On 30 September 2010 15:23, C K Kashyap <ckkash...@gmail.com> wrote:
> Hi,
> I was going over the Error Handling chapter in RWH and tried out this sample -
>
> Prelude> :m Control.Exception
> Prelude Control.Exception> let x=5 `div` 0
> Prelude Control.Exception> let y=5 `div` 1
> Prelude Control.Exception> handle (\_ -> putStrLn "Text") (print x)
>
> <interactive>:1:0:
> Ambiguous type variable `e' in the constraint:
> `Exception e'
> arising from a use of `handle' at <interactive>:1:0-39
> Probable fix: add a type signature that fixes these type variable(s)
> Prelude Control.Exception>
>
> Could someone please tell me what the problem is and how to resolve it?
RWH was written before GHC 6.10 came out, and thus is using old-style
exceptions. As of GHC 6.10.1 with base-4, the old-style exceptions
were replaced with extensible exceptions. As such, you have two
options:
* Keep using old-style exceptions. With GHC 6.10 and 6.12, import
Control.OldException instead of Control.Exception
* Manually migrate the RWH code to new-style exceptions; there are two
ways of doing this:
- For production code, you should add explicit type signatures, etc.
for your exception-handling functions passed to handle, etc.
- For just playing around, use SomeException: e.g.: handle (\
SomeException{} -> putStrLn "Text") (print x)
--
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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