Haskell中的语法错误

两个问题

正如评论中所说,你的守卫至少需要缩进一个额外的空间(所以管道更加缩进了b)

您需要的不仅仅是块let内部do.例如,您可能想要测试您的功能!

解决了这些问题:

main = do
  let bmiTell weight height
       | weight / height ^ 2 <= 18.5 = "You're underweight, you emo, you!"
       | weight / height ^ 2 <= 25.0 = "You're supposedly normal. Pffft, I bet you're ugly!"
       | weight / height ^ 2 <= 30.0 = "You're fat! Lose some weight, fatty!"
       | otherwise                   = "You're a whale, congratulations!"  
  putStrLn $ bmiTell 6 1