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Today's Topics:
1. Using IO values for computations (Dananji Liyanage)
2. Re: Using IO values for computations (Magnus Therning)
3. Re: Couldn't match expected type `(a0 -> [a0]) -> [a0]',
with actual type `[a0]' (Grzegorz Milka)
4. Re: Using IO values for computations (Kostiantyn Rybnikov)
5. Re: Using IO values for computations (Dananji Liyanage)
----------------------------------------------------------------------
Message: 1
Date: Mon, 25 May 2015 12:14:32 +0530
From: Dananji Liyanage <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Using IO values for computations
Message-ID:
<CAAPsY8zM3Kjboj6NdmPmVcHCq63jXmupV+JQGgFkjJp=3zo...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi All,
I'm writing a code, where the input is read from a text file using:
readValues = readFile "Input.txt"
Since the type of this is 'IO String', I can't use this in the consequent
functions.
For an example: I want to split this as follows within another function
extractInput url method template
| isURI url == True = getList values components
| otherwise = []
where components = splitTemplate readValues
values = getURL (splitURL url) method
This gives the following error:
Couldn't match type ?IO String? with ?[Char]?
Expected type: String
Actual type: IO String
How can I solve this?
Thanks in advance!
--
Regards,
Dananji Liyanage
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Message: 2
Date: Mon, 25 May 2015 14:29:12 +0200
From: Magnus Therning <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Using IO values for computations
Message-ID:
<caaexw5tlntdv6yxwxc5tv0tjimhucamsg1gqjyusn9ksshg...@mail.gmail.com>
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On 25 May 2015 at 08:44, Dananji Liyanage <[email protected]> wrote:
> Hi All,
>
> I'm writing a code, where the input is read from a text file using:
> readValues = readFile "Input.txt"
>
> Since the type of this is 'IO String', I can't use this in the consequent
> functions.
>
> For an example: I want to split this as follows within another function
>
> extractInput url method template
> | isURI url == True = getList values components
> | otherwise = []
> where components = splitTemplate readValues
> values = getURL (splitURL url) method
>
> This gives the following error:
>
> Couldn't match type ?IO String? with ?[Char]?
> Expected type: String
> Actual type: IO String
>
> How can I solve this?
Start with reading some basic Haskell book/tutorial. That should tell
you how to e.g. use 'do' notation, or `liftM`, to achieve what you
want.
/M
--
Magnus Therning OpenPGP: 0xAB4DFBA4
email: [email protected] jabber: [email protected]
twitter: magthe http://therning.org/magnus
------------------------------
Message: 3
Date: Sat, 23 May 2015 10:29:30 +0200
From: Grzegorz Milka <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Couldn't match expected type `(a0 ->
[a0]) -> [a0]', with actual type `[a0]'
Message-ID: <[email protected]>
Content-Type: text/plain; charset="windows-1252"
Hi,
The faulty expression is this one:
((number `mod` 10 ): acc) (toDigits' (number `mod` 10))
The first expression: ((number `mod` 10 ): acc) is a list. You
concatenate a number to acc.
Therefore what you are doing is calling a list with an argument:
(toDigits' (number `mod` 10)).
That's what the error says. The first expression is called with one
argument, but its is a list ([a0]), which is not a function, therefore
can not be called. I think you meant something like:
toDigits' acc number = toDigits' ((number `mod` 10 ): acc)
(number `div` 10)
Best,
Greg
On 23.05.2015 08:58, Roelof Wobben wrote:
> Hello,
>
> For some reasons my file's are corrupted.
> I had repair them with success except this one.
>
> Here is the code :
>
> toDigits number
> | number <= 0 = []
> | otherwise = toDigits' [] number
> where
> toDigits' acc 0 = acc
> toDigits' acc number = ((number `mod` 10 ): acc) (toDigits'
> (number `mod` 10))
>
> main = print $ toDigits 123
>
>
> and here is the error:
>
> Couldn't match expected type `(a0 -> [a0]) -> [a0]'
> with actual type `[a0]'
> The function `(number `mod` 10) : acc' is applied to one argument,
> but its type `[a0]' has none
> In the expression:
> ((number `mod` 10) : acc) (toDigits' (number `mod` 10))
> In an equation for toDigits':
> toDigits' acc number
> = ((number `mod` 10) : acc) (toDigits' (number `mod` 10))
>
>
> Roelof
>
>
> ---
> Dit e-mailbericht is gecontroleerd op virussen met Avast
> antivirussoftware.
