"do" notation and ">>"

27 Mar 2002

      Hugs appears to ignore definitions of ">>" when using "do" notation,
perhaps relying on the default definition in terms of ">>=". Here is an example.

According to the Haskell 98 Tutorial, the following two statements
should be equivalent, right?

	main = do put "hello"; put "world"

	main' = put "hello" >> put "world"

In the Hugs output below, it appears that they are not.

% hugs TestDo
__   __ __  __  ____   ___      _________________________________________
||   || ||  || ||  || ||__      Hugs 98: Based on the Haskell 98 standard
||___|| ||__|| ||__||  __||     Copyright (c) 1994-2001
||---||         ___||           World Wide Web: http://haskell.org/hugs
||   ||                         Report bugs to: hugs-bugs@haskell.org
||   || Version: December 2001  _________________________________________

...

TestDo> run main
hello
...
...
=
world
TestDo> run main'
hello
...
...
world
TestDo> :quit
[Leaving Hugs]

% cat TestDo.hs
module TestDo where

type State = [String]
newtype MyMonad a = MyMonad (State -> (a, State))

instance Monad MyMonad where
 (MyMonad m) >>= fm = MyMonad $
                    \s -> let (x, s') = m s
                              MyMonad m' = fm x
                              s'' = ">>=" : s'
                              in m' s''
 (MyMonad m) >> (MyMonad m') = MyMonad $
                               \s -> let (_, s') = m s
                                         s'' = ">>":s'
                                         in m' s''
 return x = MyMonad (\s -> (x, "return":s))

put x = MyMonad (\s -> (s, x:s))
get = MyMonad (\s -> (s,s))

run (MyMonad m) = let (_,s) = m []
                      s' = map putStrLn s
                      in sequence_  (reverse s')

main = do put "hello"; put "world"

main' = put "hello" >> put "world"

-- 
James B. White III (Trey)
Center for Computational Sciences
Oak Ridge National Laboratory
whitejbiii@ornl.gov