<cakxlvoqqwhonsy6wv1o+ezjvsn2bta7wxbwdturqhucqcw5...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Oops that was supposed to be
mapOutList list = M.fromListWith (++) $ map (second (: [])) list
On Fri, Jul 22, 2011 at 6:40 PM, Dennis Raddle <dennis.rad...@gmail.com>wrote:
> In my application I find an issue coming up frequently. I have a map
>
> Ord k => Map k [a]
>
> That is, a list of things may be stored at each key. I sometimes want to
> convert it to
>
> [(k,a)]
>
> where each value is paired with its key, and some keys may repeat in that
> form of the list. No particular order is needed.
>
> so I have the following code. Would appreciate hearing if there are any
> tricks I'm missing
>
> import qualified Map as M
>
> listOutMap :: Ord k => Map k [a] -> [(k,a)]
> listOutMap m = concatMap (\(k,a) -> zip (repeat k) a) (M.toList m)
>
> mapOutList :: Ord k => [(k,a)] -> Mpa k [a]
> mapOutList list = M.fromList $ map (second (: [])) list
>
>
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Message: 8
Date: Fri, 22 Jul 2011 21:54:10 -0400
From: David Place <d...@vidplace.com>
Subject: Re: [Haskell-beginners] a problem with maps
To: Dennis Raddle <dennis.rad...@gmail.com>
Cc: Beginners@haskell.org
Message-ID: <7548a985-5449-44bc-b6a3-b850c4bc7...@vidplace.com>
Content-Type: text/plain; charset="iso-8859-1"
On Jul 22, 2011, at 9:41 PM, Dennis Raddle wrote:
> mapOutList list = M.fromListWith (++) $ map (second (: [])) list
>
Is `second` the function from Control.Arrow?
____________________
David Place
Owner, Panpipes Ho! LLC
http://panpipesho.com
d...@vidplace.com
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