haskell - 简化 Haskell 代码

Question

我正在 Snap 框架中编写 Haskell 代码。一切正常，但现在我需要清理一些代码。然而，经过几次尝试，我似乎有了更多的代码。也许有人可以给我一些指示？

这是我最初的代码。问题是，我对一个 Maybe 值进行模式匹配，在我看来这是非常错误的。所以我想解决这个问题。但是下一行将获得一个 Maybe 值，所以我必须更改它..

handleNewUserPost :: Handler App (AuthManager App) ()
handleNewUserPost = do
    Just username <- getPostParam "login"
    exists <- usernameExists $ T.decodeUtf8 username
    case exists of
        True    -> handleNewUserGet $ Just "Sorry, this username already exist."
        False   -> do
            registerUser "login" "password"
            redirect "/new_user"

最终我想到了这一点:

handleNewUserPost :: Handler App (AuthManager App) ()
handleNewUserPost = do
    username <- getPostParam "login" -- :t username = Maybe ByteString
    validate username
    where 
      validate Nothing = redirect "/new_user"
      validate (Just username) = do exists <- existcheck username
                                    if exists
                                      then userexists
                                      else register
      existcheck :: C.ByteString -> Handler b (AuthManager b) Bool
      existcheck username = (usernameExists . T.decodeUtf8) $ username
      userexists          =  handleNewUserGet $ Just "Sorry, this username already exist."
      register            = do registerUser "login" "password"
                               redirect "/new_user"

问题是我觉得我也不应该对 Nothing 进行模式匹配。我正在做事情，所以这里应该有一些东西。另一件事是，我在 register 中也有一个 do 。任何指示表示赞赏。

Answer 1

您的新代码没有任何问题。它比原始文件长的部分原因是您正在处理 Nothing情况，您最初没有这样做(因此您的新代码更安全)。如果你想稍微整理一下，你可以写成

handleNewUserPost :: Handler App (AuthManager App) ()
handleNewUserPost =
  getPostParam "login" >>= maybe (redirect "/new_user") validateUser
 where
  validateUser username = do
    exists <- usernameExists $ T.decodeUtf8 username
    if exists
      then handerNewUserGet $ Just "Sorry, this username already exists."
      else do
        registerUser "login" "password"
        redirect "/new_user"

我在哪里使用了函数maybe来自Data.Maybe无需显式处理 Just或Nothing值(value)观。该函数刚刚定义为

maybe :: b -> (a -> b) -> Maybe a -> b
maybe b _ Nothing  = b
maybe _ f (Just a) = f a