On 10/18/2012 03:20 PM, MigMit wrote:
Why do you need "ALike x", "BLike x" etc.? Why not just "Like u x"?
Hmm, looks like a nice idea. I tried it, unfortunately I can't cope with compiler error messages: tst.hs:32:15: Context reduction stack overflow; size = 201 Use -fcontext-stack=N to increase stack size to N Upcast a b In the first argument of `(.)', namely `(upcast :: b -> a)' In the expression: (upcast :: b -> a) . (upcast :: c -> b) In the expression: (upcast :: b -> a) . (upcast :: c -> b) $ x {-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances, ExistentialQuantification, DeriveDataTypeable, MultiParamTypeClasses, FlexibleContexts, IncoherentInstances #-} import Data.Typeable import Data.Maybe data A = A {a_x :: Int} deriving (Show, Typeable) data B = B {b_x :: Int, b_a :: A} deriving (Show, Typeable) data C = C {c_z :: Int, c_b :: B} deriving (Show, Typeable) data D = D {d_w :: Int, d_c :: C, d_a :: A} deriving (Show, Typeable) class Upcast c x where upcast :: x -> c instance Upcast x x where upcast = id instance Upcast A B where upcast = b_a instance Upcast B C where upcast = c_b instance Upcast C D where upcast = d_c instance (Upcast a b, Upcast b c) => Upcast a c where upcast = (upcast :: b -> a) . (upcast :: c -> b) a1 = A 1 b1 = B 2 (A 2) c1 = C 3 b1 d1 = D 4 c1 (A 10) print_a :: Upcast A x => x -> String print_a v = "A = " ++ show (a_x $ upcast v) sum_a :: (Upcast A x, Upcast A y) => x -> y -> String sum_a v1 v2 = "A1 = " ++ show (a_x $ upcast v1) ++ " A2 = " ++ show (a_x $ upcast v2) data LikeA = forall a. (Upcast A a, Typeable a) => LikeA a --instance Upcast a LikeA where -- upcast (LikeA x) = upcast x lst_a = [LikeA a1, LikeA b1, LikeA c1, LikeA d1] get_mono :: Typeable b => [LikeA] -> [b] get_mono = catMaybes . map ((\(LikeA x) -> cast x)) data LikeC = forall c. (Upcast C c, Typeable c) => LikeC c --instance Upcast C LikeC where -- upcast (LikeC x) = upcast x lst_c = [LikeC c1, LikeC d1]