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 apush' s = local (s:)

Best, Nick

2013/10/31 David Fox

...

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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So you have some particular ts for which you wish to provide a MonadReaderinstance? In that case for each t which lacks MonadReader instance (look herehttp://hackage.haskell.org/package/mtl-2.1.2/docs/Control-Monad-Reader-Class...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

I mean m, not M.

On Thu, Oct 31, 2013 at 3:54 PM, David Fox 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 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

...

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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