> http://www.avast.com
>
> _______________________________________________
> Beginners mailing list
> [email protected]
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Message: 4
Date: Mon, 25 May 2015 19:23:04 +0300
From: Kostiantyn Rybnikov <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Using IO values for computations
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Dananji,
Haskell explicitly separates "pure" from "impure", so "readFile <file>"
returns not a string, but rather an action, which tells haskell to read a
string. In order to "use" a result of some IO action as a pure value, you
have several ways, most popula of which is a do-notation.
main :: IO ()
main = do
s <- readFile "somefile.txt"
putStrLn (show (doSplit s))
In the code above, variable "s" is "pure", it has type String and you can
use it as you want. Do-notation is essentially a sugar to callback
mechanism, where in order to use a value of some IO-computation you write a
function-callback that will be called with a pure value of computation
result. This code is the same as previous one, but without do-notation:
main :: IO ()
main =
readFile "somefile.txt" >>= (\s ->
putStrLn (show (doSplit s)))
I highly recommend reading "Learn You A Haskell" book
http://learnyouahaskell.com/ , which explained these concepts really well
to me.
On Mon, May 25, 2015 at 9:44 AM, Dananji Liyanage <[email protected]> wrote:
> Hi All,
>
> I'm writing a code, where the input is read from a text file using:
> readValues = readFile "Input.txt"
>
> Since the type of this is 'IO String', I can't use this in the consequent
> functions.
>
> For an example: I want to split this as follows within another function
>
> extractInput url method template
> | isURI url == True = getList values components
> | otherwise = []
> where components = splitTemplate readValues
> values = getURL (splitURL url) method
>
> This gives the following error:
>
> Couldn't match type ?IO String? with ?[Char]?
> Expected type: String
> Actual type: IO String
>
> How can I solve this?
>
> Thanks in advance!
>
> --
> Regards,
> Dananji Liyanage
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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Message: 5
Date: Tue, 26 May 2015 08:59:01 +0530
From: Dananji Liyanage <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Using IO values for computations
Message-ID:
<caapsy8zbvan9alo5dfb0xehrd1uj4zbc1mezh04bnowwnmh...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thank you for the nice explanation!
I understood it for some extent, and will read further on I/O.
On Mon, May 25, 2015 at 9:53 PM, Kostiantyn Rybnikov <[email protected]> wrote:
> Dananji,
>
> Haskell explicitly separates "pure" from "impure", so "readFile <file>"
> returns not a string, but rather an action, which tells haskell to read a
> string. In order to "use" a result of some IO action as a pure value, you
> have several ways, most popula of which is a do-notation.
>
> main :: IO ()
> main = do
> s <- readFile "somefile.txt"
> putStrLn (show (doSplit s))
>
> In the code above, variable "s" is "pure", it has type String and you can
> use it as you want. Do-notation is essentially a sugar to callback
> mechanism, where in order to use a value of some IO-computation you write a
> function-callback that will be called with a pure value of computation
> result. This code is the same as previous one, but without do-notation:
>
> main :: IO ()
> main =
> readFile "somefile.txt" >>= (\s ->
> putStrLn (show (doSplit s)))
>
> I highly recommend reading "Learn You A Haskell" book
> http://learnyouahaskell.com/ , which explained these concepts really well
> to me.
>
> On Mon, May 25, 2015 at 9:44 AM, Dananji Liyanage <[email protected]>
> wrote:
>
>> Hi All,
>>
>> I'm writing a code, where the input is read from a text file using:
>> readValues = readFile "Input.txt"
>>
>> Since the type of this is 'IO String', I can't use this in the
>> consequent functions.
>>
>> For an example: I want to split this as follows within another function
>>
>> extractInput url method template
>> | isURI url == True = getList values components
>> | otherwise = []
>> where components = splitTemplate readValues
>> values = getURL (splitURL url) method
>>
>> This gives the following error:
>>
>> Couldn't match type ?IO String? with ?[Char]?
>> Expected type: String
>> Actual type: IO String
>>
>> How can I solve this?
>>
>> Thanks in advance!
>>
>> --
>> Regards,
>> Dananji Liyanage
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
--
Regards,
Dananji Liyanage
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