[Haskell-cafe] Re: Phantoms

[Ben Franksen]

Jason Dagit wrote:
> On Wed, Aug 6, 2008 at 11:09 AM, Andrew Coppin
> <andrewcoppin at btinternet.com>wrote:
> 
>> I just (re)discovered that I can do things like
>>
>>  data Foo x = Foo Int Int
>>
>> Now "Foo Int" and "Foo Double" are, as far as the type checker cares, two
>> completely different types, even though in fact they are the same. This
>> is actually Quite Useful, in the particular case I'm working on.
> 
> Phantom types are indeed useful for many things, but a bit of cautionary
> advice.  If you start to depend on the phantoms for type safety AND you
> export your data constructors then you run a serious risk of being type
> unsafe.  Bonus points if you can demonstrate an equivalent of
> unsafeCoerce# this way.

This would be very bad, but I doubt it is possible.

> Example:
> fooCast :: Foo Int -> Foo Double
> fooCast (Foo x) = Foo x
> 
> On noes!  We just cast that Foo Int to a Foo Double without changing it! 

What's the problem?

> It works because the value on the RHS is consider freshly constructed and
> other than sharing x it is unrelated to the one on the LHS.

Right. You must call the data constructor Foo in order to exploit that it
has the type

  Foo :: Int -> Foo a

I don't see how this is not type safe, and I was not able to produce an
#unsafeCoerce with this technique. One would need some

  unFoo a -> a

but the  a  in  data Foo a  is phantom, i.e. there is no thing of type  a 
in a Foo.

Cheers
Ben