Control.Monad.Supervisor.Trace

This module add a MonadLoc instance to the Supervisor monad. This instance generates a trace when a uncaugh exception is raised.

. See the MonadLoc package: http://hackage.haskell.org/package/monadloc

The package control-monad-exception produces call stacks using monadloc, but the Supervisor monad produces execution traces thanks to the backtracking mechanism.

The trace is produced after the exception is raised. So it does not generate overhead in normal execution.

For more finer control of exceptions, ej. for retrowing exceptions managed outside the Supervisor monad , create your own instance

Execute the example at Demos/TraceExample.hs

 {-# OPTIONS -F -pgmF MonadLoc #-}
 module Demos.TraceExample (

 ) where

 import Control.Monad.Loc
 import Control.Monad.Supervisor.Trace
 import Control.Monad.Trans

 main= runTrace $ do
    liftIO $ print "hello"

    example

 example=
    if True
       then  do
               liftIO $ print "world"
               liftIO $ undefined

       else liftIO $ print "not there"

Produce this trace:

  "hello"
  "world"
  TraceExample.hs: TRACE (error in the last line):
  .
  main, Demos.TraceExample(DemosTraceExample.hs): (23, 18)
  main, Demos.TraceExample(DemosTraceExample.hs): (26, 4)
  example, Demos.TraceExample(DemosTraceExample.hs): (30, 13)
  example, Demos.TraceExample(DemosTraceExample.hs): (32, 15)
  exception: Prelude.undefined

Documentation

runTrace :: Supervise [String] m => Sup [String] m a -> m (Control a)

Execute an Supervisor computation and raise an error with a trace when an uncaugh exception is raised. It is necessary to preprocess the file with the monadloc-pp preprocessor.

Otherwise, it produces the same error with no trace.