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. Jos� Romildo Malaquias <[EMAIL PROTECTED]>
Departamento de Computa��o
Universidade Federal de Ouro Preto
Brasil
