Text.Show.Text.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.

Since: 0.5

Documentation

traceIO :: Text -> IO ()

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

Since: 0.5

traceIOLazy :: Text -> IO ()

Like traceIO but accepts a lazy Text argument.

Since: 0.5

trace :: Text -> a -> a

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.

Since: 0.5

traceLazy :: Text -> a -> a

Like trace but accepts a lazy Text argument.

Since: 0.5

traceId :: Text -> Text

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

Since: 0.5

traceIdLazy :: Text -> Text

Like traceId but accepts a lazy Text argument.

Since: 0.5

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

Like trace, but uses show 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 =
    traceShow (x, z) $ result
  where
    z = ...
    ...

Since: 0.5

traceShowId :: Show a => a -> a

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

Since: 0.5

traceM :: Monad m => Text -> m ()

Like trace but returning unit in an arbitrary monad. Allows for convenient use in do-notation. Note that the application of trace is not an action in the monad, as traceIO is in the IO monad.

... = do
  x <- ...
  traceM $ "x: " <> show x
  y <- ...
  traceM $ "y: " <> show y

Since: 0.5

traceMLazy :: Monad m => Text -> m ()

Like traceM but accepts a lazy Text argument.

traceShowM :: (Show a, Monad m) => a -> m ()

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

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

Since: 0.5

traceStack :: Text -> 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.

Since: 0.5

traceStackLazy :: Text -> a -> a

Like traceStack but accepts a lazy Text argument.

Since: 0.5

traceEvent :: Text -> 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.

Since: 0.5

traceEventLazy :: Text -> a -> a

Like traceEvent but accepts a lazy Text argument.

Since: 0.5

traceEventIO :: Text -> 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.

Since: 0.5

traceEventIOLazy :: Text -> IO ()

Like traceEventIO but accepts a lazy Text argument.

Since: 0.5

traceMarker :: Text -> a -> a

The traceMarker 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 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: 0.5

traceMarkerLazy :: Text -> a -> a

Like traceMarker but accepts a lazy Text argument.

Since: 0.5

traceMarkerIO :: Text -> IO ()

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: 0.5

traceMarkerIOLazy :: Text -> IO ()

Like traceMarkerIO but accepts a lazy Text argument.

Since: 0.5