15 Sep
2008
15 Sep
'08
9:25 p.m.
On Mon, Sep 15, 2008 at 3:11 PM, apfelmus
So, in other words, in order to test whether terms constructed with Equal are equal, you have to compare two terms of different type for equality. Well, nothing easier than that:
(===) :: Expr a -> Expr b -> Bool Const === Const = True (Equal a b) === (Equal a' b') = a === a' && b === b' _ === _ = False
instance Eq (Expr a) where (==) = (===)
OK. But let's modify Expr so that Const carries values of different types: data Expr :: * -> * where Const :: a -> Term a Equal :: Term a -> Term a -> Term Bool How would you then define: Const a === Const b = ... -Tom