[Haskell-beginners] exception, not in IO

[Lyndon Maydwell]

I agree that it seems uncomfortable for a seemingly pure function to
require handling exceptions in the IO monad, but if the code in question
really does have the capability of raising IO exceptions, then I think it
is reasonable to consider it as impure and belonging in the IO monad and
that it probably should not have been given a pure type signature to start
with. I'd double check to see if the function in question has the
capability of raising an exception in a more pure monad as per Kees's
suggestion. The other option is to perform the actions I've suggested and
then wrap the whole lot in unsafePerformIO to make it pure again but with a
new signature of (a -> Either YourException b), however that just makes me
feel even more queazy...

Other than that, I guess we'd need a snippet of the code and what libraries
are being used in order to provide better help.

Good luck!


On Mon, Jul 15, 2013 at 4:07 AM, Kees Bleijenberg <
k.bleijenberg at lijbrandt.nl> wrote:

> Lyndon,****
>
> You wrote:
> You can turn anything into an IO action with return, or you could catch
> the exception at a level where you are performing IO. Would this be what
> you're after?****
>
> Wait until you perform IO, seems quit uneasy to me and why? Converting a
> string to a typeable is not a IO action. Problem is that this function
> lives in a dll. The IO is done by the calling program (not a Haskell
> program). The function is not in a monad, it is a pure function. So I think
> I can’t do a return.****
>
> Kees****
>
> On Sun, Jul 14, 2013 at 7:47 PM, Kees Bleijenberg <
> k.bleijenberg at lijbrandt.nl> wrote:****
>
> The app I’am working on, converts a jsonString to another string encoding.
> ****
>
> The function I want to write is  jsonString -> (encoding, errorMsg)  so
> String-> (String, String) ****
>
> For this purpose I have a typeable datastructure Glass. Because it is
> typeable I can do (decodeJSON jsonString) :: Glass****
>
> But sometimes the jsonString is not valid (misformed or wrong fields).
> decodeJSON then throws a exception.  I want to****
>
> catch that exection and transform the result to something like (“” ,
> theErrorMsg). Unfortunately all catch functions want IO parameters. ****
>
> What can I do?****
>
>  ****
>
> Kees****
>
>  ****
>
>
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