[Haskell-cafe] how can I do this the best
Alex Hammel
ahammel87 at gmail.com
Mon Feb 23 20:23:12 UTC 2015
You can pattern match on string literals:
parseMessage :: String -> String
parseMessage s = go (words s)
where
go ("I":_:_) = "Info"
go _ = "false"
You don't need to call it "go2", btw. Functions in where-blocks don't leak
into the global scope, so there's no conflict with the `go` in your other
function.
On Mon, Feb 23, 2015 at 12:15 PM, Roelof Wobben <r.wobben at home.nl> wrote:
> Sorry ,
>
> I cannot change that function. I have to use it,
>
> But I think I found a solution for a step earlier,
>
> To parse a line and change it to another line which will be a member of
> LogMessage :
>
> -- | 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
>
> parseMessage :: String -> String
> parseMessage s = go2 (words s)
> where
> go2 (a:_:_) = case a of
> "I" -> "Info "
> _ -> "False"
>
> go2 _ = "false"
>
>
> -- | The main entry point.
> main :: IO ()
> main = do
> putStrLn $ parseMessage "I 656 He trusts to you to set them free,"
>
>
>
> But I feels wierd, to use first a pattern matching and later do a case of.
> Is this a good way or are there better ways,
>
> Roelfo
>
>
>
> Alex Hammel schreef op 23-2-2015 om 21:01:
>
> 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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>>>>
>>>>
>>>
>>>
>>>
>>>
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>>
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