Is traceIO unnecessarily specialised to the IO monad?

[Chris Seaton]

What do you call these functions? I'll put them all into a patch and open a
feature request for them all in one go.

Chris


On 21 January 2013 22:40, Roman Cheplyaka <roma at ro-che.info> wrote:

> 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
> > >
> > >
> > >
> > >
> > >_______________________________________________
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> > >
> >
> > --
> > 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/
> >
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>
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