How to read logs during program execution ?

[jfaure]

In order notably to return logging information if a client requests it

[StaelTchinda]

Hi @jfaure, if you're still looking for an answer, I found out that you can use handlers to either print it in the console or save it in a file. Here is an example from src/System/Log/Logger.hs

 import System.Log.Logger
 import System.Log.Handler.Syslog
 import System.Log.Handler.Simple
 import System.Log.Handler (setFormatter)
 import System.Log.Formatter

 -- By default, all messages of level WARNING and above are sent to stderr.
 -- Everything else is ignored.

 -- "MyApp.Component" is an arbitrary string; you can tune
 -- logging behavior based on it later.
 main = do
        debugM "MyApp.Component"  "This is a debug message -- never to be seen"
        warningM "MyApp.Component2" "Something Bad is about to happen."

        -- Copy everything to syslog from here on out.
        s <- openlog "SyslogStuff" [PID] USER DEBUG
        updateGlobalLogger rootLoggerName (addHandler s)

        errorM "MyApp.Component" "This is going to stderr and syslog."

        -- Now we'd like to see everything from BuggyComponent
        -- at DEBUG or higher go to syslog and stderr.
        -- Also, we'd like to still ignore things less than
        -- WARNING in other areas.
        --
        -- So, we adjust the Logger for MyApp.BuggyComponent.

        updateGlobalLogger "MyApp.BuggyComponent"
                           (setLevel DEBUG)

        -- This message will go to syslog and stderr
        debugM "MyApp.BuggyComponent" "This buggy component is buggy"

        -- This message will go to syslog and stderr too.
        warningM "MyApp.BuggyComponent" "Still Buggy"

        -- This message goes nowhere.
        debugM "MyApp.WorkingComponent" "Hello"

        -- Now we decide we'd also like to log everything from BuggyComponent at DEBUG
        -- or higher to a file for later diagnostics.  We'd also like to customize the
        -- format of the log message, so we use a 'simpleLogFormatter'

        h <- fileHandler "debug.log" DEBUG >>= \lh -> return $
                 setFormatter lh (simpleLogFormatter "[$time : $loggername : $prio] $msg")
        updateGlobalLogger "MyApp.BuggyComponent" (addHandler h)

        -- This message will go to syslog and stderr,
        -- and to the file "debug.log" with a format like :
        -- [2010-05-23 16:47:28 : MyApp.BuggyComponent : DEBUG] Some useful diagnostics...
        debugM "MyApp.BuggyComponent" "Some useful diagnostics..."

With a bit of reverse engineering, I think you can do it too ;)