Question

我想知道是否可以在这种情况下使用方法代替常量：

fromEvent :: Event -> Maybe WorkspaceCreated
fromEvent event =
  case (eventName $ header event) of
    "workspaceCreated" -> Just WorkspaceCreated {eventId = CoreEvent.eventId $ header event, workspaceId = aggregateId $ header event}
    _ -> Nothing

类似这样：

eventNameForWorkspaceCreated = "workspaceCreated" :: String
fromEvent :: Event -> Maybe WorkspaceCreated
fromEvent event =
  case (eventName $ header event) of
    eventNameForWorkspaceCreated -> Just WorkspaceCreated {eventId = CoreEvent.eventId $ header event, workspaceId = aggregateId $ header event}
    _ -> Nothing

在最后一个示例中，使用eventNameForWorkspaceCreated作为有意义的变量名称...

Answer 1

不直接。名称被视为不可辩驳的模式；名称的值不用作匹配的模式。

相反，您可以使用PatternSynonyms扩展名定义模式同义词。

{-# LANGUAGE PatternSynonyms -#}
pattern EventNameForWorkspaceCreated = "workspaceCreated"

fromEvent :: Event -> Maybe WorkspaceCreated
fromEvent event = case (eventName $ header event) of
                EventNameForWorkspaceCreated -> Just WorkspaceCreated {eventId = CoreEvent.event}
                _ -> Nothing

但是，如果您实际上不需要模式匹配，则保护语法可能会更简单。（可以比较String个值是否相等。）

fromEvent event | eventName $ header event == eventNameForWorkspaceCreated = Just WorkspaceCreated {eventId = CoreEvent.event}
                | otherwise = Nothing

在有fct的情况下，我们可以进行模式匹配吗？

1 个答案: