Created
December 24, 2013 07:12
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Haskell コード片鑑賞：「再帰」編 ref: http://qiita.com/nobsun/items/6c2fa63f294dbd98d5cb
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| factorial :: Integer -> Integer | |
| factorial (n+1) = (n+1) * factorial n | |
| factorial 0 = 1 |
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| append :: [a] -> [a] -> [a] | |
| append (x:xs) ys = x : append xs ys | |
| append [] ys = ys |
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| gappend :: ([a] -> [a] -> [a]) -> ([a] -> [a] -> [a]) | |
| gappend _ [] ys = ys | |
| gappend f (x:xs) ys = x : f xs ys | |
| append' :: [a] -> [a] -> [a] | |
| append' = fix gappend |
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| gfibs :: [Integer] -> [Integer] | |
| gfibs f = 0:1:zipWith (+) f (tail f) | |
| fibs' :: [Integer] | |
| fibs' = fix gfibs |
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| gpascal :: [[Integer]] -> [[Integer]] | |
| gpascal f = [1]:[ zipWith (+) ([0]++p) (p++[0]) | p <- f ] | |
| pascal' :: [[Integer]] | |
| pascal' = fix gpascal |
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| undefined' :: a | |
| undefined' = fix id |
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| fix :: (a -> a) -> a | |
| fix f = f (fix f) |
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| fibs :: [Integer] | |
| fibs = 0:1:zipWith (+) fibs (tail fibs) |
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| pascal :: [[Integer]] | |
| pascal = [1]:[ zipWith (+) ([0]++p) (p++[0]) | p <- pascal ] |
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| undefined :: a | |
| undefined = undefined |
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| gfactorial :: (Integer -> Integer) -> (Integer -> Integer) | |
| gfactorial _ 0 = 1 | |
| gfactorial f (n+1) = (n+1) * f n | |
| factorial' :: Integer -> Integer | |
| factorial' = fix gfactorial |
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| factorial' 0 | |
| ==> {- factorial' の定義 -} | |
| fix gfactorial 0 | |
| ==> {- fix の定義 -} | |
| (let x = gfactorial x in x) 0 | |
| ==> {- x を展開 -} | |
| gfactorial x 0 | |
| ==> {- gfactorial の定義 -} | |
| 1 |
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| factorial' 1 | |
| ==> {- factorial' の定義 -} | |
| fix gfactorial 1 | |
| ==> {- fix の定義 -} | |
| (let x = gfactorial x in x) 0 | |
| ==> {- let -} | |
| let x = gfactorial x in x 0 | |
| ==> {- x = gfactorial x -} | |
| let x = gfactorial x in gfactorial x 1 | |
| ==> {- x = gfactorial x -} | |
| let x = gfactorial x in gfactorial (gfactorial x) 1 | |
| ==> {- gfactorial の定義 -} | |
| let x = gfactorial x in 1 * (gfactorial x 0) | |
| ==> {- gfactorial の定義 -} | |
| let x = gfactorial x in 1 * 1 | |
| ==> {- 算術演算 -} | |
| 1 |
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| factorial' 2 | |
| ==> {- factorial' の定義 -} | |
| fix gfactorial 2 | |
| ==> {- fix の定義 -} | |
| (let x = gfactorial x in x) 2 | |
| ==> {- let -} | |
| let x = gfactorial x in x 2 | |
| ==> {- x = gfactorial x -} | |
| let x = gfactroial x in gfactorial x 2 | |
| ==> {- x = gfactorial x -} | |
| let x = gfactorial x in gfactorial (gfactorial x) 2 | |
| ==> {- gfactorial の定義 -} | |
| let x = gfactorial x in 2 * (gfactorial x 1) | |
| ==> {- x = gfactorial x -} | |
| let x = gfactorial x in 2 * (gfactorial (gfactorial x) 1) | |
| ==> {- gfactorial の定義 -} | |
| let x = gfactorial x in 2 * (1 * gfactorial x 0) | |
| ==> {- gfactorial の定義 -} | |
| let x = gfactorial x in 2 * (1 * 1) | |
| ==> {- 算術演算 -} | |
| 2 |
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