Copyright	(c) The University of Glasgow 2001
License	BSD-style (see the file libraries/base/LICENSE)
Maintainer	[email protected]
Stability	provisional
Portability	portable
Safe Haskell	Unsafe
Language	Haskell2010

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.

All these functions evaluate the message completely before printing it; so if the message is not fully defined, none of it will be printed.

The usual output stream is stderr. For Windows GUI applications (that have no stderr) the output is directed to the Windows debug console. Some implementations of these functions may decorate the string that's output to indicate that you're tracing.

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.

>>> let x = 123; f = show
>>> trace ("calling f with x = " ++ show x) (f x)
"calling f with x = 123
123"

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.

>>> traceId "hello"
"hello
hello"

Since: base-4.7.0.0

traceShow :: Show a => a -> b -> b Source

Like trace, but uses show on the argument to convert it to a String.

This makes it convenient for printing the values of interesting variables or expressions inside a function. For example here we print the value of the variables x and y:

>>> let f x y = traceShow (x,y) (x + y) in f (1+2) 5
(3,5)
8

traceShowId :: Show a => a -> a Source

Like traceShow but returns the shown value instead of a third value.

>>> traceShowId (1+2+3, "hello" ++ "world")
(6,"helloworld")
(6,"helloworld")

Since: base-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 available 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: base-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: base-4.5.0.0

traceM :: Applicative f => String -> f () Source

Like trace but returning unit in an arbitrary Applicative context. Allows for convenient use in do-notation.

Note that the application of traceM is not an action in the Applicative context, as traceIO is in the IO type. While the fresh bindings in the following example will force the traceM expressions to be reduced every time the do-block is executed, traceM "not crashed" would only be reduced once, and the message would only be printed once. If your monad is in MonadIO, liftIO . traceIO may be a better option.

>>> :{
do
    x <- Just 3
    traceM ("x: " ++ show x)
    y <- pure 12
    traceM ("y: " ++ show y)
    pure (x*2 + y)
:}
x: 3
y: 12
Just 18

Since: base-4.7.0.0

traceShowM :: (Show a, Applicative f) => a -> f () Source

Like traceM, but uses show on the argument to convert it to a String.

>>> :{
do
    x <- Just 3
    traceShowM x
    y <- pure 12
    traceShowM y
    pure (x*2 + y)
:}
3
12
Just 18

Since: base-4.7.0.0

putTraceMsg :: String -> IO () Source

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: base-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: base-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: base-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: base-4.7.0.0