#303: Rebindable syntax doesn't work as advertised
-------------------------------------+--------------------------------------
Reporter: nobody | Owner: nobody
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.8.3
Severity: normal | Resolution: duplicate
Keywords: | Difficulty: Unknown
Testcase: rebindable/T303 | Architecture: Unknown
Os: Unknown |
-------------------------------------+--------------------------------------
Changes (by simonpj):
* testcase: => rebindable/T303
* difficulty: => Unknown
* status: reopened => closed
* resolution: => duplicate
Old description:
> {{{
> --
> -- 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)
>
> }}}
New description:
{{{
--
-- 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)
}}}
Comment:
This is really a dup of #1537, which was fixed last year but never got
into 6.8. Works in HEAD (and hence the upcoming GHC 6.10). However, I've
added your program as a new test case in the regression test suite
`rebindable/T303`, so that we'll know if it ever fails in the future.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/303#comment:9>
GHC <http://www.haskell.org/ghc/>
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