[ ghc-Bugs-1119221 ] Rebindable syntax doesn't work as advertised

Bugs item #1119221, was opened at 2005-02-09 03:07
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Category: Compiler (Type checker)
Group: 6.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Rebindable syntax doesn't work as advertised

Initial Comment:
--
-- According to section 7.3.5. Rebindable syntax in the
user's manual
-- this should work (unless I totally misunderstood it).
-- It doesn't.
--
-- Compile with -fno-implicit-prelude
--
--     Lennart
--

import Prelude hiding(Monad(..))

class B a where
    b :: a

instance B Bool where
    b = False

class M m where 
    return :: a -> m a
    (>>=)  :: (B a) => m a -> (a -> m b) -> m b
    (>>)   :: (B a) =>  m a -> m b -> m b
    fail   :: String -> m a
  
    p >> q  = p >>= \ _ -> q
    fail s  = error s
   
instance M Maybe where
    return x = Just x
    (>>=) = error "bind"

test :: Maybe Bool
test = do
    x <- return True
    return (x && x)

unJust (Just x) = x

main = print (unJust test)


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