[Haskell-cafe] How do I lift Control.Monad.Reader.local?

[David Fox]

I mean m, not M.


On Thu, Oct 31, 2013 at 3:54 PM, David Fox <dsf at seereason.com> wrote:

> Hmm, I'm still having trouble implementing local for the MonadReader
> instance of M.
>
>
> On Thu, Oct 31, 2013 at 11:59 AM, Nickolay Kudasov <
> nickolay.kudasov at gmail.com> wrote:
>
>> Hi David,
>>
>> I believe the easiest way to do what you want is to use MonadReaderclass instead of explicit t
>> (Reader [String] a):
>>
>> push' :: MonadReader [String] m => String -> m a -> m apush' s = local (s:)
>>
>> Best,
>> Nick
>>
>>
>> 2013/10/31 David Fox <dsf at seereason.com>
>>
>>> I can write a stack push function in the reader monad like so:
>>>
>>>     import Control.Monad.Reader
>>>
>>>     push :: String -> Reader [String] a ->  Reader [String] a
>>>     push s action = local (s :) action
>>>
>>> How can I write a push that works inside a monad transformer?
>>>
>>>     push' :: MonadTrans t => String -> t (Reader [String]) a ->  t
>>> (Reader [String]) a
>>>     push' s action = ???
>>>
>>>
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>>
>
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