[Haskell-beginners] exceptions and stacks

Emmanuel Touzery etouzery at gmail.com
Tue Nov 6 23:15:19 CET 2012

so I have found the culprit, by defining this function:

Import Debug.Trace

headMarker :: String -> [a] -> a
headMarker marker [] = trace marker (head [])
headMarker _ array = head array

and replacing calls to head to "headMarker "XX"", "headmarker "YY"" and so
it works but I would really rather have a compiler/runtime based solution
which would not force me to modify my code to track the bug (and would be
simpler to use in case the problem comes from a library).

If anybody has an idea what I did wrong with the -xc and the -RTS I'm all
ears... I'd love to test it and find out how to make that work reliably.


On Tue, Nov 6, 2012 at 10:50 PM, Emmanuel Touzery <etouzery at gmail.com>wrote:

> you mean in combination with the RTS/profiling trick?
> If I must do something like that, I'll just mark my code with debug lines
> or code the else situations (like making a headMarker with an extra
> parameter "String marker" and it will output the marker on stdoud if the
> array is empty), seems less work...
> I was really hoping being able to fix it without having the modify the
> code, it's disappointing, I think.
> On Tue, Nov 6, 2012 at 9:47 PM, Christian Maeder <Christian.Maeder at dfki.de
> > wrote:
>> Usually, these head calls are in your code. Do "fgrep head" on your
>> sources and replace all suspicious occurrences. Use "let h : _ = l in ..."
>> and "h" instead of "head l" for proper positions.
>> HTH Christian
>> Am 06.11.2012 12:44, schrieb Emmanuel Touzery:
>>  Hello,
>>>      I'm getting really frustrated by error handling in haskell. By this
>>> I mean "assert"-like errors. Which means, something that should never
>>> happen, happens. This will happen in the development and deployment of
>>> software of any size.
>>>      When coding the feature you'll say, I don't need to handle the
>>> "Nothing" case, because it will never happen, or almost never, and this
>>> is not critical production software.
>>>      But if one day it happens you find yourself with a several thousand
>>> lines program and all you have to debug is the input data (most of the
>>> time anyway, sometimes not even that) and this error message (for
>>> instance):
>>> *** Exception: Prelude.head: empty list
>>>      And you don't know even, did you call "head" or maybe a library you
>>> are using called it.
>>>      This is really poor compared to what Java, python and others
>>> offers. I can understand that it's difficult to provide a stack trace,
>>> especially with lazy evaluation, but at least the function name and
>>> maybe a line number... Actually I don't know why it would be so
>>> difficult to also give me the values of the parameters of the
>>> function... using print and if it's a pure function it intuitively
>>> doesn't seem difficult.
>>>      I've tried ghci with :trace and ":set -fbreak-on-exception" and I
>>> got nowhere. It breaks in things which are not important (I have no idea
>>> where it breaks, it claims an exception was thrown, maybe but not in my
>>> code and it never reaches my code for sure). After hitting ":continue"
>>> many times, I get to my exception and there I didn't manage to get
>>> useful information (:hist never printed me anything).
>>>      But I would like not to have to use ghci at all. And to avoid
>>> having to write all over the place in my code these "else" and case
>>> situations for things I assume are impossible (these are cases I would
>>> not write in other modern languages).
>>>      This is a feature for which I would be ready to pay a 50% speed
>>> penalty and 50% memory use penalty.
>>>      I understand if it's the haskell way of writing all those else
>>> clauses... But I think in that case it really makes it compare
>>> negatively to other languages. It would be a real minus in my opinion
>>> (including to code readability).
>>>      I have read (quickly) those but didn't find the solution there:
>>> http://www.haskell.org/ghc/**docs/7.0.1/html/users_guide/**
>>> ghci-debugger.html<http://www.haskell.org/ghc/docs/7.0.1/html/users_guide/ghci-debugger.html>
>>> http://book.realworldhaskell.**org/read/error-handling.html<http://book.realworldhaskell.org/read/error-handling.html>
>>>      Any advice will be welcome! I really hope I missed something.
>>> Emmanuel
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