Add common utility variants of trace to Debug.Trace
As discussed on the libraries list.
'traceId' traces a value, and then returns that same value. This avoids the common case of 'trace a a'.
traceId :: String -> String traceId a = trace a a
'traceM' runs 'trace' and returns unit in an arbitrary monad. This is useful for writing traces in do-notation, as each can sit on their own line.
traceM :: (Monad m) => String -> m () traceM string = trace string $ return ()
... = do x <- ... traceM $ "x: " ++ show x y <- ... ...
That application of 'traceM' there can be temporarily commented out with a simple line comment.
Finally, equivalents of 'traceShow' for those two functions.
traceShowId :: (Show a) => a -> a traceShowId a = trace (show a) a
traceShowM :: (Show a, Monad m) => a -> m () traceShowM = traceM . show
I use these functions in my code all the time. When I discussed it on the libraries list there were several enthusiastic +1s of people doing the same.
The documentation in the patch makes it clear that traceM does not actually add a trace action to the monad you're using, which is a potential point of confusion (http://www.haskell.org/pipermail/libraries/2013-January/019322.html). I had originally suggested that 'traceIO == traceM', but it turns out this is not so.
Henning Thielemann suggested that 'traceM' was better achieved with directly using 'trace' and 'printf' (http://www.haskell.org/pipermail/libraries/2013-January/019320.html). Of course, any function can be replaced with its RHS, and whether or not the user uses 'show' or 'printf' or whatever is orthogonal.
I haven't written any tests, as it's all output generating code, so I'm really not sure how that's tested in GHC - sorry.
Status of patch:
Applies against base 3b51b741f45d4d85e79b2128d00479ce230f72f2 and compiles with GHC 388e1e82. I've run a full GHC compile cycle with it without problem. Includes documentation.