Thanks,

I see the problem.
I m trying to push too much in a function.

I see that in Haskell that a function does really 1 thing so I function for checking the format and another for making the right output.

Roelof

Alex Hammel schreef op 23-2-2015 om 21:52:
Perhaps you could split up the format checking and the unpacking into two different functions:

parseMessage :: String -> String
parseMessage s = go (words s)
    where
        go log = if isValid log
                 then parse log
                 else "false"

        parse ("I":time:msg) = "Info"
        parse ("W":time:msg) = -- etc.


On Mon, Feb 23, 2015 at 12:39 PM, Roelof Wobben <r.wobben@home.nl> wrote:
Oke,

But then I do not check on the I 
Sorry for being difficult.

Roelof



Alex Hammel schreef op 23-2-2015 om 21:33:
For that you need a guard:

parseMessage :: String -> String
parseMessage s = go (words s)
    where
        go logMessage
            | isValid logMessage = doSomething
            | otherwise          = "false"


On Mon, Feb 23, 2015 at 12:30 PM, Roelof Wobben <r.wobben@home.nl> wrote:
Thanks,

Can I somehow also test if the string is valid so that the isvalid  function is used.

Roelof

Alex Hammel schreef op 23-2-2015 om 21:23:
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@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@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@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@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@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@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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