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Today's Topics:
1. Re: Sequential IO processing (Andrew Wagner)
2. Re: Sequential IO processing (Sergey V. Mikhanov)
3. Re: Sequential IO processing (Heinrich Apfelmus)
4. Re: Re: Sequential IO processing (Sergey V. Mikhanov)
5. Re: Re: Sequential IO processing (Felipe Lessa)
6. Counting Fruits (Adolfo Builes)
7. Re: Counting Fruits (Alexander Dunlap)
----------------------------------------------------------------------
Message: 1
Date: Thu, 19 Feb 2009 10:19:35 -0500
From: Andrew Wagner <[email protected]>
Subject: Re: [Haskell-beginners] Sequential IO processing
To: "Sergey V. Mikhanov" <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
You did really well here. There's just one small detail that you missed,
which is causing the problem:
>
> sequenceIO [] = return []
> sequenceIO (x : xs) = do result <- x
> return result : sequenceIO xs
>
The problem is indeed here. The type of 'sequenceIO xs' is IO [a], but the
type of result is 'a'. You can't cons an 'a' onto an 'IO [a]'. Thus, what
you need is something like this:
sequenceIO [] = return []
sequenceIO (x : xs) = do result <- x xs'
<- sequenceIO xs -- to take the list out of the IO Monad
return result : xs'
On Thu, Feb 19, 2009 at 9:56 AM, Sergey V. Mikhanov <[email protected]>wrote:
> Hi community,
>
> I am making my first steps in Haskell, reading Haskell wikibook and
> now stuck with one of the excercises, namely this one:
>
> Implement a function sequenceIO :: [IO a] -> IO [a]. Given a list of
> actions, this function runs each of the actions in order and returns
> all their results as a list.
>
> This is what I came with:
>
> ioOne :: Num a => IO a
>
> ioOne = do print guid
> return guid
> where
> guid = 2
>
> ioTwo :: Num a => IO a
>
> ioTwo = do print guid
> return guid
> where
> guid = 3
>
> sequenceIO :: Num a => [IO a] -> IO [a]
>
> sequenceIO [] = return []
> sequenceIO (x : xs) = do result <- x
> return result : sequenceIO xs
>
> First two functions are there because of the invocation that I've
> planned: sequenceIO [getGuid, getNextGuid].
>
> However, this could not be compiled (GHC):
>
> Couldn't match expected type `[m a]' against inferred type `IO [a]'
> In the second argument of `(:)', namely `sequenceIO xs'
> In the expression: return result : sequenceIO xs
> In the expression:
> do result <- x
> return result : sequenceIO xs
>
> Fine, I thought, something wrong with the type of the 'sequenceIO xs'
> (becasue I am sure the type of 'result' is fine). So I wrote another
> program to check what happens to the result of IO action evaluation
> (namely, which type is assigned):
>
> bar :: Num a => IO a
>
> bar = do print guid
> return guid
> where
> guid = 2
>
> foo = do result <- bar
> result
>
> This could not be compiled either:
>
> No instance for (Num (IO b))
> arising from a use of `bar' at auxil.hs:8:19-21
> Possible fix: add an instance declaration for (Num (IO b))
> In a stmt of a 'do' expression: result <- bar
> In the expression:
> do result <- bar
> result
> In the definition of `foo':
> foo = do result <- bar
> result
>
> I am a newbie, so I am interpreting this like "Haskell could not
> construct Num from the result of invocation of bar, which is of type
> IO a". But why do I need this at all? When doing console I/O with
> 'result <- getLine', I do not need to reconstruct String from the
> result.
>
> What am I doing wrong? Where is the failure in reasoning?
>
> Thanks,
> Sergey
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 2
Date: Thu, 19 Feb 2009 16:27:02 +0100
From: "Sergey V. Mikhanov" <[email protected]>
Subject: Re: [Haskell-beginners] Sequential IO processing
To: beginners <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
I tried this earlier as well:
sequenceIO [] = return []
sequenceIO (x : xs) = do result <- x
resultTail <- sequenceIO xs
return result : resultTail
This results in:
Couldn't match expected type `IO [a]' against inferred type `[m a]'
In the expression: return result : resultTail
In the expression:
do result <- x
resultTail <- sequenceIO xs
return result : resultTail
In the definition of `sequenceIO':
sequenceIO (x : xs)
= do result <- x
resultTail <- sequenceIO xs
return result : resultTail
>> sequenceIO [] = return []
>> sequenceIO (x : xs) = do result <- x
>> return result : sequenceIO xs
>
> The problem is indeed here. The type of 'sequenceIO xs' is IO [a], but the
> type of result is 'a'. You can't cons an 'a' onto an 'IO [a]'. Thus, what
> you need is something like this:
> sequenceIO [] = return []
> sequenceIO (x : xs) = do result <- x
> xs' <- sequenceIO xs -- to take the
> list out of the IO Monad
> return result : xs'
------------------------------
Message: 3
Date: Thu, 19 Feb 2009 16:32:29 +0100
From: Heinrich Apfelmus <[email protected]>
Subject: [Haskell-beginners] Re: Sequential IO processing
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
Sergey V. Mikhanov wrote:
> I tried this earlier as well:
>
> sequenceIO [] = return []
> sequenceIO (x : xs) = do result <- x
> resultTail <- sequenceIO xs
> return result : resultTail
The last line should be
return (result : resultTail)
otherwise it will be parsed as
(return result) : resultTail
which is not what you want.
