trace -package:Cabal-syntax

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.

Compute the trace of a matrix

>>> trace (V2 (V2 a b) (V2 c d))
a + d

Output a debug message to sterr, using trace, if tracing is enabled. Note: this writes to stderr even in pure instances.

apply a trace arrow and issue message to stderr

Warning: trace remains in code

Warning: trace remains in code

Lens onto the Trace of a QDiagram.

Log a message.

Warning: Trace statement left in code

Internally log what occurred. Log will be re-played on failure.

see trace

Sum of the elements in the diagonal. See also getDiag. Example:

( 1 2 3 )
( 4 5 6 )
trace ( 7 8 9 ) = 15

Warning: Leaving traces in the code

Lens onto the Trace of a QDiagram.

Apply a monadic function to each element flowing through the stream and discard the results.

>>> Stream.drain $ Stream.trace print (Stream.enumerateFromTo 1 2)
1
2

Compare with tap.

Append a message to the trace log.

Warning: trace remains in code

Apply a monadic function to each element flowing through the stream and discard the results.

>>> s = Stream.enumerateFromTo 1 2

>>> Stream.fold Fold.drain $ Stream.trace print s
1
2

Compare with tap.