[Haskell-cafe] Re: Monad transformer to consume a list

[Gleb Alexeyev]

Stephan Friedrichs wrote:
> 
> Oh I see - my bad. I was somehow thinking I could prevent modification
> of the input list but that's obviously impossible when the ConsumerT
> constructor is... exported? public? how do you say that?
You can export ConsumerT as an abstract type constructor.

 > {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 > module ConsumerT(ConsumerT, runConsumerT, next) where

 > import Control.Monad.State
 > import Control.Monad.Trans

 > newtype ConsumerT c m a =  ConsumerT { runConsumerT' :: StateT [c] m a }
 >    deriving (Functor, Monad, MonadTrans)

 > runConsumerT = runStateT . runConsumerT'

 > next :: Monad m => ConsumerT a m a
 > next = ConsumerT $ StateT $ \(x:xs) -> return (x, xs)