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Today's Topics:
1. Re: Beginners Digest, Vol 7, Issue 24 (Benedikt Ahrens)
2. Re: Help! Trapped in the IO Monad! (Heinrich Apfelmus)
3. Re: Monads... (Cory Knapp)
4. Re: Monads... (Henk-Jan van Tuyl)
5. Question about Real World Haskell (Alan Cameron)
6. Compile type Error (emmanuel.delaborde)
7. Real World Haskell book authors accepting comments?
(Alan Cameron)
8. Re: Compile type Error (Rafael Gustavo da Cunha Pereira Pinto)
----------------------------------------------------------------------
Message: 1
Date: Thu, 29 Jan 2009 11:50:47 +0100
From: Benedikt Ahrens <[email protected]>
Subject: [Haskell-beginners] Re: Beginners Digest, Vol 7, Issue 24
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
hello cory,
you might want to read this article:
http://stefan-klinger.de/files/monadGuide.pdf
it helped me find and understand the monads in haskell.
ben
> Message: 2
> Date: Wed, 28 Jan 2009 20:44:30 -0600
> From: Cory Knapp <[email protected]>
> Subject: [Haskell-beginners] Monads...
> To: [email protected]
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Hello, so, I'm having a simple problem with monads: I have no idea how
> to actually use them. I understand the category theory (or, at least
> well enough to be able to explain "what is a monad"); I understand the
> way to declare something as a monad instance, but I just don't get how
> to program with them. Can anyone provide me with, or direct me towards,
> some simple monads and some ways of using (for example) the monadic
> properties of lists?
>
> Thanks,
> Cory
>
------------------------------
Message: 2
Date: Thu, 29 Jan 2009 11:58:51 +0100
From: Heinrich Apfelmus <[email protected]>
Subject: [Haskell-beginners] Re: Help! Trapped in the IO Monad!
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
Alexander Dunlap wrote:
> You can do (something like; this is untested)
>
> splitDirFile :: [FilePath] -> IO ([FilePath],[FilePath])
> splitDirFile [] = return ([],[])
> splitDirFile (f:fs) = do
> (yess,nos) <- splitDirFile fs
> exists <- doesDirectoryExist f
> return $ if exists
> then (f:yess,nos)
> else (yess,f:nos)
>
> You might also look at Control.Monad.filterM. I often define a
> function "partitionM" which is like partition except it uses a monadic
> test, just like you have.
How about
splitDirFile ps =
((map fst *** map fst) . partition snd . zip ps)
`liftM` mapM doesDirectoryExist ps
There is no need to rewrite partition , you can reuse it.
Hm, the plumbing seems slightly cumbersome to me, maybe Conal's editor
combinators
http://conal.net/blog/posts/semantic-editor-combinators/
can help.
Regards,
apfelmus
--
http://apfelmus.nfshost.com
------------------------------
Message: 3
Date: Thu, 29 Jan 2009 08:48:42 -0600
From: Cory Knapp <[email protected]>
Subject: Re: [Haskell-beginners] Monads...
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Thanks to both of you, I'll look into those.
Cory
Rafael Gustavo da Cunha Pereira Pinto wrote:
> Cory,
>
> The big hit for me was Phillip Wadler's paper "Monads for functional
> programming" I made me start thinking "well, this looks like a monad..."
>
> homepages.inf.ed.ac.uk/wadler/papers/marktoberdorf/baastad.pdf
> <http://homepages.inf.ed.ac.uk/wadler/papers/marktoberdorf/baastad.pdf>
>
> Cheers
>
> On Thu, Jan 29, 2009 at 01:38, nanothief <[email protected]
> <mailto:[email protected]>> wrote:
>
> Cory Knapp wrote:
>
> Hello, so, I'm having a simple problem with monads: I have no
> idea how to actually use them. I understand the category
> theory (or, at least well enough to be able to explain "what
> is a monad"); I understand the way to declare something as a
> monad instance, but I just don't get how to program with them.
> Can anyone provide me with, or direct me towards, some simple
> monads and some ways of using (for example) the monadic
> properties of lists?
>
> Thanks,
> Cory
> _______________________________________________
> Beginners mailing list
> [email protected] <mailto:[email protected]>
> http://www.haskell.org/mailman/listinfo/beginners
>
> I found that
> http://www.haskell.org/all_about_monads/html/index.html gave a lot
> of nice examples of using list and maybe monads. The list monad is
> particularly useful for finding possible solutions given available
> input values.
