求教等式为啥成立

newtype Parser a = Parser (String -> [(a, String)]) apply :: Parser a -> String -> [(a, String)] apply (Parser p) s = p s instance Monad Parser where return x = Parser (\s -> [(x, s)]) p >>= q = Parser (\s -> [(y, s'') | (x, s') <- apply p s, (y, s'') <- apply (q x) s']) fail = Parser (\s -> []) guard :: Bool -> Parser () guard True = return () guard False = fail sat :: (Char -> Bool) -> Parser Char sat p = do { c <- getc; guard (p c); return c } -- 以上是背景 -- 以下是问题 -- 为啥 guard (p c) >> return c = fail >> return c = fail -- p c == True 很好理解 guard (p c)结果被弃掉了 -- p c == False 为啥没走到 return c