#9795: Debug.Trace.trace is too strict -------------------------------------+------------------------------------- Reporter: jcpetruzza | Owner: Type: bug | Status: patch Priority: normal | Milestone: 7.12.1 Component: libraries/base | Version: 7.8.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: Phab:D654 -------------------------------------+------------------------------------- Comment (by simonpj): Thomie is right, and Reid is (very unusually) off the mark, unless I'm mistaken. `trace` really is defined (in `Debug.Trace` like this), just as the OP thought: {{{ trace :: String -> a -> a trace string expr = unsafePerformIO $ do traceIO string return expr }}} The call to `traceIO` really does happen before `expr` is evaluated. The culprit is `traceIO`: {{{ 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 }}} I'm not quite sure what those calls to `withCString` are doing, but I think they are strict in the string. So if the message has bottoms in it (which is the case in this example) none of it will get printed. That is not really too bad; although it should be documented. After all, you can always write {{{ trace ("in: " ++ show n) $ trace ("out: " ++ show res) $ res }}} and now you should see the first message even if you get stuck on the second. My conclusion: fix the documentation. I'll add this to the documentation for `trace`: {{{ The 'trace' function evaluates the message (i.e. the first argument) completely before printing it; so if the message is not fully defined, none of it will be printed. }}} OK? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9795#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler