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Today's Topics:
1. reverse 'lookup' (Patrick LeBoutillier)
2. Re: reverse 'lookup' (Yitzchak Gale)
3. Evaluate behaviour (Tom Hobbs)
4. Re: reverse 'lookup' (Alex Rozenshteyn)
5. Re: Evaluate behaviour (Elvio Rogelio Toccalino)
6. Re: Evaluate behaviour (Elvio Rogelio Toccalino)
7. Re: Dynamic Programming in Haskell (Heinrich Apfelmus)
----------------------------------------------------------------------
Message: 1
Date: Wed, 7 Jul 2010 13:28:03 -0400
From: Patrick LeBoutillier <[email protected]>
Subject: [Haskell-beginners] reverse 'lookup'
To: beginners <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
Hi,
The lookup function can locate something in a a-list:
lookup :: Eq a => a -> [(a,b)] -> Maybe b
is there an easy way to do a reverse lookup:
rlookup :: Eq b => b -> [(a,b)] -> Maybe a
or perhaps a builtin function to flip a pair:
(a, b) -> (b, a)
Thanks,
Patrick
--
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada
------------------------------
Message: 2
Date: Wed, 7 Jul 2010 20:54:48 +0300
From: Yitzchak Gale <[email protected]>
Subject: Re: [Haskell-beginners] reverse 'lookup'
To: Patrick LeBoutillier <[email protected]>
Cc: beginners <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
Patrick LeBoutillier wrote:
> is there... perhaps a builtin function to flip a pair:
> (a, b) -> (b, a)
The function
swap (x, y) = (y, x)
will be added to Data.Tuple in the next release
of the base libraries. In the meantime, it is easy
enough to define it yourself, and then, of course
rlookup x = lookup x . map swap
Or, you could use the find function from Data.List
and write:
rlookup x = find ((== x) . snd)
Regards,
Yitz
------------------------------
Message: 3
Date: Wed, 07 Jul 2010 20:51:40 +0100
From: "Tom Hobbs" <[email protected]>
Subject: [Haskell-beginners] Evaluate behaviour
To: "[email protected]" <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed; delsp=yes
Hi guys,
Thanks to everyone who helped me, I hit a milestone this evening and
finally got the (small) bit of functionality I was working on, working! A
huge "thank you" to everyone who has taken the time to use very small
words to explain things to me! (I just need to sort out some nicer error
handling now...)
Just one more question... for today.
Here's my code;
import Network
import System.IO
(hGetLine,hClose,hPutStrLn,hSetBuffering,BufferMode(..),Handle,stdout)
import Data.Bits
import Data.Binary
import Data.Binary.Put
import Data.Binary.Get
import qualified Data.ByteString.Lazy as L
import qualified Data.ByteString.UTF8 as UTF
import Control.Monad
import Control.Exception (evaluate)
ping a t = do
h <- connectTo a (PortNumber t)
hSetBuffering h NoBuffering
L.hPut h (encode (0xFAB10000 :: Word32))
numStrings <- fmap (fromIntegral . runGet getWord64be) $ L.hGet h 8
names <- (fmap (runGet $ readStrings numStrings) $ L.hGetContents h)
evaluate names
hClose h
return names
readStrings :: Int -> Get [String]
readStrings n = replicateM n $ do
len <- getWord32be
name <- getByteString $ fromIntegral len
return $ UTF.toString name
This works in GHCi exactly as I want it to. Note the call to "evaluate"
in the ping function which allows me to close the handle before returning
the IO [String]. This behaviour prints out the list of Strings as read
from the handle in GHCi, exactly what I wanted it to do. Here's the
strange part;
If I remove the evalute line, and instead have;
names <- evaluate (fmap (runGet $ readStrings numStrings) $ L.hGetContents
h)
GHCI claims that this is okay, but when I call ping I don't get any
output. I don't get any errors either, but I would expect to see the same
list of Strings as before.
Can anyone explain to me why that happpens? I'm assuming it's something
to do with the "<-" magic, but I don't know what.
Thanks,
Tom
--
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
------------------------------
Message: 4
Date: Wed, 7 Jul 2010 20:25:50 -0400
From: Alex Rozenshteyn <[email protected]>
Subject: Re: [Haskell-beginners] reverse 'lookup'
To: [email protected]
Cc: beginners <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="utf-8"
Incidentally (and more as a curiosity than anything else)
Prelude> :t uncurry . flip . curry $ id
uncurry . flip . curry $ id :: (b, a) -> (a, b)
On Wed, Jul 7, 2010 at 1:54 PM, Yitzchak Gale <[email protected]> wrote:
> Patrick LeBoutillier wrote:
> > is there... perhaps a builtin function to flip a pair:
> > (a, b) -> (b, a)
>
> The function
>
> swap (x, y) = (y, x)
>
> will be added to Data.Tuple in the next release
> of the base libraries. In the meantime, it is easy
> enough to define it yourself, and then, of course
>
> rlookup x = lookup x . map swap
>
> Or, you could use the find function from Data.List
> and write:
>
> rlookup x = find ((== x) . snd)
>
> Regards,
> Yitz
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
--
Alex R
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------------------------------
Message: 5
Date: Thu, 8 Jul 2010 00:56:06 -0300
From: Elvio Rogelio Toccalino <[email protected]>
Subject: Re: [Haskell-beginners] Evaluate behaviour
To: Tom Hobbs <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
This is a common behaviour, Tom. I suggest you read about the
"semi-closed" state of Handles in the documentation for System.IO.
