Rails 3：在某些部分强制HTTP，在其他部分强制使用HTTPS？(Rails 3: Forcing HTTP in some parts, and Forcing HTTPS in others?)

 由于doSomething [1]是[2,1] ，因此您的代码相当于： 
 
afterThreeTurns = do
   first <- ["test"]
   x <- [2,1]
   return first
 
 这与列表理解[ first | first <- ["test"], x <- [2,1] ] [ first | first <- ["test"], x <- [2,1] ] ，这就解释了为什么你得到长度为2的列表。 
 
 请注意，变量x在任何地方都没有被引用，所以这也可以写成： 
 
afterThreeTurns = do
   first <- ["test"]
   _ <- [2,1]
   return first
 
 这是使用IO monad的一个类似情况。 代码： 
 
thirdLine = do
  getLine
  getLine
  x <- getLine
  putStrLn $ "The third line is: " ++ x
 
 是相同的： 
 
thirdLine = do
  _ <- getLine
  _ <- getLine
  x <- getLine
  putStrLn $ "The third line is: " ++ x
 
 您可以-fwarn-unused-do-bind编译器标志来让ghc标记这些类型的-fwarn-unused-do-bind语句。 在你的例子中，ghc会发出警告： 
 
...: Warning:
    A do-notation statement discarded a result of type ‘Int’
    Suppress this warning by saying ‘_ <- doSomething [1]’
    or by using the flag -fno-warn-unused-do-bind

Since doSomething [1] is [2,1], your code is equivalent to:
 
afterThreeTurns = do
   first <- ["test"]
   x <- [2,1]
   return first
 
This is the same as the list comprehension [ first | first <- ["test"], x <- [2,1] ] which explains why you are getting a list of length 2.
 
Note that the variable x is not referenced anywhere, so this could also be written:
 
afterThreeTurns = do
   first <- ["test"]
   _ <- [2,1]
   return first
 
Here is an analogous case using the IO monad. The code:
 
thirdLine = do
  getLine
  getLine
  x <- getLine
  putStrLn $ "The third line is: " ++ x
 
is the same as:
 
thirdLine = do
  _ <- getLine
  _ <- getLine
  x <- getLine
  putStrLn $ "The third line is: " ++ x
 
You can get ghc to flag these kinds of monadic statements with the -fwarn-unused-do-bind compiler flag. In your example ghc will emit the warning:
 
...: Warning:
    A do-notation statement discarded a result of type ‘Int’
    Suppress this warning by saying ‘_ <- doSomething [1]’
    or by using the flag -fno-warn-unused-do-bind