メモ付きのFibonacci関数(1)
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-- 「つまずきの記憶 メモ化(2)」のソースコード | |
-- メモ付きのFibonacci関数 [mfib-1.hs] | |
-- 型の定義 | |
-- 「メモ化(1)」の `a`,`b`,`s` がここの`A`,`B`,`S`に対応している。 | |
type A = Int | |
type B = Integer | |
type S = [(A,B)] | |
type St = S -> (B, S) | |
-- メモ付きFibonacci関数 | |
mfib :: A -> St | |
mfib 0 = \s-> (0, s) | |
mfib 1 = \s-> (1, s) | |
mfib n = \s-> | |
let | |
(n1, s1) = memo mfib (n-1) s | |
(n2, s2) = memo mfib (n-2) s1 | |
in | |
(n1 + n2, s2) | |
-- メモ化関数 | |
memo :: (A -> St) -> A -> St | |
memo f n = \s -> | |
case getResult n s of | |
Just v -> (v, s) -- 計算済み | |
Nothing -> let (v, s1) = f n s | |
in (v, putResult (n, v) s1) -- 新規登録 | |
-- 「メモ操作関数」 | |
getResult :: A -> S -> Maybe B | |
getResult = lookup | |
putResult :: (A, B) -> S -> S | |
putResult = (:) | |
sEmpty :: S | |
sEmpty = [] | |
-- メモ付きの関数を走らせ、結果からメモを除去する。 | |
fib :: Int -> Integer | |
fib n = fst $ mfib n sEmpty |
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