trace -package:matrix package:base
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.
Functions for tracing and monitoring execution.
These can be useful for investigating bugs or performance problems.
They should not be used in production code.
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.
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.
Like
traceEvent, but emits the result of calling a function on
its argument.
The
traceIO function outputs the trace message from the IO
monad. This sequences the output with respect to other IO actions.
Like
trace but returns the message instead of a third value.
>>> traceId "hello"
hello
"hello"
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
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.
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.
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
Like
traceShow but returns the shown value instead of a third
value.
>>> traceShowId (1+2+3, "hello" ++ "world")
(6,"helloworld")
(6,"helloworld")
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
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]
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.
Like
trace, but outputs the result of calling a function on the
argument.
>>> traceWith fst ("hello","world")
hello
("hello","world")
Emits an event via the RTS tracing framework. The contents of the
event is the binary object passed as the first argument with the given
length passed as the second argument. The event will be emitted to the
.eventlog file.
Emits an event via the RTS tracing framework. The contents of the
event is the zero-terminated byte string passed as the first argument.
The event will be emitted either to the .eventlog file, or to
stderr, depending on the runtime RTS flags.
Emits a marker event via the RTS tracing framework. The contents of
the event is the zero-terminated byte string passed as the first
argument. The event will be emitted either to the .eventlog
file, or to stderr, depending on the runtime RTS flags.
trace nonmoving GC heap census samples