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

Bugs item #1119221, was opened at 2005-02-09 03:07
Message generated for change (Comment added) made by nobody
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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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Comment By: Nobody/Anonymous (nobody)
Date: 2005-02-10 10:55

Message:
Logged In: NO 

My motivation is identical to Amr's.  I was trying out his
quantum computation stuff, and then you need an additional
constraint on the bind operation in the monad.  So I tried
using my own monad definition, but it failed.

It's not really a big deal, but if it's reasonably easy to
fix...

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Comment By: Simon Peyton Jones (simonpj)
Date: 2005-02-10 08:50

Message:
Logged In: YES 
user_id=50165

This is a bug, and I plan to fix it.  

To date I only knew of a couple of people who cared (Amr 
Sabry, Arthur van Leeuwen, perhpas Ashley Yakely).  This 
makes one more.  Can you say a bit more about why it'd be 
useful to you. To help motivate me...

This won't be in 6.4, but I will do it, esp if more people yell.

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Comment By: Nobody/Anonymous (nobody)
Date: 2005-02-10 08:45

Message:
Logged In: NO 

You're not alone:

http://www.haskell.org/pipermail/haskell/2005-January/015113.html

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Comment By: Nobody/Anonymous (nobody)
Date: 2005-02-10 08:41

Message:
Logged In: NO 

I would much prefer if the implementation had changed to
match the documentation. :)

  -- Lennart


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Comment By: Nobody/Anonymous (nobody)
Date: 2005-02-10 08:24

Message:
Logged In: NO 

The documentation was wrong - it now says that >>= etc have
to be polymorphic.

----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=108032&aid=1119221&group_id=8032