做标记法

do-notation 是 monad 的语法糖。以下是规则：

do x <- mx                               do x <- mx
   y <- my       is equivalent to           do y <- my
   ...                                         ...

do let a = b                             let a = b in
   ...           is equivalent to          do ...

do m                                     m >> (
   e             is equivalent to          e)

do x <- m                                m >>= (\x ->
   e             is equivalent to          e)

do m             is equivalent to        m

例如，这些定义是等效的：

example::IO Integer
example =
  putStrLn "What's your name?" >> (
    getLine >>= (\name ->
      putStrLn ("Hello, " ++ name ++ ".") >> (
        putStrLn "What should we return?" >> (
          getLine >>= (\line ->
            let n = (read line::Integer) in
              return (n + n))))))

example::IO Integer
example = do
  putStrLn "What's your name?"
  name <- getLine
  putStrLn ("Hello, " ++ name ++ ".")
  putStrLn "What should we return?"
  line <- getLine
  let n = (read line::Integer)
  return (n + n)