Regards,
apfelmus
--
http://apfelmus.nfshost.com
------------------------------
Message: 4
Date: Thu, 19 Feb 2009 16:35:08 +0100
From: "Sergey V. Mikhanov" <[email protected]>
Subject: Re: [Haskell-beginners] Re: Sequential IO processing
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
Now works!
Thank you very much -- a nice lesson learned for me.
S.
>> I tried this earlier as well:
>>
>> sequenceIO [] = return []
>> sequenceIO (x : xs) = do result <- x
>> resultTail <- sequenceIO xs
>> return result : resultTail
>
> The last line should be
>
> return (result : resultTail)
>
> otherwise it will be parsed as
>
> (return result) : resultTail
>
> which is not what you want.
>
>
> Regards,
> apfelmus
------------------------------
Message: 5
Date: Thu, 19 Feb 2009 13:06:00 -0300
From: Felipe Lessa <[email protected]>
Subject: Re: [Haskell-beginners] Re: Sequential IO processing
To: "Sergey V. Mikhanov" <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
Just a small comment, it's great to see the beginners list being so
useful! Thanks, everyone, beginners and 'otherwise'.
--
Felipe.
------------------------------
Message: 6
Date: Thu, 19 Feb 2009 23:15:33 +0000
From: Adolfo Builes <[email protected]>
Subject: [Haskell-beginners] Counting Fruits
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hello everyone ,
I wanted to do a small program which read a txt with fruit's name in each
line and then print how many fruits I have of each type. something like
these:
apple
apple
and then
[(apple,2)]
I came up whit this
import qualified Data.Map as Map
import Data.List
import System.IO
main =
do
file <- readFile "fruits.txt"
let answer = proccessFile $ lines file
putStrLn (show answer)
proccessFile :: [String] -> [(String,Int)]
proccessFile file = Map.toAscList $ parseFile Map.empty file
where parseFile fruits [] = fruits
parseFile fruits_map (x:xs) = parseFile (Map.insertWith (+)
x 1 fruits_map) xs
It works, but I would like to know how would you do it ?, Share different
points of view, different code. Was it a good idea to use a Map ?, Did I
separate the code in a proper way, I mean pure - impure ? How can we
improve the performance ?
Best Regards for everyone.
Adolfo
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Message: 7
Date: Thu, 19 Feb 2009 21:01:24 -0800
From: Alexander Dunlap <[email protected]>
Subject: Re: [Haskell-beginners] Counting Fruits
To: Adolfo Builes <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
On Thu, Feb 19, 2009 at 3:15 PM, Adolfo Builes
<[email protected]> wrote:
> Hello everyone ,
> I wanted to do a small program which read a txt with fruit's name in each
> line and then print how many fruits I have of each type. something like
> these:
> apple
> apple
>
> and then
>
> [(apple,2)]
>
> I came up whit this
>
> import qualified Data.Map as Map
> import Data.List
> import System.IO
>
> main =
> do
> file <- readFile "fruits.txt"
> let answer = proccessFile $ lines file
>
>
> putStrLn (show answer)
>
> proccessFile :: [String] -> [(String,Int)]
> proccessFile file = Map.toAscList $ parseFile Map.empty file
> where parseFile fruits [] = fruits
> parseFile fruits_map (x:xs) = parseFile (Map.insertWith (+) x 1
> fruits_map) xs
>
>
> It works, but I would like to know how would you do it ?, Share different
> points of view, different code. Was it a good idea to use a Map ?, Did I
> separate the code in a proper way, I mean pure - impure ? How can we
> improve the performance ?
>
> Best Regards for everyone.
>
>
> Adolfo
You spelled "process" incorrectly ;). Not a big deal of course, just
makes your code a bit cleaner-looking.
In terms of actual code, I think "proccessFruits" could probably be
rewritten more clearly (and perhaps more efficiently, due to rewrite
rules) using a fold. You might also check out the "group" function
from Data.List, although that approach would probably be a bit slower.
Hope that helps.
Alex
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