在为卫士执行模块表达式时，我的Haskell代码中出现了三个“变量不在作用域”错误

-- 错误示例
main = do
    print x -- x 未定义

-- 正确示例
main = do
    let x = 10
    print x

-- 错误示例
module Main where

x = 10

main = do
    print y -- y 未定义

-- 正确示例
module Main where

x = 10

main = do
    let y = x + 5
    print y

-- 错误示例
main = do
    print Data.List.length [1, 2, 3] -- 未导入 Data.List

-- 正确示例
import Data.List (length)

main = do
    print $ length [1, 2, 3]

module Main where

main = do
    let x = 10
    let y = z + 5 -- z 未定义
    print y

module Main where

main = do
    let x = 10
    let z = 20
    let y = z + 5
    print y