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