It's kind of Zen :D
Basically, you are reading lazily from a bytestring (1 chunk at a
time)... Although it may seem as your program read and processed the
whole bytestring, it didn't (it's lazy!). If you close the handle
before "using" the output (when you 'return' it from your main
function), you're shutting down the incoming data channel before a
single request for a chunk of the bytestring is made.
Check it out for yourself... don't hClose the handle, just leave it
be. (It's messy, I know, but experiment with it.)
2010/7/7, Tom Hobbs <[email protected]>:
> Hi guys,
>
> Thanks to everyone who helped me, I hit a milestone this evening and
> finally got the (small) bit of functionality I was working on, working! A
> huge "thank you" to everyone who has taken the time to use very small
> words to explain things to me! (I just need to sort out some nicer error
> handling now...)
>
> Just one more question... for today.
>
> Here's my code;
>
> import Network
> import System.IO
> (hGetLine,hClose,hPutStrLn,hSetBuffering,BufferMode(..),Handle,stdout)
> import Data.Bits
> import Data.Binary
> import Data.Binary.Put
> import Data.Binary.Get
> import qualified Data.ByteString.Lazy as L
> import qualified Data.ByteString.UTF8 as UTF
> import Control.Monad
> import Control.Exception (evaluate)
>
> ping a t = do
> h <- connectTo a (PortNumber t)
> hSetBuffering h NoBuffering
> L.hPut h (encode (0xFAB10000 :: Word32))
> numStrings <- fmap (fromIntegral . runGet getWord64be) $ L.hGet h 8
> names <- (fmap (runGet $ readStrings numStrings) $ L.hGetContents h)
> evaluate names
> hClose h
> return names
>
> readStrings :: Int -> Get [String]
> readStrings n = replicateM n $ do
> len <- getWord32be
> name <- getByteString $ fromIntegral len
> return $ UTF.toString name
>
> This works in GHCi exactly as I want it to. Note the call to "evaluate"
> in the ping function which allows me to close the handle before returning
> the IO [String]. This behaviour prints out the list of Strings as read
> from the handle in GHCi, exactly what I wanted it to do. Here's the
> strange part;
>
> If I remove the evalute line, and instead have;
>
> names <- evaluate (fmap (runGet $ readStrings numStrings) $ L.hGetContents
> h)
>
> GHCI claims that this is okay, but when I call ping I don't get any
> output. I don't get any errors either, but I would expect to see the same
> list of Strings as before.
>
> Can anyone explain to me why that happpens? I'm assuming it's something
> to do with the "<-" magic, but I don't know what.
>
> Thanks,
>
> Tom
>
>
> --
> Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 6
Date: Thu, 8 Jul 2010 01:17:48 -0300
From: Elvio Rogelio Toccalino <[email protected]>
Subject: Re: [Haskell-beginners] Evaluate behaviour
To: Tom Hobbs <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
...Oh, I just saw this:
"""
instead have;
names <- evaluate (fmap (runGet $ readStrings numStrings) $ L.hGetContents h)
"""
My apologizes, I was tackling a different question.
I believe the magic attributed to the (<-) operator results from the following:
let FUN be "(fmap (runGet $ readStrings numStrings) $ L.hGetContents h)"
then FUN :: IO [String]
This:
names <- FUN
evaluate names
bounds the result of the FUN computation to 'names'. Evaluate is run
on names, therefore 'evaluate'ing the result of the FUN computation.
On the other hand, this:
names <- evaluate FUN
bounds to 'names' the results of 'evaluate'ing FUN. The action is
executed and the argument (FUN) is evaluated to WHNF. Let's see,
evaluate :: a -> IO a
evaluate FUN :: IO (IO [String])
names <- evaluate FUN
... and 'names' is bound to a [String] which has been evaluated to
WHNF. You end up closing a handle before reading your input.
If my reasoning is wrong, please (please!) let me know, you'd be
helping me greatly.
2010/7/8, Elvio Rogelio Toccalino <[email protected]>:
> This is a common behaviour, Tom. I suggest you read about the
> "semi-closed" state of Handles in the documentation for System.IO.
