[Haskell-cafe] How to "instance MonadIO Identity"?

[Magicloud Magiclouds]

Wow, this explains a lot. Thanks.

On Tue, Dec 28, 2010 at 2:05 PM, Antoine Latter <aslatter at gmail.com> wrote:
> There are a couple of ways to handle this - either call
> getSystemEncoding on the outside, before calling 'runWriter', and then
> pass it in to your writer computation, or use 'WriterT IO' instead of
> 'Writer'.
>
> That would be something like:
>
> main = do
>  result <- runWriterT ( computation involving 'tell' and 'liftIO')
>  ( computation using result)
>
> The error message in your case can be difficult to understand - the
> 'Writer' type is a synonym for the type 'WriterT Identity'. Since none
> of these types involve IO, then liftIO can't be used.
>
> Antoine
>
>
> On Tue, Dec 28, 2010 at 12:47 AM, Magicloud Magiclouds
> <magicloud.magiclouds at gmail.com> wrote:
>>  Ah, that is a bad news.
>>  I am using Control.Monad.Writer and Data.Encoding. Code like (not compilable)
>> 43|instance WithMessage String where
>> 44|  append s = (liftIO $ getSystemEncoding) >>= (\e -> tell $
>> encodeLazyByteString e s)
>>  May I know how to make this work?
>>
>> 2010/12/28 Michael Snoyman <michael at snoyman.com>:
>>> The only way to create such an instance would be with unsafePerformIO,
>>> which in this case would be a Very Bad Idea (tm).
>>>
>>> Michael
>>>
>>> On Tue, Dec 28, 2010 at 7:29 AM, Magicloud Magiclouds
>>> <magicloud.magiclouds at gmail.com> wrote:
>>>> Hi,
>>>>  From another thread in this list, I got code as:
>>>>> instance MonadIO Identity where
>>>>>     liftIO = id
>>>>  Well, it does not work for me as:
>>>> Message.hs:22:12:
>>>>    Couldn't match expected type `Identity a' with actual type `IO a'
>>>>    Expected type: IO a -> Identity a
>>>>      Actual type: IO a -> IO a
>>>>    In the expression: id
>>>>    In an equation for `liftIO': liftIO = id
>>>> --
>>>> 竹密岂妨流水过
>>>> 山高哪阻野云飞
>>>>
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>>>
>>
>>
>>
>> --
>> 竹密岂妨流水过
>> 山高哪阻野云飞
>>
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