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

[David Fox]

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 MonadReader class
> 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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