[Haskell-cafe] Phantoms

[Trevor Elliott]

Hi Andrew,

Here's an example that might help:

> import Data.Typeable
>
> data Test a = Test Int
>
> instance Typeable a => Show (Test a) where
>   show t@(Test i) = "Test " ++ show (typeOf (ty t)) ++ " " ++ show i
>     where
>       ty :: Test a -> a,
>       ty  = undefined


and its use:

*Main> show (Test 10 :: Test Int)
"Test Int 10"
*Main> show (Test 10 :: Test String)
"Test [Char] 10"


--trevor

On Wed, 2008-08-06 at 19:09 +0100, Andrew Coppin 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.
> 
> Here's the thing though... How do I get it so that "Foo Int" and "Foo 
> Double" produce slightly different strings when printed?
> 
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