> For example, with the problem
> x + 8y = 114
> 3x - 8y + 4z = 182
> x < y < z < 100
> Find solutions for x,y,z
>
> The program:
> res :: [(Int,Int,Int)]
> res = do
> x <- [1..100]
> y <- [1..100]
> z <- [1..100]
> guard $ x + 8 * y == 114
> guard $ 3*x - 8*y + 4*z == 182
> guard $ x < y
> guard $ y < z
> return (x,y,z)
>
> will output all the possible solutions. Note how close the program
> is to the actual problem. The values of x,y, and z are chosen from
> the value [1..100], but if a guard statement fails, the (x,y,z)
> choice is abandoned.
>
> Another example (taken from
> http://www.mathsisfun.com/puzzles/sum-of-digits-is-43-solution.html )
> *The Puzzle:* I am thinking of a 6-digit number. The sum of the
> digits is 43.
>
> And only two of the following three statements about the number
> are true:
>
> (1) it's a square number,
> (2) it's a cube number, and
> (3) the number is under 500000.
>
> the program
> answer = do
> d1 <- [0..9]
> d2 <- [0..9]
> d3 <- [0..9]
> d4 <- [0..9]
> d5 <- [0..9]
> d6 <- [0..9]
> let digitSum = d1 + d2 + d3 + d4 + d5 + d6
> let value = d1 + d2*10 + d3*100 + d4*1000 + d5*10000 + d6*100000
> guard $ digitSum == 43
> let lessThan500000 = digitSum < 500000
> let isSquare = (round $ sqrt (fromIntegral value)) ^ 2 == value
> let isCube = (round $ (fromIntegral value) ** (1/3)) ^ 3 == value
> guard $ length (filter id [lessThan500000,isSquare,isCube]) == 2
> return value
>
> will output the three answers (not that the author only found one
> solution!).
>
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected] <mailto:[email protected]>
> http://www.haskell.org/mailman/listinfo/beginners
>
>
>
>
> --
> Rafael Gustavo da Cunha Pereira Pinto
> Electronic Engineer, MSc.
------------------------------
Message: 4
Date: Thu, 29 Jan 2009 16:06:07 +0100
From: "Henk-Jan van Tuyl" <[email protected]>
Subject: Re: [Haskell-beginners] Monads...
To: "Cory Knapp" <[email protected]>, [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; format=flowed; delsp=yes;
charset=iso-8859-15
I have written a reference manual for the basic Haskell monad functions,
"A Tour of the Haskell Monad functions". It contains a lot of examples.
You can find it at:
http://members.chello.nl/hjgtuyl/tourdemonad.html
Regards,
Henk-Jan van Tuyl
--
http://functor.bamikanarie.com
http://Van.Tuyl.eu/
--
On Thu, 29 Jan 2009 03:44:30 +0100, Cory Knapp <[email protected]>
wrote:
> Hello, so, I'm having a simple problem with monads: I have no idea how
> to actually use them. I understand the category theory (or, at least
> well enough to be able to explain "what is a monad"); I understand the
> way to declare something as a monad instance, but I just don't get how
> to program with them. Can anyone provide me with, or direct me towards,
> some simple monads and some ways of using (for example) the monadic
> properties of lists?
>
> Thanks,
> Cory
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
--
------------------------------
Message: 5
Date: Thu, 29 Jan 2009 15:21:24 -0000
From: "Alan Cameron" <[email protected]>
Subject: [Haskell-beginners] Question about Real World Haskell
To: <[email protected]>
Message-ID: <c431f46580464bb1bb9e2c2fa1b32...@alanxps>
Content-Type: text/plain; charset="us-ascii"
In Chapter 3 of Real World Haskell it is not clear to me how to get the
myInfo into the ghci system there appears to be two definitions for
ch03/BookStore.hs can anyone help?
