[Haskell-cafe] M1 + M2 = M3 where both computations in M1 and M2 can be used?

[sam lee]

 > tInfer :: MonadTypeInfer m => Expr -> m Type
 > eval   :: MonadEval      m => Expr -> m Expr

That solves!
I should've left out type annotation.


On Mon, May 12, 2008 at 10:38 AM, Twan van Laarhoven <twanvl at gmail.com> wrote:
> sam lee wrote:
>
>
> >
> > Hi.
> >
> > I want to compose two monads to build another monad where
> > computations of the two monads can be used inside.
> >
> > I have:
> >
> > - MonadTypeInfer : interface (class) for TypeInfer monad
> > - TypeInfer : a monad that has Map String Type (association of names and
> types)
> > - TypeInferT : transformer of above monad
> > - MonadEval : interface (class) for Eval monad
> > - Eval : a monad that has Map String Expr (association of names and
> > code/function body)
> > - EvalT : transformer of Eval
> > - tInfer :: Expr -> TypeInfer Type -- given expr, returns type of it
> > in TypeInfer monad
> > - eval :: Expr -> Eval Expr -- given expr, returns normalized expr in Eval
> monad
> >
> >
> > Is there a way to build a monad where you could use sub-monads'
> > (monads used to build current monad) computations?
> >
>
>  A solution to this problem is to use type classes, and in particular
> MonadTrans. You can then give an instance of MonadTypeInfer for EvalT m
> where m is an instance of MonadTypeInfer, and similarly an instance
> MonadEval for TypeInferT m. How this is implemented depends on the Monads in
> question, but if you use the monad transformer library with newtype deriving
> you can just add "deriving MonadTrans".
>
>   class Monad m => MonadTypeInfer m where
>       -- functions --
>       tiStuff :: X -> m Whatever
>
>   class Monad m => MonadEval m where
>       -- functions --
>
>   instance Monad m => MonadTypeInfer (TypeInferT m) where
>       -- implementation --
>       tiStuff = ...
>
>   instance Monad m => MonadEval (EvalT m) where
>       -- implementation --
>
>   instance MonadEval m => MonadTypeInfer (EvalT m) where
>       -- lift the functions from TypeInfer through the EvalT type,
>       -- MonadTrans from the mtl might help here
>       tiStuff x = lift (tiStuff x)
>
>   tInfer :: MonadTypeInfer m => Expr -> m Type
>   eval   :: MonadEval      m => Expr -> m Expr
>
>
>  Twan
>