| Portability | portable | 
|---|---|
| Stability | provisional | 
| Maintainer | libraries@haskell.org | 
| Safe Haskell | Unsafe | 
Debug.Trace
Contents
Description
Functions for tracing and monitoring execution.
These can be useful for investigating bugs or performance problems. They should not be used in production code.
- trace :: String -> a -> a
- traceShow :: Show a => a -> b -> b
- traceStack :: String -> a -> a
- traceIO :: String -> IO ()
- putTraceMsg :: String -> IO ()
- traceEvent :: String -> a -> a
- traceEventIO :: String -> IO ()
Tracing
The trace, traceShow and traceIO functions print messages to an output
 stream. They are intended for "printf debugging", that is: tracing the flow
 of execution and printing interesting values.
The trace function outputs the trace message given as its first argument,
before returning the second argument as its result.
For example, this returns the value of f x but first outputs the message.
 trace ("calling f with x = " ++ show x) (f x)
The trace function should only be used for debugging, or for monitoring
execution. The function is not referentially transparent: its type indicates
that it is a pure function but it has the side effect of outputting the
trace message.
traceStack :: String -> a -> a
like trace, but additionally prints a call stack if one is
 available.
In the current GHC implementation, the call stack is only
 availble if the program was compiled with -prof; otherwise
 traceStack behaves exactly like trace.  Entries in the call
 stack correspond to SCC annotations, so it is a good idea to use
 -fprof-auto or -fprof-auto-calls to add SCC annotations automatically.
The traceIO function outputs the trace message from the IO monad.
 This sequences the output with respect to other IO actions.
putTraceMsg :: String -> IO ()
Deprecated: Use Debug.Trace.traceIO
Deprecated. Use traceIO.
Eventlog tracing
Eventlog tracing is a performance profiling system. These functions emit extra events into the eventlog. In combination with eventlog profiling tools these functions can be used for monitoring execution and investigating performance problems.
Currently only GHC provides eventlog profiling, see the GHC user guide for details on how to use it. These function exists for other Haskell implementations but no events are emitted. Note that the string message is always evaluated, whether or not profiling is available or enabled.
traceEvent :: String -> a -> a
The traceEvent function behaves like trace with the difference that
 the message is emitted to the eventlog, if eventlog profiling is available
 and enabled at runtime.
It is suitable for use in pure code. In an IO context use traceEventIO
 instead.
Note that when using GHC's SMP runtime, it is possible (but rare) to get
 duplicate events emitted if two CPUs simultaneously evaluate the same thunk
 that uses traceEvent.
traceEventIO :: String -> IO ()
The traceEventIO function emits a message to the eventlog, if eventlog
 profiling is available and enabled at runtime.
Compared to traceEvent, traceEventIO sequences the event with respect to
 other IO actions.