So you have some particular ts for which you wish to provide a MonadReader instance?

In that case for each t which lacks MonadReader instance (look here for existing instances) you need to implement it:

instance MonadReader r m => MonadReader r (t m) where
    ask = lift ask
    local = ...

The way to implement local is individual for each t, so you might want to specify which transformers are you working with.

Nick

2013/11/1 David Fox <dsf@seereason.com>

I mean m, not M.
On Thu, Oct 31, 2013 at 3:54 PM, David Fox <dsf@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@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 a
push' s = local (s:)
Best,
Nick
2013/10/31 David Fox <dsf@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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