我在 IO monad haskell 中做阶乘函数。我按照示例 mod3 使用 IO monad 执行阶乘函数。我不明白为什么我的代码是错误的。我想看看n>=1,然后对n做阶乘。最后添加到 (n,r2) 并返回。任何人都可以帮助理解吗?
while :: IO Bool -> IO () -> IO ()
while test body =
do b <- test
if b
then do {body ; while test body} -- same-line syntax for do
else return ()
-- remainder when integer-dividing n by 3
mod3 :: Integer -> IO Integer
mod3 n = do r <- newIORef n
while
(do {v <- readIORef r; return (v >= 3)})
(do {v <- readIORef r; writeIORef r (v-3)})
readIORef r
-- ghci> fact 4
-- (4,24)
fact :: Integer -> IO (Integer, Integer)
fact n = do r2 <- newIORef n
while
(do {v2 <- readIORef r2; return (v2 >= 1)})
(do {v2 <- readIORef r2; writeIORef r2 (v2*fact(v2-1))})
readIORef (n,r2)