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

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 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 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@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 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@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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