[Haskell-cafe] How to understand such a newtype ?
z_axis at 163.com
Sat Nov 21 23:03:47 EST 2009
Felipe Lessa wrote:
> On Sat, Nov 21, 2009 at 06:20:40PM -0800, zaxis wrote:
>> newtype X a = X (ReaderT XConf (StateT XState IO) a)
>> #ifndef __HADDOCK__
>> deriving (Functor, Monad, MonadIO, MonadState XState, MonadReader
>> In `X (ReaderT XConf (StateT XState IO) a)`, X is a type constructor, how
>> understand `(ReaderT XConf (StateT XState IO) a)` ?
> Well, “ReaderT XConf (StateT XState IO) a” is *the* type :).
> It's a monad that is a Reader of XConf and has a State of XState.
> This means you can use, for example,
> ask :: X XConf
> get :: X XState
>> And why use `#ifndef __HADDOCK__` ?
> Because Haddock used to have difficulties in processing some
> directives, like that “deriving (..., MonadState XState, ...)”
> which is part of the GeneralizedNewtypeDeriving extension.
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
fac n = foldr (*) 1 [1..n]
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