GHC.Utils.Trace

Tracing utilities

Documentation

pprTrace :: String -> SDoc -> a -> a

If debug output is on, show some SDoc on the screen

pprTraceM :: Applicative f => String -> SDoc -> f ()

pprTraceDebug :: String -> SDoc -> a -> a

pprTraceIt :: Outputable a => String -> a -> a

pprTraceIt desc x is equivalent to pprTrace desc (ppr x) x

pprTraceWith :: String -> (a -> SDoc) -> a -> a

pprTraceWith desc f x is equivalent to pprTrace desc (f x) x. This allows you to print details from the returned value as well as from ambient variables.

pprSTrace :: HasCallStack => SDoc -> a -> a

If debug output is on, show some SDoc on the screen along with a call stack when available.

pprTraceException :: ExceptionMonad m => String -> SDoc -> m a -> m a

pprTraceException desc x action runs action, printing a message if it throws an exception.

warnPprTrace :: HasCallStack => Bool -> String -> SDoc -> a -> a

Just warn about an assertion failure, recording the given file and line number.

pprTraceUserWarning :: HasCallStack => SDoc -> a -> a

For when we want to show the user a non-fatal WARNING so that they can report a GHC bug, but don't want to panic.

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 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.