[Haskell-cafe] phantom types

[TP]

Hi,

I am currently reading documentation on Generalized Algebraic Data Types:

http://en.wikibooks.org/wiki/Haskell/GADT

I have a question concerning this page. Let us consider the following code 
proposed in the page:

----------------------------------
-- Phantom type variable a (does not appear in any Expr: it is just a
-- dummy variable).
data Expr a = I Int
            | B Bool
            | Add (Expr a) (Expr a)
            | Mul (Expr a) (Expr a)
            | Eq (Expr a) (Expr a)
            deriving (Show)

-- Smart constructors
add :: Expr Int -> Expr Int -> Expr Int
add = Add

i :: Int  -> Expr Int
i = I

b :: Bool -> Expr Bool
b = B

eval :: Expr a -> a
eval (I n) = n
----------------------------------

I obtain the following error:

Phantom.hs:27:14:
    Couldn't match type `a' with `Int'
      `a' is a rigid type variable bound by
          the type signature for eval :: Expr a -> a at Phantom.hs:27:1
    In the expression: n
    In an equation for `eval': eval (I n) = n

The wiki page explains:

"""
But alas, this does not work: how would the compiler know that encountering 
the constructor I means that a = Int?
"""

I don't understand. When we write "eval (I n) = n", as I is a constructor 
which takes an Int as argument, we are able to deduce that the type of n is 
Int; so the type of eval should be in this case "Expr Int -> Int".
What do I miss?

Thanks,

TP
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