Is traceIO unnecessarily specialised to the IO monad?

[Roman Cheplyaka]

While we're at it, the trace functions I miss are

  \x -> trace x x

and

  \x -> trace (show x) x

Roman

* Andreas Abel <andreas.abel at ifi.lmu.de> [2013-01-21 23:31:24+0100]
> +1.  I also had to define traceM for the same purposes. --Andreas
> 
> On 21.01.13 6:41 PM, Chris Seaton wrote:
> >Yes, I suppose that traceIO does not have the semantics I assumed.
> >Still, I think it is useful to have a trace that one can easily insert
> >into an arbitrary monad. Here's how I use it:
> >
> >--------
> >
> >import Debug.Trace
> >
> >main :: IO ()
> >main = putStrLn $ show foo
> >
> >foo :: Maybe Int
> >foo = do
> >     x <- bar 14
> >     traceM $ show x
> >     y <- bar 2
> >     traceM $ show y
> >     return $ x + y
> >
> >bar :: Int -> Maybe Int
> >bar x = Just $ 2*x
> >
> >traceM :: (Monad m) => String -> m ()
> >traceM message = trace message $ return ()
> >
> >----------
> >
> >I think it is cleaner and more obvious than without the abstraction.
> >Plus it is very easy to comment out. It is really good for list
> >comprehensions written in do notation, as I often want to peek at
> >intermediate values of those. I know I always add it to my projects, so
> >I thought it may be wanted in base.
> >
> >As Henning Thielemann said, you can use printf or whatever with it, but
> >I think that is an orthogonal issue.
> >
> >Regards,
> >
> >Chris
> >
> >
> >
> >On 21 January 2013 17:09, Herbert Valerio Riedel <hvr at gnu.org
> ><mailto:hvr at gnu.org>> wrote:
> >
> >    Chris Seaton <chris at chrisseaton.com <mailto:chris at chrisseaton.com>>
> >    writes:
> >
> >     > I use printf-style debugging a lot, so I am always adding and
> >    removing
> >     > applications of trace. There is the Debug.Trace.traceIO function
> >    that makes
> >     > this easy to do in the IO monad (it just applies hPutStrLn
> >    stderr), but is
> >     > that specialisation to IO unnecessary?
> >     >
> >     > I find myself always using this utility function:
> >     >
> >     > traceM :: (Monad m) => String -> m ()
> >     > traceM message = trace message $ return ()
> >     >
> >     > Which can be used to implement traceIO.
> >     >
> >     > traceIO :: String -> IO ()
> >     > traceIO = traceM
> >
> >    btw, that wouldn't have the same semantics as the existing
> >    `Debug.Trace.traceIO` which is more or less something similiar to a
> >    `hPutStrLn stderr` whose side-effect gets triggered at monad-execution
> >    time, whereas the side-effect of `traceM` occurs at monad-construction
> >    time; consider the following program:
> >
> >    --8<---------------cut here---------------start------------->8---
> >    import Control.Monad
> >    import Debug.Trace
> >
> >    traceM :: (Monad m) => String -> m ()
> >    traceM message = trace message $ return ()
> >
> >    traceIO' :: String -> IO ()
> >    traceIO' = traceM
> >
> >    main = replicateM_ 5 $ do
> >              trace1
> >              trace2
> >       where
> >         trace1 = traceIO' "trace1"
> >         trace2 = traceIO  "trace2"
> >    --8<---------------cut here---------------end--------------->8---
> >
> >    when run via runghc (or compiled with -O0) for GHC 7.6, this emits
> >
> >    --8<---------------cut here---------------start------------->8---
> >    trace1
> >    trace2
> >    trace2
> >    trace2
> >    trace2
> >    trace2
> >    --8<---------------cut here---------------end--------------->8---
> >
> >    only when using -O1 or -O2 the output results in
> >
> >    --8<---------------cut here---------------start------------->8---
> >    trace1
> >    trace2
> >    trace1
> >    trace2
> >    trace1
> >    trace2
> >    trace1
> >    trace2
> >    trace1
> >    trace2
> >    --8<---------------cut here---------------end--------------->8---
> >
> >    (I'm guessing this due to `trace1` being inlined for -O1/-O2 -- but I
> >    haven't checked)
> >
> >    cheers,
> >       hvr
> >
> >
> >
> >
> >_______________________________________________
> >Libraries mailing list
> >Libraries at haskell.org
> >http://www.haskell.org/mailman/listinfo/libraries
> >
> 
> -- 
> Andreas Abel  <><      Du bist der geliebte Mensch.
> 
> Theoretical Computer Science, University of Munich
> Oettingenstr. 67, D-80538 Munich, GERMANY
> 
> andreas.abel at ifi.lmu.de
> http://www2.tcs.ifi.lmu.de/~abel/
> 
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries