我提供了this question的答案,并且想到了使用Cont
monad的想法。我对Haskell不够了解,无法解释为什么该程序不起作用
import Control.Monad.Cont
fib1 n = runCont (slow n) id
where
slow 0 = return 0
slow 1 = return 1
slow n = do
a <- slow (n - 1)
b <- slow (n - 2)
return a + b
main = do
putStrLn $ show $ fib1 10
错误-
main.hs:10:18: error:
• Occurs check: cannot construct the infinite type: a2 ~ m a2
• In the second argument of ‘(+)’, namely ‘b’
In a stmt of a 'do' block: return a + b
In the expression:
do a <- slow (n - 1)
b <- slow (n - 2)
return a + b
• Relevant bindings include
b :: a2 (bound at main.hs:9:7)
a :: a2 (bound at main.hs:8:7)
slow :: a1 -> m a2 (bound at main.hs:5:5)
|
10 | return a + b
|
但这对我来说没有意义。为什么有a2
和m a2
?我期望a
和b
属于同一类型。
这让我很烦,因为同一程序在JavaScript中工作得很好。也许Haskell需要一个类型提示?
const runCont = m => k =>
m (k)
const _return = x =>
k => k (x)
const slow = n =>
n < 2
? _return (n)
: slow (n - 1) (a =>
slow (n - 2) (b =>
_return (a + b)))
const fib = n =>
runCont (slow(n)) (console.log)
fib (10) // 55
答案 0 :(得分:5)
return a + b
解析为(return a) + b
,而您想要return (a + b)
。请记住,函数应用程序的绑定比任何infix运算符都更紧密。
(写return $ a + b
也很常见,这等同于同一件事)