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{-# LANGUAGE GADTs #-}
module Main where
data Rep t where
Rint :: Rep Int
Rchar :: Rep Char
Runit :: Rep ()
Rpair :: Rep a -> Rep b -> Rep (a,b)
Rsum :: Rep a -> Rep b -> Rep (Either a b)
Rcon :: String -> Rep t -> Rep t
Rdata :: Rep i -> (t -> i) -> (i -> t) -> Rep t
int :: Rep Int
int = Rint
list :: Rep a -> Rep [a]
list x = Rdata (Rsum (Rcon "[]" Runit) (Rcon ":" (Rpair x (list x)))) h g
where
h [] = Left ()
h (x:xs) = Right (x,xs)
g (Left ()) = []
g (Right (x,xs)) = x:xs
addUp :: Rep a -> a -> Int
addUp Rint n = n
addUp (Rpair r s) (x,y) = addUp r x + addUp s y
addUp (Rsum r s) (Left x) = addUp r x
addUp (Rsum r s) (Right x) = addUp s x
addUp (Rdata i f g) x = addUp i (f x)
addUp (Rcon s r) x = addUp r x
addUp v x = 0
main = print (addUp (list int) [1,2,4])
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