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Today's Topics:
1. Re: SOLVED - Stack overflow, but hard to understand
(Magnus Therning)
2. Re: SOLVED - Stack overflow, but hard to understand
(Michael Mossey)
3. Haskell Output Help (Chandni Navani)
4. Re: Haskell Output Help (aditya siram)
5. Re: Haskell Output Help (aditya siram)
6. how to set command args in ghci (Kui Ma)
7. Re: how to set command args in ghci (Yusaku Hashimoto)
8. Re: how to set command args in ghci (Yusaku Hashimoto)
----------------------------------------------------------------------
Message: 1
Date: Wed, 21 Oct 2009 16:49:23 +0100
From: Magnus Therning <[email protected]>
Subject: Re: [Haskell-beginners] SOLVED - Stack overflow, but hard to
understand
To: Michael Mossey <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
On Wed, Oct 21, 2009 at 9:44 AM, Michael Mossey <[email protected]> wrote:
>
>
> Magnus Therning wrote:
>>
>> On 20/10/09 20:32, Michael Mossey wrote:
>>>
>>> Okay, I figured this out. mapM is not lazy, at least not for a monad that
>>> has state and under the circumstance you demand the state out the other
>>> side.
>>
>> If I understand what you are saying then this behaviour isn't unexpected.
>> Â If
>> you have a monad with state, and you ask for the final state, then it's
>> likely
>> that everything happening in that particular monad will has to be
>> evaluated.
>
> Hi Magnus,
>
> Yes, that's what I was trying to imply. I realized it is mathematically
> impossible for mapM to be lazy for a monad with state.
>
> mapM doesn't seem to be lazy anywhere. For example, this program
>
> main = do
> Â buffers <- forM ["file1.txt","file2.txt","file3.txt"] readFile
> Â print . take 1 . concat $ buffers
>
> will, according to my experiments with the profiler, read all three files.
> It's possible to imagine lazy behavior here, but no doubt there is a
> technical reason against it.
Laziness in combination with IO is a difficult thing. Just search the
archives to see the number of threads about it.
>>> I may rewrite the program. Or I may consider the ISS principle.
>>> ("Increase the stack, stupid.")
>>
>> You may also be able to improve the situation by adding a bit of
>> strictness.
>> In some cases the thunks resulting from laziness can take up a lot of
>> space.
>> Forcing evaluation, at well thought out locations in your program, may
>> both
>> speed things up and reduce memory/stack usage.
>
> You are probably right... I am having trouble spanning the gap between "my
> current understanding" and "well thought-out locations." Real World Haskell
> has a chapter on this, so I may look there.
>
> I was able to get the program to run by removing all places that I had
> created a chain of Rand computations that forced evaluation of the last one.
> I replaced these with computations that did not require evaluation of the
> last one.
>
> However, my program was still forced to read more files than it really
> needed in the end, as the above snippet demonstrates.
Just out of curiosity, did you verify that all three files were
completely *read*, as opposed to all three opened, but only the first
line of the first file being read?
/M
--
Magnus Therning (OpenPGP: 0xAB4DFBA4)
magnusï¼ therningï¼org Jabber: magnusï¼ therningï¼org
http://therning.org/magnus identi.ca|twitter: magthe
------------------------------
Message: 2
Date: Wed, 21 Oct 2009 12:46:55 -0700
From: Michael Mossey <[email protected]>
Subject: Re: [Haskell-beginners] SOLVED - Stack overflow, but hard to
understand
To: beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
Magnus Therning wrote:
>
> Just out of curiosity, did you verify that all three files were
> completely *read*, as opposed to all three opened, but only the first
> line of the first file being read?
>
> /M
>
Actually, no I didn't verify that, in this example. I was thinking about my
other program in which I used mapM over the characters of the file onto a
Rand monad. That one definitely read all the characters of the
file---certain processing routines were called about 30,000 times
(according to the profiler). This makes sense to me now, because I was
asking for the state out the other side of the mapM.
