MWE:
import Control.Monad.State.Lazy
fibStep :: State (Integer, Integer) ()
fibStep = state $ \(a, b) -> ((), (b, a + b))
execStateN :: Int -> State s a -> s -> s
execStateN n m s
| n == 1 = execState m s
| n > 1 = let s' = execState m s in
execStateN (n - 1) m s'
-- | n > 1 = execStateN (n - 1) m s' where s' = execState m s
| otherwise = error "undefined behaviour"
它可以工作,但是一旦我取消注释 where
变量并注释 let
变量,就会出现语法错误:
error: parse error on input ‘|’
我检查了缩进,它们很好。怎么了?
最佳答案
where
的范围涵盖所有守卫,因此您将其放在守卫的末尾,例如:
execStateN :: Int -> State s a -> s -> s
execStateN n m s
| n == 1 = execState m s
| n > 1 = execStateN (n - 1) m s'
| otherwise = error "undefined behaviour"
<b>where</b> s' = execState m s
关于haskell - 带防护功能 : syntax error when using "where",我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/58583869/