<div dir="ltr">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.<div><br></div><div>Chris</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">
On 21 January 2013 22:40, Roman Cheplyaka <span dir="ltr"><<a href="mailto:roma@ro-che.info" target="_blank">roma@ro-che.info</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
While we're at it, the trace functions I miss are<br>
<br>
\x -> trace x x<br>
<br>
and<br>
<br>
\x -> trace (show x) x<br>
<br>
Roman<br>
<br>
* Andreas Abel <<a href="mailto:andreas.abel@ifi.lmu.de">andreas.abel@ifi.lmu.de</a>> [2013-01-21 23:31:24+0100]<br>
<div class="HOEnZb"><div class="h5">> +1. I also had to define traceM for the same purposes. --Andreas<br>
><br>
> On 21.01.13 6:41 PM, Chris Seaton wrote:<br>
> >Yes, I suppose that traceIO does not have the semantics I assumed.<br>
> >Still, I think it is useful to have a trace that one can easily insert<br>
> >into an arbitrary monad. Here's how I use it:<br>
> ><br>
> >--------<br>
> ><br>
> >import Debug.Trace<br>
> ><br>
> >main :: IO ()<br>
> >main = putStrLn $ show foo<br>
> ><br>
> >foo :: Maybe Int<br>
> >foo = do<br>
> > x <- bar 14<br>
> > traceM $ show x<br>
> > y <- bar 2<br>
> > traceM $ show y<br>
> > return $ x + y<br>
> ><br>
> >bar :: Int -> Maybe Int<br>
> >bar x = Just $ 2*x<br>
> ><br>
> >traceM :: (Monad m) => String -> m ()<br>
> >traceM message = trace message $ return ()<br>
> ><br>
> >----------<br>
> ><br>
> >I think it is cleaner and more obvious than without the abstraction.<br>
> >Plus it is very easy to comment out. It is really good for list<br>
> >comprehensions written in do notation, as I often want to peek at<br>
> >intermediate values of those. I know I always add it to my projects, so<br>
> >I thought it may be wanted in base.<br>
> ><br>
> >As Henning Thielemann said, you can use printf or whatever with it, but<br>
> >I think that is an orthogonal issue.<br>
> ><br>
> >Regards,<br>
> ><br>
> >Chris<br>
> ><br>
> ><br>
> ><br>
> >On 21 January 2013 17:09, Herbert Valerio Riedel <<a href="mailto:hvr@gnu.org">hvr@gnu.org</a><br>
> ><mailto:<a href="mailto:hvr@gnu.org">hvr@gnu.org</a>>> wrote:<br>
> ><br>
> > Chris Seaton <<a href="mailto:chris@chrisseaton.com">chris@chrisseaton.com</a> <mailto:<a href="mailto:chris@chrisseaton.com">chris@chrisseaton.com</a>>><br>
> > writes:<br>
> ><br>
> > > I use printf-style debugging a lot, so I am always adding and<br>
> > removing<br>
> > > applications of trace. There is the Debug.Trace.traceIO function<br>
> > that makes<br>
> > > this easy to do in the IO monad (it just applies hPutStrLn<br>
> > stderr), but is<br>
> > > that specialisation to IO unnecessary?<br>
> > ><br>
> > > I find myself always using this utility function:<br>
> > ><br>
> > > traceM :: (Monad m) => String -> m ()<br>
> > > traceM message = trace message $ return ()<br>
> > ><br>
> > > Which can be used to implement traceIO.<br>
> > ><br>
> > > traceIO :: String -> IO ()<br>
> > > traceIO = traceM<br>
> ><br>
> > btw, that wouldn't have the same semantics as the existing<br>
> > `Debug.Trace.traceIO` which is more or less something similiar to a<br>
> > `hPutStrLn stderr` whose side-effect gets triggered at monad-execution<br>
> > time, whereas the side-effect of `traceM` occurs at monad-construction<br>
> > time; consider the following program:<br>
> ><br>
> > --8<---------------cut here---------------start------------->8---<br>
> > import Control.Monad<br>
> > import Debug.Trace<br>
> ><br>
> > traceM :: (Monad m) => String -> m ()<br>
> > traceM message = trace message $ return ()<br>
> ><br>
> > traceIO' :: String -> IO ()<br>
> > traceIO' = traceM<br>
> ><br>
> > main = replicateM_ 5 $ do<br>
> > trace1<br>
> > trace2<br>
> > where<br>
> > trace1 = traceIO' "trace1"<br>
> > trace2 = traceIO "trace2"<br>
> > --8<---------------cut here---------------end--------------->8---<br>
> ><br>
> > when run via runghc (or compiled with -O0) for GHC 7.6, this emits<br>
> ><br>
> > --8<---------------cut here---------------start------------->8---<br>
> > trace1<br>
> > trace2<br>
> > trace2<br>
> > trace2<br>
> > trace2<br>
> > trace2<br>
> > --8<---------------cut here---------------end--------------->8---<br>
> ><br>
> > only when using -O1 or -O2 the output results in<br>
> ><br>
> > --8<---------------cut here---------------start------------->8---<br>
> > trace1<br>
> > trace2<br>
> > trace1<br>
> > trace2<br>
> > trace1<br>
> > trace2<br>
> > trace1<br>
> > trace2<br>
> > trace1<br>
> > trace2<br>
> > --8<---------------cut here---------------end--------------->8---<br>
> ><br>
> > (I'm guessing this due to `trace1` being inlined for -O1/-O2 -- but I<br>
> > haven't checked)<br>
> ><br>
> > cheers,<br>
> > hvr<br>
> ><br>
> ><br>
> ><br>
> ><br>
> >_______________________________________________<br>
> >Libraries mailing list<br>
> ><a href="mailto:Libraries@haskell.org">Libraries@haskell.org</a><br>
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> ><br>
><br>
> --<br>
> Andreas Abel <>< Du bist der geliebte Mensch.<br>
><br>
> Theoretical Computer Science, University of Munich<br>
> Oettingenstr. 67, D-80538 Munich, GERMANY<br>
><br>
> <a href="mailto:andreas.abel@ifi.lmu.de">andreas.abel@ifi.lmu.de</a><br>
> <a href="http://www2.tcs.ifi.lmu.de/~abel/" target="_blank">http://www2.tcs.ifi.lmu.de/~abel/</a><br>
><br>
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</div></div></blockquote></div><br></div>