[Haskell-cafe] how can I do this the best

[Konstantine Rybnikov]

As Alex mentioned, isValid returns Bool, while type for putStrLn is `String
-> IO ()`. So, in order to print something of type Bool, you need to first
convert it to String. For example, via a function `show`:

putStrLn (show True)

As Alex mentioned, there's a `print` function, which does exactly this:

print x = putStrLn (show x)

You can use it.

On Mon, Feb 23, 2015 at 7:19 PM, Roelof Wobben <r.wobben at home.nl> wrote:

>  And when Im trying this:
>
> {-# OPTIONS_GHC -Wall #-}
>
> module LogAnalysis where
>
> import Log;
> import Data.Char (isLetter, isDigit)
>
> isValid :: String -> Bool
> isValid s = go (words s)
>     where
>       go ([a]:b:_) = isLetter a && all isDigit b
>       go _         = False
>
>
> -- | The main entry point.
> main :: IO ()
> main = do
>     putStrLn $ isValid "I 4764 He trusts to you to set them free,"
>
>
> I see this error message :
>
> src/LogAnalysis.hs at 19:16-19:67
> Couldn't match type
> Bool
>  with
> [Char]
>  Expected type: String Actual type: Bool … In the second argument of
> ‘($)’, namely ‘isValid "I 4764 He trusts to you to set them free,"’ In a
> stmt of a 'do' block: putStrLn $ isValid "I 4764 He trusts to you to set
> them free,"
>
> Roelof
>
>
>
>
>
> Roelof Wobben schreef op 23-2-2015 om 17:19:
>
> I tried it another way more like explained on this page :
> http://www.seas.upenn.edu/~cis194/spring13/lectures/02-ADTs.html
>
> so I tried this :
>
> parseMessage :: [Char] -> [Char]
> parseMessage s
> case Errornumber of
>     IsDigit Errornumber  -> "Geldige string"
>     otherwise            -> "Ongeldige string"
>   where
>       Error = s words
>       Errornumber = Error(ErrorNumber _ _ )
>       Errorcode = Error(_ Errorcode _ )
>
> but now I cannot use where :(
>
> Roelof
>
>
>
>
> Roelof Wobben schreef op 23-2-2015 om 16:10:
>
> Oke,
>
> Then I make there a mistake,
>
> What I try to do is to send the file to parseMessage and let IsValid check
> if it´s have the right format.
>
> Then after the where I try to check if the function isValid returns true
> or false.
>
> Roelof
>
>
> Konstantine Rybnikov schreef op 23-2-2015 om 16:03:
>
>   Roelof,
>
>  You defined isValid function in the upper-scope first, and then you
> defined a symbol (variable) that re-wrote that name to something different
> (string "Geldige string"). That's why you get an error saying it doesn't
> expect arguments.
>
>  My suggestion is to rename second isValid.
>
>  Good luck.
>
> On Mon, Feb 23, 2015 at 4:50 PM, Roelof Wobben <r.wobben at home.nl> wrote:
>
>>  Chaddaï Fouché schreef op 23-2-2015 om 13:20:
>>
>>  Note that Roelof is doing the CIS 194 Homework
>> http://www.seas.upenn.edu/~cis194/fall14/hw/03-ADTs.pdf (the older
>> version of fall2014, not the one currently running). This is much clearer
>> than Roelof's description, and gives among other information an algebraic
>> datatype to represent log messages.
>>
>>
>> --
>>  Jedaï
>>
>>
>>  Correct and Im trying to do exercise 1 of Week 2,
>>
>> I have tried this solution :
>>
>> -- | Main entry point to the application.
>> {-# OPTIONS_GHC -Wall #-}
>>
>> module LogAnalysis where
>>
>> import Log;
>> import Data.Char (isLetter, isDigit)
>>
>> isValid :: [Char] -> Bool
>> isValid s = go (words s)
>>     where
>>       go ([a]:b:_) = isLetter a && all isDigit b
>>       go _         = False
>>
>> parseMessage :: [Char] -> [Char]
>> parseMessage s = isValid s
>>     where
>>         isValid = "Geldige string"
>>         _       = "Ongeldige string"
>>
>> -- | The main entry point.
>> main :: IO ()
>> main = do
>>     putStrLn $ parseMessage "I 4764 He trusts to you to set them free,"
>>
>>
>> but I see this error message :
>>
>> src/LogAnalysis.hs at 16:18-16:27
>> Couldn't match expected type ‘[Char] -> [Char]’ with actual type
>> [Char]
>>  The function
>> isValid
>>  is applied to one argument, but its type
>> [Char]
>>  has none … In the expression: isValid s In an equation for
>> ‘parseMessage’: parseMessage s = isValid s where isValid = "Geldige string"
>> _ = "Ongeldige string"
>>
>>
>>
>>
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>>
>>
>
>
>
>
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