Thanks,
Mike
------------------------------
Message: 3
Date: Thu, 22 Oct 2009 12:11:07 -0700 (PDT)
From: Chandni Navani <[email protected]>
Subject: [Haskell-beginners] Haskell Output Help
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
I have a list of lists which all contain strings. [[String]]. I need to
figure out how to print them so that after each individual string, there is a
new line.
If this is the initial list [["abc", "cde"] ["fgh", "ghi"]]
[["abc"
"cde"]
["fgh",
"ghi"]]
Can anyone help me figure this out? Thanks.
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Message: 4
Date: Thu, 22 Oct 2009 21:53:57 -0500
From: aditya siram <[email protected]>
Subject: Re: [Haskell-beginners] Haskell Output Help
To: [email protected]
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Is this what you need?
print $ foldl (++) [] [["abc", "cde"], ["fgh", "ghi"]]
-deech
On Thu, Oct 22, 2009 at 2:11 PM, Chandni Navani <[email protected]>wrote:
> I have a list of lists which all contain strings. [[String]]. I need to
> figure out how to print them so that after each individual string, there is
> a new line.
>
> If this is the initial list [["abc", "cde"] ["fgh", "ghi"]]
> [["abc"
> "cde"]
> ["fgh",
> "ghi"]]
>
> Can anyone help me figure this out? Thanks.
>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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------------------------------
Message: 5
Date: Thu, 22 Oct 2009 21:59:22 -0500
From: aditya siram <[email protected]>
Subject: Re: [Haskell-beginners] Haskell Output Help
To: [email protected]
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Sorry, forgot about the newline. How's this?
mapM putStrLn $ concat [["abc", "cde"], ["fgh", "ghi"]]
-deech
On Thu, Oct 22, 2009 at 9:53 PM, aditya siram <[email protected]>wrote:
> Is this what you need?
>
> print $ foldl (++) [] [["abc", "cde"], ["fgh", "ghi"]]
>
> -deech
>
> On Thu, Oct 22, 2009 at 2:11 PM, Chandni Navani <[email protected]>wrote:
>
>> I have a list of lists which all contain strings. [[String]]. I need to
>> figure out how to print them so that after each individual string, there is
>> a o line.
>>
>> If this is the initial list [["abc", "cde"] ["fgh", "ghi"]]
>> [["abc"
>> "cde"]
>> ["fgh",
>> "ghi"]]
>>
>> Can anyone help me figure this out? Thanks.
>>
>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
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Message: 6
Date: Fri, 23 Oct 2009 17:59:51 +0800
From: Kui Ma <[email protected]>
Subject: [Haskell-beginners] how to set command args in ghci
To: <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="gb2312"
Is it possible to set the Command Arguments when running the main function in
ghci?
Thanks,
Kui
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Message: 7
Date: Fri, 23 Oct 2009 19:40:46 +0900
From: Yusaku Hashimoto <[email protected]>
Subject: Re: [Haskell-beginners] how to set command args in ghci
To: Kui Ma <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
:set args foo bar
should work. FYI, type :? to ghci to see all commands.
HTH
-nwn
2009/10/23 Kui Ma <[email protected]>:
> Is it possible to set the Command Arguments when running the main function
> in ghci?
>
> Thanks,
> Kui
>
>
>
> ________________________________
> Windows Live: Friends get your Flickr, Yelp, and Digg updates when they
> e-mail you.
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
------------------------------
Message: 8
Date: Fri, 23 Oct 2009 19:43:22 +0900
From: Yusaku Hashimoto <[email protected]>
Subject: Re: [Haskell-beginners] how to set command args in ghci
To: Kui Ma <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
or
:ma foo bar
On Fri, Oct 23, 2009 at 7:40 PM, Yusaku Hashimoto <[email protected]> wrote:
> :set args foo bar
>
> should work. FYI, type :? to ghci to see all commands.
>
> HTH
>
> -nwn
>
> 2009/10/23 Kui Ma <[email protected]>:
>> Is it possible to set the Command Arguments when running the main function
>> in ghci?
>>
>> Thanks,
>> Kui
>>
>>
>>
>> ________________________________
>> Windows Live: Friends get your Flickr, Yelp, and Digg updates when they
>> e-mail you.
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
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