Tristan Allwood got quite a long way with this a couple of years ago. http://research.microsoft.com/en-us/um/people/simonpj/papers/stack-trace/Deb... But not enough to finish it off! The paper describes the tricky points... Simon M is more of an expert than I. Moreover the work Simon has done on improving profiling is relevant too. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe- | bounces@haskell.org] On Behalf Of Evan Laforge | Sent: 25 April 2012 17:08 | To: Michael Snoyman | Cc: Haskell Cafe | Subject: Re: [Haskell-cafe] Printing call site for partial functions | | > And then have the compiler automatically include (optional) package | > name, module name, and line number where `headContext` was called. How | > about we borrow a bit from rewrite rules, and have a pragma such as: | > | >    {-# WITH_CONTEXT head headContext #-} | | This seems similar to the SRCLOC_ANNOTATE pragma supported by jhc. | | I'd love to have this feature. I'd just like to point out it's not just | useful for partial functions like 'head', but also for logging and "expected" | exceptions like Left. It's also extremely useful for tests, when I get | "check failed: 3 /= 4" it's really important to see filename and line number. | | There are various hacks out there to get this, but they all involve | annotating the call sites with some magic, either an 'assert' (and then | runtime overhead on every function that uses it) or a TH splice (and then you | have to turn TH on in every module). I have literally thousands of lines of | tests that would all need an extra boilerplate argument. | | I implemented a preprocessor that is effectively like SRCLOC_ANNOTATE and it | works ok for me, but it's not a good general solution because it's hardcoded | to certain symbols and slows down compilation too much. | And being specific to my app of course it wouldn't help in your case | :) | | But it won't happen unless one of us few people who care about it just goes | and implements it. I don't think it's much of a priority with others. | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

On Wed, Apr 25, 2012 at 7:28 PM, Ozgur Akgun wrote:

Hi,

On 25 April 2012 16:36, Michael Snoyman wrote:

...

   Prelude.head: empty list

Recent versions of GHC actually generate a very helpful stack trace, if the program is compiled with profiling turned on and run with -xc.

See: http://community.haskell.org/~simonmar/slides/HIW11.pdf (Ironically titled "Prelude.head: empty list")

HTH, Ozgur

I certainly think that stack traces are going to be a powerful debugging tool, and I'm looking forward to using them. But I think this feature would be useful in and of itself. I'm presuming it won't be the default to have stack traces turned on due to performance overhead (is that a faulty assumption?). This feature would be available for all builds and give you a very high amount of useful information- though not quite as much as a stack trace- without any (serious) performance impact. Michael

On Thu, Apr 26, 2012 at 5:05 PM, Joachim Breitner wrote:

Hi,

Am Mittwoch, den 25.04.2012, 18:36 +0300 schrieb Michael Snoyman:

...

I'm sure there are many better ways to approach the problem, and I can't speak to the complexity of implementation within GHC. I *can* say, however, that this would have saved me a lot of time in the example I gave above, and I'd bet many Haskellers have similar stories. This could be a huge debugging win across the board.

using TH (which I only reluctantly advocate for general usage) you can get good location information behaviour, see http://hackage.haskell.org/packages/archive/stm-stats/0.2.0.0/doc/html/Contr... (and its source) for one example. One would use this approach maybe with http://hackage.haskell.org/packages/archive/safe/0.3.3/doc/html/Safe.html#v:...

Greetings, Joachim

-- Joachim Breitner  e-Mail: mail@joachim-breitner.de  Homepage: http://www.joachim-breitner.de  Jabber-ID: nomeata@joachim-breitner.de

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We actually use a similar technique in Yesod for logging[1]. But it requires actively using it in all libraries. In other words, unless the author of the library in question explicitly used $headLoc, this wouldn't help. [1] http://hackage.haskell.org/packages/archive/yesod-core/1.0.1.1/doc/html/Yeso...

On Mon, 30 Apr 2012 10:31:01 +0200, Ketil Malde wrote:

One half-baked quasi-solution is to use:

#define head (\xs -> case xs of { (x:_) -> x ; _ -> error("head: empty list at"++__FILE__++show __LINE__)})

Downsides are that it depends on CPP, and, CPP being a C preprocessor, it doesn't blend well with lines with single apostrophes on them (e.g.: head x')

-k

There is a Haskell solution: cpphs[0], you can invoke this preprocessor by specifying the flags -cpp -pgmPcpphs -optP--cpp for GHC. Regards, Henk-Jan van Tuyl [0] http://hackage.haskell.org/package/cpphs -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --