TextShow.Debug.Trace

Functions for tracing and monitoring execution.

These can be useful for investigating bugs or performance problems. They should not be used in production code.

If you do not wish to require TextShow instances for your trace functions, the TextShow.Debug.Trace.TH and Text.Show.Text.Debug.Trace.Generic modules exist to convert the input to a debug message using Template Haskell or generics, respectively.

Since: 2

Tracing

The tracet(l), traceTextShow and tracet(l)IO 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 Text that's output to indicate that you're tracing.

tracet :: Text -> a -> a

The tracet 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.

tracet ("calling f with x = " <> showt x) (f x)

The tracet 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.

Since: 2

tracetl :: Text -> a -> a

Like tracet but accepts a lazy Text argument.

Since: 2

tracetId :: Text -> Text

Like tracet but returns the message instead of a third value.

Since: 2

tracetlId :: Text -> Text

Like tracetId but accepts a lazy Text argument.

Since: 2

traceTextShow :: TextShow a => a -> b -> b

Like tracet, but uses showt on the argument to convert it to a Text.

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 z:

f x y =
    traceTextShow (x, z) $ result
  where
    z = ...
    ...

Since: 2

traceTextShowId :: TextShow a => a -> a

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

Since: 2

tracetStack :: Text -> a -> a

Like tracet 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 tracetStack behaves exactly like tracet. 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: 2

tracetlStack :: Text -> a -> a

Like tracetStack but accepts a lazy Text argument.

Since: 2

tracetIO :: Text -> IO ()

The tracetIO function outputs the trace message from the IO monad. This sequences the output with respect to other IO actions.

Since: 2

tracetlIO :: Text -> IO ()

Like tracetIO but accepts a lazy Text argument.

Since: 2

tracetM :: Applicative f => Text -> f ()

Like tracet but returning unit in an arbitrary Applicative context. Allows for convenient use in do-notation. Note that the application of tracet is not an action in the Applicative context, as tracetIO is in the IO type.

... = do
  x <- ...
  tracetM $ "x: " <> showt x
  y <- ...
  tracetM $ "y: " <> showt y

Since: 2

tracetlM :: Applicative f => Text -> f ()

Like tracetM but accepts a lazy Text argument.

traceTextShowM :: (TextShow a, Applicative f) => a -> f ()

Like tracetM, but uses showt on the argument to convert it to a Text.

... = do
  x <- ...
  traceTextShowM x
  y <- ...
  traceTextShowM $ x + y

Since: 2

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 Text message is always evaluated, whether or not profiling is available or enabled.

tracetEvent :: Text -> a -> a

The tracetEvent function behaves like tracet 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 tracetEventIO 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: 2

tracetlEvent :: Text -> a -> a

Like tracetEvent but accepts a lazy Text argument.

Since: 2

tracetEventIO :: Text -> IO ()

The tracetEventIO function emits a message to the eventlog, if eventlog profiling is available and enabled at runtime.

Compared to tracetEvent, tracetEventIO sequences the event with respect to other IO actions.

Since: 2

tracetlEventIO :: Text -> IO ()

Like tracetEventIO but accepts a lazy Text argument.

Since: 2

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.

tracetMarker :: Text -> a -> a

The tracetMarker function emits a marker to the eventlog, if eventlog profiling is available and enabled at runtime. The Text 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 tracetMarkerIO 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: 2

tracetlMarker :: Text -> a -> a

Like tracetMarker but accepts a lazy Text argument.

Since: 2

tracetMarkerIO :: Text -> IO ()

The tracetMarkerIO function emits a marker to the eventlog, if eventlog profiling is available and enabled at runtime.

Compared to tracetMarker, tracetMarkerIO sequences the event with respect to other IO actions.

Since: 2

tracetlMarkerIO :: Text -> IO ()

Like tracetMarkerIO but accepts a lazy Text argument.

Since: 2