[Haskell-cafe] Monad Transformer question

2005-11-22 Thread Fan Wu 
Hi Haskell gurus,

I'm learning Haskell now and here I'm quite puzzled over some code
about Monad Transformers. The code is like:


   type NDS a = StateT ProblemState [] a

   getVar :: Var - NDS (Maybe Value)
   getVar v = do vs - gets vars
 return $ lookup v vs


What puzzles me is that, I think the Monad of the do block shall be
the NDS (Maybe Value) in declaration, but the type of gets is

gets :: (MonadState s m) = (s - a) - m a

So gets returns a Monad of type m ([] is this case), which seems to
be different from NDS (Maybe Value), but GHC does not complain about
it.


If I comment out the type declaration of getVar :: Var - NDS (Maybe
Value) and let GHC interpret the type, then the type of getVar is
like:

getVar :: (MonadState ProblemState m) = Var - m (Maybe Value)

So does it mean StateT ProblemState m and the m as in MonadState
ProblemState m is the same thing?

I guess I must missed something when trying to understand the Monad
Transformers. Please give me some insights.

Thanks,
Fan
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Re: [Haskell-cafe] Monad Transformer question

2005-11-22 Thread Andrew Pimlott 
On Tue, Nov 22, 2005 at 05:26:00PM -0700, Fan Wu wrote:
type NDS a = StateT ProblemState [] a
 
getVar :: Var - NDS (Maybe Value)
getVar v = do vs - gets vars
  return $ lookup v vs
 
 
 What puzzles me is that, I think the Monad of the do block shall be
 the NDS (Maybe Value) in declaration, but the type of gets is
 
 gets :: (MonadState s m) = (s - a) - m a
 
 So gets returns a Monad of type m ([] is this case), which seems to
 be different from NDS (Maybe Value), but GHC does not complain about
 it.
 
 
 If I comment out the type declaration of getVar :: Var - NDS (Maybe
 Value) and let GHC interpret the type, then the type of getVar is
 like:
 
 getVar :: (MonadState ProblemState m) = Var - m (Maybe Value)
 
 So does it mean StateT ProblemState m and the m as in MonadState
 ProblemState m is the same thing?

No!  Check out the instance:

instance (Monad m) = MonadState s (StateT s m) where ...

So you can see that the m in MonadState ProblemState m is actually
StateT ProblemState m' where m' is the inner monad, [] in your case.
So a correct type for gets as you use it is

gets :: (ProblemState - a) - MonadState ProblemState [] a

Andrew
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