[haskell] redoing Hlist.hs
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Template metaprogramming in C++
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#include<iostream>
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#include<list>
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template<unsigned n>
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struct factorial {
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enum { val = n * factorial<n-1>::val };
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};
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template<>
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struct factorial<0> {
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enum { val = 1 };
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};
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int main() {
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std::cout << factorial<0>::val << std::endl;
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std::cout << factorial<1>::val << std::endl;
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std::cout << factorial<2>::val << std::endl;
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std::cout << factorial<3>::val << std::endl;
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std::cout << factorial<4>::val << std::endl;
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std::cout << factorial<5>::val << std::endl;
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std::cout << factorial<6>::val << std::endl;
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std::cout << factorial<7>::val << std::endl;
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std::cout << factorial<8>::val << std::endl;
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std::cout << factorial<9>::val << std::endl;
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std::cout << factorial<10>::val << std::endl;
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std::cout << factorial<11>::val << std::endl;
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std::cout << factorial<12>::val << std::endl;
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}
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// Output
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/*
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1
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1
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2
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6
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24
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120
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720
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5040
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40320
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362880
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3628800
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39916800
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479001600
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*/
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE GADTs #-}
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{-# LANGUAGE TypeOperators #-}
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{-# LANGUAGE TypeFamilies #-}
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-- -- Method 1
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-- data family Hlist (l :: [*])
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-- data instance Hlist '[] = HNil
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-- data instance Hlist (t ': ts) = HCons t (Hlist ts)
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-- hd :: Hlist (t ': ts) -> t
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-- hd (HCons x _) = x
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-- tl :: Hlist (t ': ts) -> Hlist ts
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-- tl (HCons _ xs) = xs
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-- idx :: Hlist ts -> (n :: Int) -> (ts '!! n)
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-- -- Can't make it work
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-- -- app :: Hlist ts1 -> Hlist ts2 -> Hlist (ts1 '++ ts2)
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-- -- app HNil hls2 = hls2
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-- -- Can't make it work
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-- -- len :: Hlist ts -> Int
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-- -- len HNil = 0
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-- -- len (HCons _ xs) = 1 + (len xs)
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-- -- Method 2 ??
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-- data HNil = HNil
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-- data HCons a b = HCons a b
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-- a = HCons 3 HNil
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-- b = HCons 3 (HCons True HNil)
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-- -- Method 3
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-- data Hlist :: [*] -> * where
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-- HNil :: Hlist '[]
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-- HCons :: t -> Hlist ts -> Hlist (t : ts)
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-- data Member x (l':ls) where
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-- First :: Member x (l:ls)
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-- Next :: Member x ls -> Member x (l:ls)
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-- data Member x (l':ls) where
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-- First :: Member x (l:ls)
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-- Next :: Member x ls -> Member x (l:ls)
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-- len :: Hlist ts -> Int
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-- len HNil = 0
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-- len (HCons _ xs) = 1 + (len xs)
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-- a = HCons 3 HNil
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-- b = HCons 3 (HCons True HNil)
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@ -1,57 +1,58 @@
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE GADTs #-}
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{-# LANGUAGE TypeOperators #-}
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{-# LANGUAGE GADTs #-} -- Enabled by default apparently
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{-# LANGUAGE TypeOperators #-} -- Enabled by default apparently
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{-# LANGUAGE TypeFamilies #-}
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-- -- Method 1
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-- data family Hlist (l :: [*])
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-- data instance Hlist '[] = HNil
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-- data instance Hlist (t ': ts) = HCons t (Hlist ts)
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-- hd :: Hlist (t ': ts) -> t
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-- hd (HCons x _) = x
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-- tl :: Hlist (t ': ts) -> Hlist ts
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-- tl (HCons _ xs) = xs
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-- idx :: Hlist ts -> (n :: Int) -> (ts '!! n)
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-- -- Can't make it work
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-- -- app :: Hlist ts1 -> Hlist ts2 -> Hlist (ts1 '++ ts2)
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-- -- app HNil hls2 = hls2
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-- -- Can't make it work
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-- -- len :: Hlist ts -> Int
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-- -- len HNil = 0
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-- -- len (HCons _ xs) = 1 + (len xs)
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-- -- Method 2 ??
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-- data HNil = HNil
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-- data HCons a b = HCons a b
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-- a = HCons 3 HNil
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-- b = HCons 3 (HCons True HNil)
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-- Method 3
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data Hlist :: [*] -> * where
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HNil :: Hlist '[]
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HCons :: t -> Hlist ts -> Hlist (t : ts)
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data Member x (l':ls) where
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First :: Member x (l:ls)
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Next :: Member x ls -> Member x (l:ls)
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data Member :: a -> [*] -> * where
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First :: Member x (x':l)
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Next :: Member x l -> Member x (x':l)
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-- data Member x (l':ls) where
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-- First :: Member x (l:ls)
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-- Next :: Member x ls -> Member x (l:ls)
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-- len :: Hlist ts -> Int
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-- len HNil = 0
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-- len (HCons _ xs) = 1 + (len xs)
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hd :: Hlist (t ': ts) -> t
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hd (HCons x _) = x
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-- a = HCons 3 HNil
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-- b = HCons 3 (HCons True HNil)
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tl :: Hlist (t ': ts) -> Hlist ts
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tl (HCons _ xs) = xs
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len :: Hlist ts -> Int
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len HNil = 0
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len (HCons _ xs) = 1 + (len xs)
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type family (++) (l1 :: [*]) (l2 :: [*]) where
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'[] ++ y = y
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(x':xs) ++ y = x':(xs ++ y)
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cat :: Hlist ts1 -> Hlist ts2 -> Hlist (ts1 ++ ts2)
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cat HNil y = y
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cat (HCons x xs) y = HCons x (cat xs y)
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-- Example hlists
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a = HCons 3 HNil
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b = HCons 3 (HCons True HNil)
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-- λ> a
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-- a :: Hlist '[Integer]
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-- λ> b
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-- b :: Hlist '[Integer, Bool]
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-- λ> tl a
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--
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-- tl a :: Hlist '[]
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-- λ> tl b
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-- tl b :: Hlist '[Bool]
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-- λ> hd a
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-- 3
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-- λ> hd (tl b)
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-- True
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-- λ> hd $ tl (tl b) -- Error
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--
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-- λ> cat b b
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-- cat b b :: Hlist '[Integer, Bool, Integer, Bool]
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