Alan Cameron
------------------------------
Message: 6
Date: Thu, 29 Jan 2009 15:25:59 +0000
From: "emmanuel.delaborde" <[email protected]>
Subject: [Haskell-beginners] Compile type Error
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
Hello
I have the following snippet :
----------------------------------------------------------
module Main where
import System.Environment (getArgs)
import Data.Digest.OpenSSL.MD5 (md5sum)
import Data.Digest.Pure.SHA (sha1, showDigest)
import qualified Data.ByteString as BS (readFile)
-- sha1 :: ByteString -> Digest
-- readFile :: FilePath -> IO ByteString
-- md5sum :: ByteString -> String
-- showDigest :: Digest -> String
checkHash :: String -> String -> String -> IO ()
checkHash codec hash file =
let f = case codec of
--"md5" -> md5sum
"sha1" -> showDigest . sha1
_ -> error "Codec must be md5 or sha1 !" in
BS.readFile file >>= \fileBS ->
let fileHash = f fileBS in
print (hash == fileHash)
main =
getArgs >>= \as ->
case as of
(codec:hash:file:_) -> checkHash codec hash file
_ -> usage
usage = print "checksum codec hash file"
----------------------------------------------------------
which fails to compile, this is the error I get :
checksum.hs:20:19:
Couldn't match expected type
`Data.ByteString.Lazy.Internal.ByteString'
against inferred type `Data.ByteString.Internal.ByteString'
In the first argument of `f', namely `fileBS'
In the expression: f fileBS
In the definition of `fileHash': fileHash = f fileBS
it looks like (showDigest . sha1) expects
Data.ByteString.Lazy.Internal.ByteString but gets
Data.ByteString.Internal.ByteString
What can I do ?
Thanks
E.
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------------------------------
Message: 7
Date: Thu, 29 Jan 2009 15:54:20 -0000
From: "Alan Cameron" <[email protected]>
Subject: [Haskell-beginners] Real World Haskell book authors accepting
comments?
To: <[email protected]>
Message-ID: <a4e979a646454446979d9c91ff2ed...@alanxps>
Content-Type: text/plain; charset="us-ascii"
The online version of the book seems to have stopped accepting comments - at
least from me.
Alan Cameron
------------------------------
Message: 8
Date: Thu, 29 Jan 2009 14:19:38 -0200
From: Rafael Gustavo da Cunha Pereira Pinto
<[email protected]>
Subject: Re: [Haskell-beginners] Compile type Error
To: "emmanuel.delaborde" <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
I didn't compile, but, by the looks of it, I would change
import qualified Data.ByteString as BS (readFile)
for
import qualified Data.ByteString.Lazy as BS (readFile)
as it seems sha1 needs a lazy bytestring
On Thu, Jan 29, 2009 at 13:25, emmanuel.delaborde <
[email protected]> wrote:
> Hello
>
> I have the following snippet :
>
> ----------------------------------------------------------
>
> module Main where
>
> import System.Environment (getArgs)
> import Data.Digest.OpenSSL.MD5 (md5sum)
> import Data.Digest.Pure.SHA (sha1, showDigest)
> import qualified Data.ByteString as BS (readFile)
>
> -- sha1 :: ByteString -> Digest
> -- readFile :: FilePath -> IO ByteString
> -- md5sum :: ByteString -> String
> -- showDigest :: Digest -> String
>
> checkHash :: String -> String -> String -> IO ()
> checkHash codec hash file =
> let f = case codec of
> --"md5" -> md5sum
> "sha1" -> showDigest . sha1
> _ -> error "Codec must be md5 or sha1 !" in
> BS.readFile file >>= \fileBS ->
> let fileHash = f fileBS in
> print (hash == fileHash)
>
> main =
> getArgs >>= \as ->
> case as of
> (codec:hash:file:_) -> checkHash codec hash file
> _ -> usage
>
> usage = print "checksum codec hash file"
>
> ----------------------------------------------------------
>
> which fails to compile, this is the error I get :
>
>
> checksum.hs:20:19:
> Couldn't match expected type `Data.ByteString.Lazy.Internal.ByteString'
> against inferred type `Data.ByteString.Internal.ByteString'
> In the first argument of `f', namely `fileBS'
> In the expression: f fileBS
> In the definition of `fileHash': fileHash = f fileBS
>
>
>
>
> it looks like (showDigest . sha1) expects
> Data.ByteString.Lazy.Internal.ByteString but gets
> Data.ByteString.Internal.ByteString
>
> What can I do ?
>
> Thanks
>
> E.
>
>
>
>
>
>
>
>
> -----------------------------------------------------------------------------------------------
>
> This e-mail (and any attachments) is confidential and may containpersonal
> views which are not the views of Cimex Media Ltd andany affiliated
> companies, unless specifically stated. It is intendedfor the use of the
> individual or group to whom it is addressed. Ifyou have received it in
> error, please delete it from your system,do not use, copy or disclose the
> information in any way nor act inreliance on it and please notify
> [email protected]
>
> A company registered in England Wales. Company Number 03765711
> Registered Office : The Olde Bakehouse, 156 Watling Street East, Towcester,
> Northants NN12 6DB
>
> This email was scanned by Postini, the leading provider in Managed Email
> Security.
>
> _______________________________________________
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>
--
Rafael Gustavo da Cunha Pereira Pinto
Electronic Engineer, MSc.
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