>
> It's kind of Zen :D
>
> Basically, you are reading lazily from a bytestring (1 chunk at a
> time)... Although it may seem as your program read and processed the
> whole bytestring, it didn't (it's lazy!). If you close the handle
> before "using" the output (when you 'return' it from your main
> function), you're shutting down the incoming data channel before a
> single request for a chunk of the bytestring is made.
>
> Check it out for yourself... don't hClose the handle, just leave it
> be. (It's messy, I know, but experiment with it.)
>
> 2010/7/7, Tom Hobbs <[email protected]>:
>> Hi guys,
>>
>> Thanks to everyone who helped me, I hit a milestone this evening and
>> finally got the (small) bit of functionality I was working on, working!
>> A
>> huge "thank you" to everyone who has taken the time to use very small
>> words to explain things to me! (I just need to sort out some nicer error
>> handling now...)
>>
>> Just one more question... for today.
>>
>> Here's my code;
>>
>> import Network
>> import System.IO
>> (hGetLine,hClose,hPutStrLn,hSetBuffering,BufferMode(..),Handle,stdout)
>> import Data.Bits
>> import Data.Binary
>> import Data.Binary.Put
>> import Data.Binary.Get
>> import qualified Data.ByteString.Lazy as L
>> import qualified Data.ByteString.UTF8 as UTF
>> import Control.Monad
>> import Control.Exception (evaluate)
>>
>> ping a t = do
>> h <- connectTo a (PortNumber t)
>> hSetBuffering h NoBuffering
>> L.hPut h (encode (0xFAB10000 :: Word32))
>> numStrings <- fmap (fromIntegral . runGet getWord64be) $ L.hGet h 8
>> names <- (fmap (runGet $ readStrings numStrings) $ L.hGetContents h)
>> evaluate names
>> hClose h
>> return names
>>
>> readStrings :: Int -> Get [String]
>> readStrings n = replicateM n $ do
>> len <- getWord32be
>> name <- getByteString $ fromIntegral len
>> return $ UTF.toString name
>>
>> This works in GHCi exactly as I want it to. Note the call to "evaluate"
>> in the ping function which allows me to close the handle before returning
>> the IO [String]. This behaviour prints out the list of Strings as read
>> from the handle in GHCi, exactly what I wanted it to do. Here's the
>> strange part;
>>
>> If I remove the evalute line, and instead have;
>>
>> names <- evaluate (fmap (runGet $ readStrings numStrings) $
>> L.hGetContents
>> h)
>>
>> GHCI claims that this is okay, but when I call ping I don't get any
>> output. I don't get any errors either, but I would expect to see the
>> same
>> list of Strings as before.
>>
>> Can anyone explain to me why that happpens? I'm assuming it's something
>> to do with the "<-" magic, but I don't know what.
>>
>> Thanks,
>>
>> Tom
>>
>>
>> --
>> Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>
------------------------------
Message: 7
Date: Thu, 08 Jul 2010 10:20:29 +0200
From: Heinrich Apfelmus <[email protected]>
Subject: [Haskell-beginners] Re: Dynamic Programming in Haskell
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
Daniel Fischer wrote:
> Heinrich Apfelmus wrote:
>> I didn't need to debug this code, because it's obviously correct. Put
>> differently, instead of spending my effort on debugging, I have spent it
>> on making the solution elegant.
>
> Well done.
> Chapeau!
:)
Well, apart from the fact that the "obviousness" of this code also
depends on the correctness of this particular division into subproblems,
there is one piece of the code that is quite error-prone, namely the
definition of chain and chain' :
chain = memoize n chain'
chain' i j
| i == j = Matrix (dimensions ! i)
| otherwise = best [mul (chain i k) (chain (k+1) j)
| k <- [i..j-1] ]
It is really easy to accidentally write chain' instead of chain and
vice versa, leading to a black hole or a loss of memoization. (Using
more distinct names doesn't necessarily help because there is almost no
semantic difference between the two functions.) Trouble is that the type
system won't catch such mistakes because the two functions have the same
type.
However, we can give chain and chain' different types by using the
fixed point combinator and writing:
chain = fix (memoize n . chain')
chain' chain i j
| i == j = Matrix (dimensions ! i)
| otherwise = best [mul (chain i k) (chain (k+1) j)
| k <- [i..j-1] ]
Shadowing the variable chain in the definition of chain' is both
intentional and harmless, since both variables chain will be bound to
the very same function. In any case, chain' and chain now have
different types and the burden of checking whether we've used them
correctly is left to the compiler.
(For those who don't know the fixed point combinator yet: I have
recently made a short video about it:
http://apfelmus.nfshost.com/blog/2010/07/02-fixed-points-video.html
)
Regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
------------------------------
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