[web-devel] Error logging in Yesod

[Sven Koschnicke (GFXpro)]

Hello,

I would add a log level "Debug" which only gets printed to the log when 
in development mode. I also found the log output of SQL queries 
generated by the database layer quite helpful in development. Another 
thing is performance measurements, the time it takes to answer an 
request. These ideas are all coming from the way RoR does logging, but I 
found it very helpful when developing with Rails, so it should be a nice 
feature in Yesod, too. Here is an example:

   Instrument Load (0.4ms)   SELECT * FROM "instruments" WHERE 
("instruments"."id" = 86) ORDER BY LOWER(instruments.name)
Rendered overview/_notification (2.1ms)
   CACHE (0.0ms)   SELECT * FROM "instruments" WHERE ("instruments"."id" 
= 86) ORDER BY LOWER(instruments.name)
Rendered overview/_notification (1.5ms)
   CACHE (0.0ms)   SELECT * FROM "instruments" WHERE ("instruments"."id" 
= 90) ORDER BY LOWER(instruments.name)
Rendered overview/_notification (1.4ms)
   CACHE (0.0ms)   SELECT * FROM "instruments" WHERE ("instruments"."id" 
= 62) ORDER BY LOWER(instruments.name)
Rendered overview/_notification (1.5ms)
   DepotGroup Load (27.2ms)   SELECT * FROM "depot_groups" ORDER BY ordinal
Completed in 3530ms (View: 166, DB: 2051) | 200 OK [http://localhost/]


Sven

On 04/01/2011 08:20 AM, Michael Snoyman wrote:
> Hi all,
>
> Hot on the heels of the decision to remove logging from WAI, I would
> like to add it to Yesod. I think (at least to start with), we really
> need two modifications: a new method in the Yesod typeclass that
> specifies what to do with log messages, and a function living in the
> Handler monad that calls that method. I do not have any strong
> opinions on this topic, so I would appreciate input. Here's a strawman
> proposal:
>
> data LogLevel = LevelInfo | LevelWarn | LevelError | LevelOther Text
>
> class Yesod a where
>      messageLogger :: a ->  LogLevel ->  Text ->  IO ()
>      messageLogger _ level msg = formatLogMessage level msg>>=
> Data.Text.Lazy.IO.hPutStrLn System.IO.stderr
>
> formatLogMessage :: LogLevel ->  Text ->  IO Text
> formatLogMessage = ...
> -- print timestamp, log level, message, maybe process ID? to stderr
> -- make sure to expose the
>
> logInfo :: MonadIO m =>  Text ->  GGHandler sub master m ()
> logInfo t = do
>      y<  getYesod
>      liftIO $ messageLogger y LevelInfo t
>
> ... and so on and so forth
>
> This should give completely flexibility to users to send their logs
> anywhere (database, printer, missiles) and in whatever format they
> want.
>
> Other ideas:
>
> * It would be nice to include file/line number information, but I'm
> not sure how easy/possible that will be without requiring the user to
> manually enter CPP macros.
> * Actually, that was my only other idea :)
>
> Michael
>
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