Copyright | (c) The University of Glasgow 2001 |
---|---|
License | BSD-style (see the file libraries/base/LICENSE) |
Maintainer | libraries@haskell.org |
Stability | provisional |
Portability | portable |
Safe Haskell | Unsafe |
Language | Haskell2010 |
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
- traceId :: String -> String
- traceShow :: Show a => a -> b -> b
- traceShowId :: Show a => a -> a
- traceStack :: String -> a -> a
- traceIO :: String -> IO ()
- traceM :: Monad m => String -> m ()
- traceShowM :: (Show a, Monad m) => a -> m ()
- putTraceMsg :: String -> IO ()
- traceEvent :: String -> a -> a
- traceEventIO :: String -> IO ()
- traceMarker :: String -> a -> a
- traceMarkerIO :: 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.
trace :: String -> a -> a Source
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.
traceId :: String -> String Source
Like trace
but returns the message instead of a third value.
Since: 4.7.0.0
traceShowId :: Show a => a -> a Source
Like traceShow
but returns the shown value instead of a third value.
Since: 4.7.0.0
traceStack :: String -> a -> a Source
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.
Since: 4.5.0.0
traceIO :: String -> IO () Source
The traceIO
function outputs the trace message from the IO monad.
This sequences the output with respect to other IO actions.
Since: 4.5.0.0
traceShowM :: (Show a, Monad m) => a -> m () Source
putTraceMsg :: String -> IO () Source
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 Source
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
.
Since: 4.5.0.0
traceEventIO :: String -> IO () Source
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.
Since: 4.5.0.0
Execution phase markers
When looking at a profile for the execution of a program we often want to be able to mark certain points or phases in the execution and see that visually in the profile.
traceMarker :: String -> a -> a Source
The traceMarker
function emits a marker to the eventlog, if eventlog
profiling is available and enabled at runtime. The String
is the name of
the marker. The name is just used in the profiling tools to help you keep
clear which marker is which.
This function is suitable for use in pure code. In an IO context use
traceMarkerIO
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 traceMarker
.
Since: 4.7.0.0
traceMarkerIO :: String -> IO () Source
The traceMarkerIO
function emits a marker to the eventlog, if eventlog
profiling is available and enabled at runtime.
Compared to traceMarker
, traceMarkerIO
sequences the event with respect to
other IO actions.
Since: 4.7.0.0