带警卫的功能：使用“ where”时的语法错误

Question

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变体并对其进行注释let，就会出现语法错误：

错误：解析输入“ |”上的错误

我检查了缩进，它们很好。怎么了？

Answer 1

将where范围限定在所有的卫兵，所以你把它放在后卫，喜欢的结尾：

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"
  where s' = execState m s