Take a look at http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap
Tom Hawkins wrote:
On Mon, Sep 15, 2008 at 3:11 PM, apfelmus <[EMAIL PROTECTED]> wrote: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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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