Created
June 1, 2018 03:40
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{-# LANGUAGE UndecidableInstances #-} | |
{-# LANGUAGE DeriveFunctor #-} | |
module Main where | |
import Control.Arrow | |
import Data.Functor.Classes | |
data Fix f = In { out :: (f (Fix f)) } | |
instance (Show (f (Fix f))) => Show (Fix f) where | |
show (In f) = "In (" ++ show f ++ ")" | |
main :: IO () | |
main = print "hello" | |
bottomUp :: Functor f => (Fix f -> Fix f) -> Fix f -> Fix f | |
bottomUp fn = | |
out -- 1) unpack | |
>>> fmap (bottomUp fn) -- 2) recurse | |
>>> In -- 3) repack | |
>>> fn -- 4) apply | |
data Stump a f = Node a | Branch f f deriving (Functor, Show) | |
type Tree a = Fix (Stump a) | |
tree :: Tree Int | |
tree = In (Branch | |
(In (Branch (In (Node 1)) (In (Node 2)))) | |
(In (Branch (In (Node 3)) (In (Node 4))))) | |
mappo f (In (Node x)) = In (Node (f x)) | |
mappo f x = x | |
inc = mappo succ | |
tree2 = bottomUp inc tree | |
summo (In (Node x)) = In (Node x) | |
summo (In (Branch (In (Node x)) (In (Node y)))) = In (Node (x + y)) | |
summo _ = In (Node 0) | |
foldo f (In (Node x)) = In (Node x) | |
foldo f (In (Branch (In (Node x)) (In (Node y)))) = In (Node (f x y)) | |
foldo f t = t | |
tree3 = bottomUp summo tree | |
tree4 = bottomUp summo tree2 | |
factorialtree5 = bottomUp (foldo (*)) tree2 |
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