| Hi, Ryan. Since I'm trying to understand Reader, I wanted to be aware of all cases of Reader. ========== From the docs (and tuts) newtype creates a new type out of an existing type and gives a single constructor for doing so. From: http://www.haskell.org/tutorial/moretypes.html newtype Natural = MakeNatural Integer This creates an entirely new type, Natural, whose only constructor contains a single Integer. The constructor MakeNatural converts between an Natural and an Integer: toNatural :: Integer -> Natural toNatural x | x < 0 = error "Can't create negative naturals!" | otherwise = MakeNatural x fromNatural :: Natural -> Integer fromNatural (MakeNatural i) = i In the above case the existing type is Integer. The new type behaves like the existing type, but we can pattern match with the new type. ++++++++++ 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? Michael --- On Wed, 12/29/10, Ryan Ingram <ryani.spam@gmail.com> wrote:
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