[Haskell-cafe] Re: How do I simulate dependent types using phantom types?

Twan van Laarhoven writes:

The solution is to use a dummy parameter:

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class IntegerType a where value :: a -> Integer And call it like: f = value (undefined :: Two)

So for instance:

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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:

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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?