counter.hs

module Counter where

-- カウンタを表わす型
data Counter a = Counter { val       -- 値
                         , step :: a -- 増分
                         } deriving Show

-- カウンタの値を増加させる
next :: Num a => Counter a -> (a, Counter a)
next (Counter v s) = let v' = v + s
                     in (v', Counter v' s)

-- カウンタ操作型を定義
type CounterOp a b = Counter a -> (b, Counter a)

-- カウンタ操作型をつなげる関数
comb :: CounterOp a b -> (b -> CounterOp a c ) -> CounterOp a c
comb m n = \counter0 -> let (val1, counter1) = m counter0
                            (val2, counter2) = n val1 counter1
                        in  (val2, counter2)

comb_ :: CounterOp a b -> CounterOp a c -> CounterOp a c
comb_ m n = m `comb` \_ -> n

ret :: b -> CounterOp a b
ret x = \counter -> (x, counter)

-- next を 3 回つなげた関数
next3 = next `comb_` next `comb_` next

-- next を 3 回つなげて、カウンタの値を足し合わせる関数
next3' = next `comb` \x1 ->
         next `comb` \x2 ->
         next `comb` \x3 ->
         ret $ x1+x2+x3