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

[Nickolay Kudasov]

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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