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. kind regards, Pieter On Tue, Sep 9, 2008 at 10:35 PM, 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 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

"The future is here. It's just not evenly distributed yet." W. Gibson

-- Pieter Laeremans

"The future is here. It's just not evenly distributed yet." W. Gibson

-- Pieter Laeremans "The future is here. It's just not evenly distributed yet." W. Gibson

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

"Justin Bailey" 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

Justin Bailey on 2008-09-09 14:12:38 -0700:

2008/9/9 Pieter Laeremans :

...

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

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/msg00008.html and here: http://www.haskell.org/pipermail/glasgow-haskell-users/2006-November/011549.... 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 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

"The future is here. It's just not evenly distributed yet." W. Gibson

-- Pieter Laeremans

"The future is here. It's just not evenly distributed yet." W. Gibson

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