module Chap8 where {
  import Chap3;

  member x []     = False;
  member x (y:ys) = x==y || member x ys;

  subset xs ys    = all (\ x -> member x ys) xs;

  instance Eq a => Eq (Tree a) where {
    Empty           == Empty           = True;
    Branch l1 n1 r1 == Branch l2 n2 r2 = l1 == l2 && n1 == n2 && r1 == r2;
    _               == _               = False;  
  };

  type Eq' a = (a -> a -> Bool, a -> a -> Bool);

  eq' :: Eq' a -> (a -> a -> Bool);
  eq' = \ (e, _)  ->  e; 

  ne' :: Eq' a -> (a -> a -> Bool);
  ne' = \ (_, n) -> n;

  member' :: Eq' a -> a -> [a] -> Bool;
  member' d x []     = False;
  member' d x (y:ys) = eq' d x y || member' d x ys;

  subset' :: Eq' a -> [a] -> [a] -> Bool;
  subset' d xs ys = all (\ x -> member' d x ys) xs;

{- Prelude‚É’č‹`ŤĎ‚Ý
  data Maybe a = Just a | Nothing; 
 -}

  myLookup :: Eq a => a -> [(a, b)] -> Maybe b;    
  myLookup x ((n,v):rest) = if n==x then Just v else myLookup x rest;
  myLookup x []           = Nothing;

  fix :: (a -> a) -> a;
  fix f = f (fix f)
}