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