Re: [Haskell-cafe] Re: Haskell stacktrace
I was going to suggest using the -xc option of the GHC runtime (if you
are using GHC), but it seems that it doesn't always give meaningful
results as indicated here:
http://osdir.com/ml/lang.haskell.glasgow.bugs/2006-09/msg8.html
and here:
http://www.haskell.org/pipermail/glasgow-haskell-users/2006-November/011549.html
You might want to try it anyway. It's documented in the GHC manual:
http://haskell.org/ghc/docs/latest/html/users_guide/runtime-control.html
Other than that, there are also haskell debuggers like hat, but I
haven't used them myself so I can't really tell if they could help here.
Best,
Michał
On Tue, 2008-09-09 at 22:35 +0200, Pieter Laeremans wrote:
Woops , I hit the send button to early.
The java approach to locate the error would be
try { ... }catch(Exception e ){
// log error
throw new RuntimeException(e);
}
...
What 's the best equivalent haskell approach ?
thanks in advance,
Pieter
On Tue, Sep 9, 2008 at 10:30 PM, Pieter Laeremans
[EMAIL PROTECTED] wrote:
Hello,
I've written a cgi script in haskell, it crashes sometimes
with the error message Prelude . tail : empty list
In Java we would use this approach to log the erro
try {
} catch (Exception e) {
}
--
Pieter Laeremans [EMAIL PROTECTED]
The future is here. It's just not evenly distributed yet. W.
Gibson
--
Pieter Laeremans [EMAIL PROTECTED]
The future is here. It's just not evenly distributed yet. W. Gibson
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Re: [Haskell-cafe] Re: Haskell stacktrace
2008/9/9 Pieter Laeremans [EMAIL PROTECTED]: What 's the best equivalent haskell approach ? thanks in advance, Pieter The preferred approach is to look at your code, figure out where you are using tail (or could be calling something that uses tail) and use the trace function to output logging info. Don't forget that output is buffered with trace so you might get some strange ordering. A quick search of the API docs should show you where trace lives. Techniques that worked for Java don't work very well when debugging haskell. Others will tell you about flags and possibly using the debugger but I would count on eyeballing and printing as the least painful method. Justin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell stacktrace
Justin Bailey [EMAIL PROTECTED] writes:
are using tail (or could be calling something that uses tail) and use
the trace function to output logging info.
Another cheap trick is to use CPP with something like:
#define head (\xs - case xs of { (x:_) - x ; _ - error(head failed at
line++__FILE__++show __LINE__)})
-k
--
If I haven't seen further, it is by standing in the footprints of giants
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Re: [Haskell-cafe] Re: Haskell stacktrace
Or define your own ghead and gtail:
ghead msg [] = error ghead ++ msg ++ []
ghead _ (x:xs) = x
gtail msg [] = error gtail ++ msg ++ []
gtail msg (x:xs) = xs
and you can call them with a name of a function to give you an idea where
the error is occurring:
myHead = ghead myHead []
Chris.
On Tue, 9 Sep 2008, Ketil Malde wrote:
Justin Bailey [EMAIL PROTECTED] writes:
are using tail (or could be calling something that uses tail) and use
the trace function to output logging info.
Another cheap trick is to use CPP with something like:
#define head (\xs - case xs of { (x:_) - x ; _ - error(head failed at
line++__FILE__++show __LINE__)})
-k
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Re: [Haskell-cafe] Re: Haskell stacktrace
On Tue, Sep 09, 2008 at 11:06:43PM +0200, Pieter Laeremans wrote: This : Prelude let f = (\x - return something went wrong) :: IOError - IO String Prelude let t = return $ show $ too short list !! 100 :: IO String Prelude catch t f *** Exception: Prelude.(!!): index too large How about: module Main where import Control.Exception import Prelude hiding (catch) f :: Exception - IO String f = const $ return sthg went wrong g :: String g = show $ too short list !! 100 h :: IO String h = do print $ head [0 .. -1] return huh? main = do mapM_ print = sequence [ h `catch` f , evaluate g `catch` f , (return $! g) `catch` f , (return g) `catch` f ] Output: [EMAIL PROTECTED]:/tmp$ runhaskell test.lhs sthg went wrong sthg went wrong sthg went wrong test.lhs: Prelude.(!!): index too large Check documentation of catch and evaluate functions in Control.Exception. Regards, -- Krzysztof Kościuszkiewicz Skype: dr.vee, Gadu: 111851, Jabber: [EMAIL PROTECTED] Simplicity is the ultimate sophistication -- Leonardo da Vinci ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell stacktrace
Justin Bailey on 2008-09-09 14:12:38 -0700: 2008/9/9 Pieter Laeremans [EMAIL PROTECTED]: What 's the best equivalent haskell approach ? thanks in advance, Pieter The preferred approach is to look at your code, figure out where you are using tail (or could be calling something that uses tail) and use the trace function to output logging info. A nice way to automate that is using LocH from Hackage. The original announcement: http://www.haskell.org/pipermail/haskell/2006-November/018729.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell stacktrace
Pieter Laeremans wrote: This : Prelude let f = (\x - return something went wrong) :: IOError - IO String Prelude let t = return $ show $ too short list !! 100 :: IO String Prelude catch t f *** Exception: Prelude.(!!): index too large doesn't work. As others've said, the right answer is to correct the bug rather than doing exception handling, but in as far as knowing how to do exception handling for pure functions, consider: http://code.haskell.org/~wren/wren-extras/Control/Exception/Extras.hs The trickiest thing to watch out for is that you must strictly evaluate the expression or else the return/evaluate function will just lazily thunk up the expression. Which means that when you finally run the IO, you'll have already stripped off the IO wrapper that can catch the exception prior to evaluating the expression (which then throws an exception out past the catcher). Another thing to watch out for is that Prelude.catch doesn't do what you want because the H98 spec declares these exceptions to be uncatchable. The Control.Exception.catch function does what you want and is portable even though it's not H98. If you're doing the unsafePerformIO trick to purify your exception handling be sure to give a NOINLINE pragma to prevent potentially buggy inlining. You should also be sure that the handler is something which is actually safe to unsafePerformIO. Finally, to ensure that other optimizations or evaluation orders don't accidentally mess you up, you should take the higher-order approach of `safely` to ensure that you don't accidentally apply the function prior to wrapping it up in a catcher. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
