[Haskell-cafe] Reader monad

[Henning Thielemann]

On Wed, 29 Dec 2010, michael rice wrote:

> In the case of ReaderT and StateT
> 
> newtype ReaderT r m a = ReaderT {
>         -- | The underlying computation, as a function of the environment.
>         runReaderT :: r -> m a
>     }
> 
> newtype StateT s m a = StateT { runStateT :: s -> m (a, s) }
> 
> what is the existing type?


The existing type is 'r -> m a'. You could also write

> newtype ReaderT r m a = ReaderT (r -> m a)

This would be the same type as above, but it would have no accessor 
function 'runReaderT'.