Restricted Data Types: A reformulation

[John Meacham]

On Tue, Feb 07, 2006 at 05:54:51PM -0800, Ashley Yakeley wrote:
> John Meacham wrote:
> 
> >newtype Eq a => Set a = Set (List a)
> >singleton :: Eq a => a -> Set a
> >class Monad m where
> >        return :: a -> m a
> >
> >instance Monad Set where
> >        return x = singleton x
> >
> >okay, our goal is to make this typesafe. 
> 
> You shouldn't be able to, should you? Monad makes a promise that Set 
> can't keep. In particular:
> 
>  returnid :: (Monad m) => m (a -> a)
>  returnid = return id
> 
>  bad :: Set (a -> a)
>  bad = returnid

however, (Set (a -> a)) is malformed. since a _requirement_ is that Set
can only be applied to a type with an Eq constraint so the instance you
try to do something like

returnid :: Set (a -> a) -- ^ static error! you need

returnid :: Eq (a -> a) => Set (a -> a)

the instant you introduce 'Set' you introduce the 'Eq' constraint. as
long as you are just working on generic monads then there is no need for
the Eq constraint.

        John


-- 
John Meacham - ⑆repetae.net⑆john⑈