[Haskell-cafe] Re: Phantoms
ben.franksen at online.de
Sat Aug 16 16:12:51 EDT 2008
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.
> 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.
More information about the Haskell-Cafe