[Haskell-cafe] Re: How do I simulate dependent types using
polyomino at f2s.com
Sat Aug 18 16:27:30 EDT 2007
Twan van Laarhoven <twanvl <at> gmail.com> writes:
> The solution is to use a dummy parameter:
> > class IntegerType a where
> > value :: a -> Integer
> And call it like:
> > f = value (undefined :: Two)
> So for instance:
> > instance IntegerType d => Show (QF d) where
> > show (QF a b) = show a ++ " + " ++ show b ++ " sqrt "
> > ++ show (value (undefined::d))
Thanks to all respondents for this suggestion. That works great.
> The problem is that this doesn't work, because d is not in scope, you
> need the scoped type variables extension:
> > valueOfQF :: forall a. IntegerType a => QF a -> Integer
> > valueOfQF qf = value (undefined :: a)
Well actually, your first attempt *did* work for me (using GHC 6.6.1). Is this
not behaviour that I can rely on?
More information about the Haskell-Cafe