haskell - 带防护功能 : syntax error when using "where"

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？

Answer 1

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