Hi, I'm looking for an example of idiomatic usage of the fixpoint library[1].
[1]: http://hackage.haskell.org/package/fixpoint-0.1.1 Here's what I managed to get: {-# LANGUAGE TypeFamilies, FlexibleContexts, UndecidableInstances, FlexibleInstances #-} import Data.Fixpoint newtype Expr = Expr { unExpr :: Pre Expr Expr } instance Functor (Pre Expr) => Fixpoint Expr where data Pre Expr a = Add a a | Const Int project = unExpr inject = Expr instance Functor (Pre Expr) where fmap f (Const x) = Const x fmap f (Add x1 x2) = Add (f x1) (f x2) eval = cata eval' where eval' (Const x) = x eval' (Add x1 x2) = x1 + x2 There are some issues with this code, compared to simply using newtype Fix f = In { out :: f (Fix f) } to build an Expr. 1. Since 'Pre' is a data (not type) family, we cannot simply make use of a functor defined elsewhere. We need to define the functor inside the instance declaration (or at least wrap an existing functor). 2. I wasn't able to derive the Functor instance, getting an error Derived instance `Functor (Pre Expr)' requires illegal partial application of data type family Pre In the data type instance declaration for `Pre' 3. Having to use UndecidableInstances makes me feel a bit uncomfortable. This makes me think that the intended usage is somewhat different. So, could someone give an example? -- Roman I. Cheplyaka :: http://ro-che.info/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
