Question

有没有办法更简洁地写这个？我有很多看起来像这样的功能。它们中的每一个都有一些布尔条件，然后返回一个值或Nothing

rootMiddleware :: Application -> Application
rootMiddleware app req respond =
    fromMaybe next . fmap respond $
          serveIndex ["questionnaire"] "../app/answer/answer.html" req
      <|> serveIndex ["survey"] "../app/builder/builder.html" req
      <|> redirect [] "/survey/forms" req

  where
    next = app req respond

serveIndex :: [Text] -> FilePath -> Request -> Maybe Response
serveIndex prefix fp req =
    if prefix `isPrefixOf` pathInfo req
      then Just $ responseFile status200 [("Content-Type", "text/html")] fp Nothing
      else Nothing

redirect :: [Text] -> ByteString -> Request -> Maybe Response
redirect pathParts url req =
    if pathParts == pathInfo req
      then Just $ redirectTo url
      else Nothing

when非常接近，但它不允许您在应用中返回值。这种情况有同等的东西吗？

Answer 1

听起来你正在寻找guard：

guard :: Alternative f => Bool -> f ()
guard c = if c then pure () else empty

然后你可以重写

if c then Just x else Nothing

作为

x <$ guard c

如果您不直接Just，请考虑

guard c *> e

也适用于do表示法。感谢Daniel Wagner的<$表达。

另请注意

fromMaybe x . fmap f

写得更好

maybe x f

函数如“when”但返回值？

1 个答案: