Debug.Trace

base-4.11.1.0: Basic libraries

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

Debug.Trace

Contents

- Tracing
- Eventlog tracing
- Execution phase markers

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.

Synopsis

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

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 #

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 #

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 #

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 #

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 #

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 () #

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 () #

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 () #

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 () #

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`.

Since: base-4.5.0.0

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.

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 #

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 () #

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