GHC.Internal.Debug.Trace

ghc-internal-9.1400.0: 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

GHC.Internal.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
- traceWith :: (a -> String) -> a -> a
- traceShowWith :: Show b => (a -> b) -> 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
- traceEventWith :: (a -> String) -> a -> a
- traceEventIO :: String -> IO ()
- flushEventLog :: IO ()
- traceMarker :: String -> a -> a
- traceMarkerIO :: String -> IO ()

# 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` and outputs the message to stderr. Depending on your terminal (settings), they may or may not be mixed.

    >>> 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 values of the variables `x` and `y`:

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

Note in this example we also create simple labels just by including some strings.

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

traceWith :: (a -> String) -> a -> a Source #

Like `trace`, but outputs the result of calling a function on the argument.

    >>> traceWith fst ("hello","world")
    hello
    ("hello","world")

Since: base-4.18.0.0

traceShowWith :: Show b => (a -> b) -> a -> a Source #

Like `traceWith`, but uses `show` on the result of the function to convert it to a `String`.

    >>> traceShowWith length [1,2,3]
    3
    [1,2,3]

Since: base-4.18.0.0

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 #

Deprecated: Use `traceIO`

# Eventlog tracing

traceEvent :: String -> a -> a Source #

The `traceEvent` function behaves like `trace` with the difference that the message is emitted to the eventlog, if eventlog tracing is available and user event tracing is 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

traceEventWith :: (a -> String) -> a -> a Source #

Like `traceEvent`, but emits the result of calling a function on its argument.

Since: base-4.18.0.0

traceEventIO :: String -> IO () Source #

The `traceEventIO` function emits a message to the eventlog, if eventlog tracing is available and user event tracing is enabled at runtime.

Compared to `traceEvent`, `traceEventIO` sequences the event with respect to other IO actions.

Since: base-4.5.0.0

flushEventLog :: IO () Source #

Immediately flush the event log, if enabled.

Since: base-4.15.0.0

# Execution phase markers

traceMarker :: String -> a -> a Source #

The `traceMarker` function emits a marker to the eventlog, if eventlog tracing 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 tracing is available and user event tracing is enabled at runtime.

Compared to `traceMarker`, `traceMarkerIO` sequences the event with respect to other IO actions.

Since: base-4.7.0.0