[GHC] #7626: Add common utility variants of trace to Debug.Trace

25 Jan 2013

      #7626: Add common utility variants of trace to Debug.Trace
-----------------------------+----------------------------------------------
Reporter:  chrisseaton       |          Owner:                  
    Type:  feature request   |         Status:  new             
Priority:  normal            |      Component:  libraries/base  
 Version:  7.6.1             |       Keywords:  trace           
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------
 As discussed on the libraries list.

 http://www.haskell.org/pipermail/libraries/2013-January/019319.html

 '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 ()
 }}}

 For example:

 {{{
     ... = 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
 }}}

 Advantages:

 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.

 Disadvantages:

 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 388e1e825f79f2d16536fc583a48e5ce9c191b06. I've run a full GHC
 compile cycle with it without problem. Includes documentation.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7626
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