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  1. -- http://stackoverflow.com/a/29880293/849891
  2.  
  3. {-
  4. zipw :: [a] -> [a] -> [a]
  5. zipw xs ys = (zipper xs) (zipper ys) where
  6.     zipper xs q = foldr (\ x r q -> x : q r) (const []) xs q
  7.                         --- c -------------- --- n ----
  8.  -- zipper [x1,x2,x3] (zipper ys) =
  9.  -- c x1 (c x2 (c x3 n)) (zipper ys)
  10.        --- r --------  --- q -----  tr ~ tq ; q r :: [a]
  11.                                  -- => r :: tq -> [a] 
  12.                                  --  tr ~   tr -> [a]         <<------<<
  13. -}
  14. newtype TR a = Pack { unpack :: TR a -> [a] }      --  !!!!!! <<------<<
  15.  
  16. zipw :: [a] -> [a] -> [a]
  17. zipw xs ys = unpack (zipper xs) (zipper ys) where
  18.     zipper :: [a] -> TR a
  19.     zipper = foldr (\ x r -> Pack $ \q -> x : unpack q r)
  20.                    (Pack $ const [])
  21.  
  22. main1 = print . take 6 $ zipw [1..] [10,20..]
  23. -- [1,10,2,20,3,30]
  24.  
  25. -- http://stackoverflow.com/q/29885983/849891
  26. {-
  27. zipp :: [a] -> [b] -> [(a,b)]
  28. zipp xs ys = zip1 xs (zip2 ys)
  29.   where
  30.      -- zip1 :: [a] -> tq -> [(a,b)]          -- zip1 xs :: tr ~ tq -> [(a,b)]
  31.      zip1 xs q = foldr (\ x r q -> q x r ) n xs q 
  32.                        -------- c --------
  33.      n    q  = []
  34.  
  35.      -- zip2 :: [b] -> a -> tr -> [(a,b)]     -- zip2 ys :: tq ~ a -> tr -> [(a,b)]
  36.      zip2 ys x r = foldr (\ y q x r -> (x,y) : r q ) m ys x r  
  37.                          ---------- k --------------
  38.      m  x r  = []
  39. -}
  40.  
  41. -- tr ~ tq -> [(a,b)]
  42. -- tq ~ a -> tr -> [(a,b)]
  43.  
  44. newtype Tr a b = PackR { unpackR ::  Tq a b -> [(a,b)] }
  45. newtype Tq a b = PackQ { unpackQ ::  a -> Tr a b -> [(a,b)] }
  46.  
  47. main = print $ zipp [1..] [10,20,30]
  48. -- [(1,10),(2,20),(3,30)]
  49.  
  50. zipp :: [a] -> [b] -> [(a,b)]
  51. zipp xs ys = unpackR (zip1 xs) (zip2 ys)
  52.   where
  53.      -- zip1 :: [a] -> Tr a b
  54.      zip1 = foldr (\ x r -> PackR $ \q -> unpackQ q x r ) n 
  55.      n = PackR (\_ -> [])
  56.  
  57.      -- zip2 :: [b] -> Tq a b
  58.      zip2 = foldr (\ y q -> PackQ $ \x r -> (x,y) : unpackR r q ) m 
  59.      m = PackQ (\_ _ -> [])
Success #stdin #stdout 0s 4656KB
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https://ideone.com/tgAK0A
http://stackoverflow.com/questions/29885983/infinite-type-error-when-defining-zip-with-foldr-only-can-it-be-fixed -- RECURSIVE TYPES
language:
Haskell (ghc 8.4.4)
created:
8 years ago
visibility:
public

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