[cpp] try some template metaprogramming

cpp/tmp/hello.cpp

@@ -0,0 +1,27 @@
+#include<iostream>
+
+template<int n>
+struct Nat {
+  enum { val = 1 + Nat<n-1>::val };
+};
+
+template<>
+struct Nat<0> {
+  enum { val = 0 };
+};
+
+/*
+ * 'class' has variables 'private' by default. 'struct' has it 'public'. More
+ * convenient here.
+ */
+
+int main() {
+  std::cout << Nat<2>::val << std::endl;
+  std::cout << Nat<23>::val << std::endl;
+}
+
+// Output
+/*
+2
+23
+*/

cpp/tmp/nat.cpp

@@ -0,0 +1,32 @@
+#include<iostream>
+
+struct Zero {
+  enum { val = 0 };
+};
+
+template<typename N>
+struct Succ {
+  enum { val = 1 + N::val};
+};
+
+int main() {
+  // O = 0
+  std::cout << Zero::val << std::endl;
+
+  // S O = 1
+  std::cout << Succ<Zero>::val << std::endl;
+
+  // S (S O) = 2
+  std::cout << Succ<Succ<Zero>>::val << std::endl;
+
+  // S (S (S O) = 3
+  std::cout << Succ<Succ<Succ<Zero>>>::val << std::endl;
+}
+
+// Output
+/*
+0
+1
+2
+3
+*/

haskell/Hlist.hs

@@ -0,0 +1,57 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE TypeFamilies #-}
+
+
+-- -- Method 1
+-- data family Hlist (l :: [*])
+-- data instance Hlist '[] = HNil
+-- data instance Hlist (t ': ts) = HCons t (Hlist ts)
+
+-- hd :: Hlist (t ': ts) -> t  
+-- hd (HCons x _) = x
+
+-- tl :: Hlist (t ': ts) -> Hlist ts
+-- tl (HCons _ xs) = xs
+
+-- idx :: Hlist ts -> (n :: Int) -> (ts '!! n)
+
+-- -- Can't make it work
+-- -- app :: Hlist ts1 -> Hlist ts2 -> Hlist (ts1 '++ ts2)
+-- -- app HNil hls2 = hls2
+
+-- -- Can't make it work
+-- -- len :: Hlist ts -> Int
+-- -- len HNil = 0
+-- -- len (HCons _ xs) = 1 + (len xs)
+
+
+-- -- Method 2 ??
+-- data HNil = HNil
+-- data HCons a b = HCons a b 
+
+-- a = HCons 3 HNil 
+-- b = HCons 3 (HCons True HNil)
+  
+
+
+-- Method 3
+data Hlist :: [*] -> * where
+  HNil :: Hlist '[]
+  HCons :: t -> Hlist ts -> Hlist (t : ts)
+
+data Member x (l':ls) where
+  First :: Member x (l:ls)
+  Next :: Member x ls -> Member x (l:ls)
+
+-- data Member x (l':ls) where
+--   First :: Member x (l:ls)
+--   Next :: Member x ls -> Member x (l:ls)
+
+-- len :: Hlist ts -> Int
+-- len HNil = 0
+-- len (HCons _ xs) = 1 + (len xs)
+
+-- a = HCons 3 HNil 
+-- b = HCons 3 (HCons True HNil)