箭头/HXT 和类型签名
我正在尝试学习 Haskell 中的 Arrow，因此我正在使用基于 XML 的 HXT 库的箭头编写一个简单的应用程序。 HXT wiki 和教程中的示例放弃了函数类型签名。然而，我非常喜欢类型，并且正在尝试研究如何使用它们。这就是我遇到的绊脚石。给定这些函数：
readXml str = runX (readString [withValidate no] str)
atTag tag = deep (isElem >>> hasName tag)
我认为应该为它们分配以下签名：
readXml ∷ String → IO [XmlTree]
atTag ∷ ArrowXml a ⇒ String → a XmlTree XmlTree
我正在尝试使用箭头语法将它们连接在一起：
parseItem = proc str -> do
desc <- text <<< atTag "description" <<< arr readXml -< str
...
但是，如果我的类型签名是正确的（GHC 没有抱怨），我需要一种方法其中结合 monad 语法和箭头语法来获取 XmlTree 并返回到 IO。
我不确定如何继续。有人有任何见解吗？
I am trying to learn Arrows in Haskell, so I am writing a simple application with the arrow based HXT library for XML. The examples in the HXT wikis and tutorials forgo function type signatures. However, I am quite fond of types and am trying to work out how to use them. This is where I have met a stumbling block. Given these functions:
readXml str = runX (readString [withValidate no] str)
atTag tag = deep (isElem >>> hasName tag)
I figure they should be assigned the following signatures:
readXml ∷ String → IO [XmlTree]
atTag ∷ ArrowXml a ⇒ String → a XmlTree XmlTree
I am trying to hook these together using arrow syntax as such:
parseItem = proc str -> do
desc <- text <<< atTag "description" <<< arr readXml -< str
...
However, if my my type-signatures are correct (GHC hasn't complained), I would need a way in which to combine monad syntax and arrow syntax to get the XmlTree out of and returned to IO.
I am unsure how to proceed. Anyone have any insights?
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（1）
在
readXml的定义中使用runX将箭头“转换”为函数，并在 parseItem 的定义中使用arr将该函数转换回函数又是一个箭头。现在，这样做就可以了，除了readString返回一个IOStateArrow（IOSLA的特殊类型别名 - IO State List Arrow )，它不仅应该被视为一个Arrow，更具体地说，应该被视为一个IOArrow；同时，您可以使用arr重新包装它，将其视为纯Arrow。这里有两个选择：
readXml = readString [withValidate no]，以便readXml :: String -> IOStateArrow sb XmlTree。然后你就可以做... <<<。parseItem中的 readXml str在这种情况下我会使用选项 1，因为如果没有特殊原因，执行此箭头包装-展开似乎是多余的。
Using
runXin the definition ofreadXml"converts" an arrow into a function, and usingarrin the definition of parseItem converts that function back into an arrow again. Now, doing it like this would be fine, except thatreadStringreturns anIOStateArrow(a special type alias for theIOSLA- IO State List Arrow), which should be treated not only as anArrow, but more specifically as anIOArrow; meanwhile, you are treating it as a pureArrowby rewrapping it usingarr.You have two options here:
readXml = readString [withValidate no], so thatreadXml :: String -> IOStateArrow s b XmlTree. Then you can just do... <<< readXml strinparseItem.arrIOto lift readXml into an IO arrow, which lets you use it the way you intended.I would use option 1 in this case, since it seems superfluous to do this arrow-wrapping-unwrapping if there's no special reason for it.