Works for me under ghc4.05; for hugs you need to:
You have it right, except you need to
1) explicitly type test,
test:: Reader [Char] Char
2) have Reader derive Show
3) use the -98 option at startup
John Atwood
---------------------
=?iso-8859-1?Q?Jos=E9_Romildo_Malaquias?= wrote:
>
> I came across an implementation of reader monads by Andy Gill,
> <http://www.cse.ogi.edu/~andy/monads/MonadReader.htm>
> inspired by the paper "Functional Programming with Overloading
> and Higher-Order Polymorphism" (by Mark P Jones)
> <http://www.cse.ogi.edu/~mpj/pubs/springschool.html>:
>
> -- ------------------------------------------------------------
> -- Reader monads.
> -- A class of monads for describing computations that
> -- consult some fixed environment.
>
> class (Monad m) => ReaderMonad s m where
> -- asks for the (internal non-mutable) state
> ask :: m s
>
> -- this allows you to provide a projection function
> asks :: (ReaderMonad s m) => (s -> a) -> m a
> asks f = do s <- ask
> return (f s)
>
> -- a parametarized reader monad
> newtype Reader w a = Reader { runReader :: w -> a }
>
> instance Functor (Reader w) where
> fmap f m = Reader ( \w -> f (runReader m w) )
>
> instance Monad (Reader w) where
> return v = Reader ( \w -> v )
>
> p >>= f = Reader ( \w -> runReader (f (runReader p w)) w )
>
> fail str = Reader ( \w -> error str )
>
> instance ReaderMonad w (Reader w) where
> ask = Reader ( \w -> w )
> -- -------------------------------------------------------------
>
> Would someone write a simple Haskell program that ilustrates how
> one can use this reader monad?
>
> I have tried the following, but it fails at compilation:
>
> ---------------
> test = do env <- ask
> if env == "choose a"
> then return 'a'
> else return 'b'
>
> do_test = runReader test "choose a"
>
> main = putStr (show do_test)
> ---------------
>
> Thanks.
>
> Romildo
> --
> Prof. José Romildo Malaquias <[EMAIL PROTECTED]>
> Departamento de Computação
> Universidade Federal de Ouro Preto
> Brasil
>
>
