-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 The first thing I thought of was to try to apply one of the recursion schemes in the category-extras package. Here is what I managed using catamorphism. - - Jake - -------------------------------------------------------------------------------- data Expr' a = Quotient a a | Product a a | Sum a a | Difference a a | Lit Double | Var Char type Expr = FixF Expr' instance Functor Expr' where fmap f (a `Quotient` b) = f a `Quotient` f b fmap f (a `Product` b) = f a `Product` f b fmap f (a `Sum` b) = f a `Sum` f b fmap f (a `Difference` b) = f a `Difference` f b fmap _ (Lit x) = Lit x fmap _ (Var x) = Var x identity = cata ident where ident (a `Quotient` InF (Lit 1)) = a ident (a `Product` InF (Lit 1)) = a ident (InF (Lit 1) `Product` b) = b ident (a `Sum` InF (Lit 0)) = a ident (InF (Lit 0) `Sum` b) = b ident (a `Difference` InF (Lit 0)) = a ident (Lit x) = InF $ Lit x ident (Var x) = InF $ Var x -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.8 (Darwin) iEYEARECAAYFAkjYhjwACgkQye5hVyvIUKnwhgCgypz0ppFgqn2dMhoJPUzO4+J1 BMUAni277vm9d2e5wTFt2Qrx+DDVjs6z =0SHe -----END PGP SIGNATURE-----