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

[Alex Hammel]

Constructors with names like 'Unknown' are a code smell, IMO. I'd just
define the data type like this:

data LogMessage = LogMessage MessageType TimeStamp String

and use `Either String LogMessage` in contexts where parsing can fail:

parseLogEntry :: String -> Either String LogMesage
parseLogEntry str
  | isValid str = Right $ mkLogMessage str
  | otherwise = Left $ "Poorly-formatted log entry: " ++ str

As for implementing mkLogMessage: you already know how to unpack the parts
of a log message with `words` and pattern matching. After that it's just a
matter of type-casting everything correctly and passing it to the
`LogMessage` constructor.

On Mon, Feb 23, 2015 at 10:14 AM, Roelof Wobben <r.wobben at home.nl> wrote:

>  Thanks,
>
> This works  :
>
> -- | Main entry point to the application.
> {-# OPTIONS_GHC -Wall #-}
>
> module LogAnalysis where
>
> 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 $ ( show (isValid "I 656 He trusts to you to set them free,"))
>
>
> Now I have to ty to find out how I can check if a has the contents of
> I/W/E and how to make the right output  (Error/Warning/Info  22)   " Text"
> )
> and then make it work with this datatype :
>
> data LogMessage = LogMessage MessageType TimeStamp String
>                 | Unknown String
>   deriving (Show, Eq)
>
> Roelof
>
> Konstantine Rybnikov schreef op 23-2-2015 om 18:49:
>
>   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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>>> Haskell-Cafe at haskell.org
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>>>
>>>
>>
>>
>>
>>
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