[Haskell-cafe] ErrorT and catchError question

[Adam Smyczek]

On Jan 15, 2008, at 8:07 PM, Jonathan Cast wrote:

> On 15 Jan 2008, at 7:54 PM, Adam Smyczek wrote:
>
>>
>> On Jan 15, 2008, at 7:34 PM, Brandon S. Allbery KF8NH wrote:
>>
>>>
>>> On Jan 15, 2008, at 22:05 , Adam Smyczek wrote:
>>>
>>>> Ups, resend, first response did not make into the list.
>>>> On Jan 14, 2008, at 9:33 PM, Brandon S. Allbery KF8NH wrote:
>>>>
>>>>>
>>>>> On Jan 15, 2008, at 0:28 , Adam Smyczek wrote:
>>>>>
>>>>>> It's probably a trivial question, but I cannot figure out
>>>>>> how to implement the catchError function in:
>>>>>>
>>>>>> instance MonadError String Shell where
>>>>>> 	throwError      = error . ("Shell failed: "++)
>>>>>> 	catchError l h = ???
>>>>>
>>>>> Take a look at Control.Exception.catch for starters.
>>>>
>>>> No, did not help and
>>>> going over the source code of Control.Monad.Error did not
>>>> help as well. Does someone have other tips for me?
>>>
>>> Perhaps you could explain what you're looking for?  Your  
>>> typeclass doesn't tell us anything about the semantics.
>>
>> The type declaration:
>>
>> newtype Loader a = Loader
>>   { load :: ErrorT String IO a }
>>   deriving (Functor, Monad, MonadIO)
>>
>> instance MonadError String Loader where
>>   throwError = error . ("Error: " ++)
>
> I don't think this is what you want; you're throwing away the  
> benefit of using ErrorT.  You probably want
>
>    throwError = Loader . throwError
>
>>   l `catchError` h = ??? how do I implement this ???
>
>    l `catchError` h = Loader (load l `catchError` load . h)
>

Thanks Jonathan, this is exactly what I was looking for
and I got the concept of ErrorT now, I think.

> jcc
>