{-# LANGUAGE Unsafe #-} {-# LANGUAGE MagicHash, UnboxedTuples #-} ----------------------------------------------------------------------------- -- | -- Module : Debug.Trace -- Copyright : (c) The University of Glasgow 2001 -- License : BSD-style (see the file libraries/base/LICENSE) -- -- Maintainer : libraries@haskell.org -- Stability : provisional -- Portability : portable -- -- Functions for tracing and monitoring execution. -- -- These can be useful for investigating bugs or performance problems. -- They should /not/ be used in production code. -- ----------------------------------------------------------------------------- module Debug.Trace ( -- * Tracing -- $tracing trace, traceId, traceShow, traceShowId, traceStack, traceIO, traceM, traceShowM, putTraceMsg, -- * Eventlog tracing -- $eventlog_tracing traceEvent, traceEventIO, -- * Execution phase markers -- $markers traceMarker, traceMarkerIO, ) where import Prelude import System.IO.Unsafe import Control.Monad import Foreign.C.String import GHC.Base import qualified GHC.Foreign import GHC.IO.Encoding import GHC.Ptr import GHC.Stack import Data.List -- $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. -- The usual output stream is 'System.IO.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. -- | The 'traceIO' function outputs the trace message from the IO monad. -- This sequences the output with respect to other IO actions. -- -- /Since: 4.5.0.0/ traceIO :: String -> IO () traceIO msg = do withCString "%s\n" $ \cfmt -> do -- NB: debugBelch can't deal with null bytes, so filter them -- out so we don't accidentally truncate the message. See Trac #9395 let (nulls, msg') = partition (=='\0') msg withCString msg' $ \cmsg -> debugBelch cfmt cmsg when (not (null nulls)) $ withCString "WARNING: previous trace message had null bytes" $ \cmsg -> debugBelch cfmt cmsg -- don't use debugBelch() directly, because we cannot call varargs functions -- using the FFI. foreign import ccall unsafe "HsBase.h debugBelch2" debugBelch :: CString -> CString -> IO () -- | putTraceMsg :: String -> IO () putTraceMsg = traceIO {-# DEPRECATED putTraceMsg "Use 'Debug.Trace.traceIO'" #-} -- deprecated in 7.4 {-# NOINLINE trace #-} {-| 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. > trace ("calling f with x = " ++ show x) (f x) 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. -} trace :: String -> a -> a trace string expr = unsafePerformIO $ do traceIO string return expr {-| Like 'trace' but returns the message instead of a third value. /Since: 4.7.0.0/ -} traceId :: String -> String traceId a = trace a a {-| 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 @z@: > f x y = > traceShow (x, z) $ result > where > z = ... > ... -} traceShow :: (Show a) => a -> b -> b traceShow = trace . show {-| Like 'traceShow' but returns the shown value instead of a third value. /Since: 4.7.0.0/ -} traceShowId :: (Show a) => a -> a traceShowId a = trace (show a) a {-| Like 'trace' but returning unit in an arbitrary monad. Allows for convenient use in do-notation. Note that the application of 'trace' is not an action in the monad, as 'traceIO' is in the 'IO' monad. > ... = do > x <- ... > traceM $ "x: " ++ show x > y <- ... > traceM $ "y: " ++ show y /Since: 4.7.0.0/ -} traceM :: (Monad m) => String -> m () traceM string = trace string $ return () {-| Like 'traceM', but uses 'show' on the argument to convert it to a 'String'. > ... = do > x <- ... > traceMShow $ x > y <- ... > traceMShow $ x + y /Since: 4.7.0.0/ -} traceShowM :: (Show a, Monad m) => a -> m () traceShowM = traceM . show -- | like 'trace', but additionally prints a call stack if one is -- available. -- -- In the current GHC implementation, the call stack is only -- availble 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: 4.5.0.0/ traceStack :: String -> a -> a traceStack str expr = unsafePerformIO $ do traceIO str stack <- currentCallStack when (not (null stack)) $ traceIO (renderStack stack) return expr -- $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. {-# NOINLINE traceEvent #-} -- | 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: 4.5.0.0/ traceEvent :: String -> a -> a traceEvent msg expr = unsafeDupablePerformIO $ do traceEventIO msg return expr -- | 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: 4.5.0.0/ traceEventIO :: String -> IO () traceEventIO msg = GHC.Foreign.withCString utf8 msg $ \(Ptr p) -> IO $ \s -> case traceEvent# p s of s' -> (# s', () #) -- $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. -- For example, a program might have several distinct phases with different -- performance or resource behaviour in each phase. To properly interpret the -- profile graph we really want to see when each phase starts and ends. -- -- Markers let us do this: we can annotate the program to emit a marker at -- an appropriate point during execution and then see that in a profile. -- -- Currently this feature is only supported in GHC by the eventlog tracing -- system, but in future it may also be supported by the heap profiling or -- other profiling tools. These function exists for other Haskell -- implementations but they have no effect. Note that the string message is -- always evaluated, whether or not profiling is available or enabled. {-# NOINLINE traceMarker #-} -- | 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: 4.7.0.0/ traceMarker :: String -> a -> a traceMarker msg expr = unsafeDupablePerformIO $ do traceMarkerIO msg return expr -- | 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: 4.7.0.0/ traceMarkerIO :: String -> IO () traceMarkerIO msg = GHC.Foreign.withCString utf8 msg $ \(Ptr p) -> IO $ \s -> case traceMarker# p s of s' -> (# s', () #)