Anyway, if the idea is to ultimately wrap every value in an expression like ([1,2]+[3,4]) in a 'run' application, that doesn't sound very useful. Program structure might be improved, but it would be bloated out by these calls. Also, I don't know what would happen to the readability of type checker errors.
I think it would be more useful if the compiler took care of this automatically. I think it would be worthwhile just for making imperative code more readable. Frederik P.S. By the way, did you misunderstand what I meant by 'automatic lifting'? Note that I'm talking about "lift" as in 'liftM', not 'lift' from MonadTrans. On Fri, Sep 09, 2005 at 01:17:57PM -0700, Frederik Eaton wrote: > On Thu, Sep 08, 2005 at 09:34:33AM +0100, Keean Schupke wrote: > > Can't you do automatic lifting with a "Runnable" class: > > > > class Runnable x y where > > run :: x -> y > > > > instance Runnable (m a) (m a) where > > run = id > > > > instance Runnable (s -> m a) (s -> m a) where > > run = id > > instance (Monad m,Monad n,MonadT t m,Runnable (m a) (n a)) => Runnable > > (t m a) (n a) where > > run = run . down > > Interesting... > > > instance (Monad m,MonadT t m,Monad (t m)) => Runnable (t m a) (m a) > > where > > run = down > > The above is redundant, right? > > > Where: > > > > class (Monad m,Monad (t m)) => MonadT t m where > > up :: m a -> t m a > > down :: t m a -> m a > > > > For example for StateT: > > ... > > So, 'run' is more like a form of casting than running, right? > > How do I use it to add two lists? Where do the 'run' applications go? > Do you have an explicit example? > > I was trying to test things out, and I'm running into problems with > the type system, for instance when I declare: > > class Cast x y where > cast :: x -> y > > instance Monad m => Cast x (m x) where > cast = return > > p1 :: (Monad m, Num a) => m (a -> a -> a) > p1 = cast (+) > > it says: > > Could not deduce (Cast (a1 -> a1 -> a1) (m (a -> a -> a))) > from the context (Monad m, Num a) > arising from use of `cast' at runnable1.hs:14:5-8 > > But this should match the instance I declared, I don't understand what > the problem is. > > Frederik > > -- > http://ofb.net/~frederik/ > -- http://ofb.net/~frederik/ _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
