[Haskell-cafe] Re: AT solution: rebinding >>= for restricted monads

Pepe Iborra mnislaih at gmail.com
Tue Feb 27 03:39:13 EST 2007

>> Did anyone with knowledge of Associated Types pursue this solution?
> Where did you get this from.  My haskell-cafe mail folder doesn't seem
> to have the thread you are replying to.

Sorry I replied from gmane; I should have included a link to the  
original thread, but I really expected gmane to do that. The thread  
is at:


>> It doesn't work with GHC head, and I can't really do anything  
>> about that.
>> Mostly curiosity.
> The main reason this doesn't work with the head is because the
> implementation of associated type *synonyms* (as opposed to associated
> data types) is still incomplete.  (See also
> <http://haskell.org/haskellwiki/GHC/Indexed_types>.)  We are  
> working at
> the implementation, but I just relocated from New York to Sydney,  
> which
> is why not much happened in the last two months.
> But I also don't quite understand the intention of this code:

I will try to fill in the details, but surely it is all expanded in  
the original thread.

The idea is that we have an indexed/effectful monad where bind and  
return have a parameterized type:

class WitnessMonad m where
   (>>=) :: m a b x -> (x -> m b c y) -> m a c y
   return :: x -> m a a x

We can produce instances of WitnessMonad from an existing Monad using  
an adaptor

newtype WitnessAdaptor m a b x = W {unW::m x}
instance Monad m => WitnessMonad (WitnessAdaptor m)

And rebind the do syntax to our WitnessMonad.
But using vanilla Monads via this trick requires to lift and unlift  
every monadic action with the adaptor. An example in the IO monad:

test1 :: IO String
test1 = unW$ do
   msg <- W getLine
   W$ putStrLn "Thanks!"
   return msg

 From here on the intent was on producing a solution using ATs that  
hides this explicit wrapping. I don't really know the details of the  
proposed solution.


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