Haskell Hxt和解析可选元素

我正在尝试在以下xml片段中使用HXT解析可选元素：

<ns9:accountBankingInfo>
  <key>
    <accountId>10</accountId>
    <bankingId>01</bankingId>
  </key>
  <bankingInfo>
    <description>Bigbank Co.</description>
    <eft>
        <bankCode>222</bankCode>
        <transit>111</transit>
        <accountNumber>3333333</accountNumber>
    </eft>
   </bankingInfo>
   <defaultType>ACCOUNT</defaultType> <!-- optional -->
</ns9:accountBankingInfo>

我正在解析accountBankingInfo元素，如下所示：

bankParser :: ArrowXml a => a XmlTree BankingInfo
bankParser = deep (isElem >>> hasLocalPart "accountBankingInfo") >>> proc x -> do
                   i <- getText <<< getChildren <<< deep (hasName "bankingId") -< x
                   d <- getText <<< getChildren <<< deep (hasName "description") -< x
                   b <- getText <<< getChildren <<< deep (hasName "bankCode") -< x
                   t <- getText <<< getChildren <<< deep (hasName "transit") -< x
                   a <- getText <<< getChildren <<< deep (hasName "accountNumber") -< x
                   g <- nc "defaultType" -< x

                   returnA -< BankingInfo i d b t a g

nc name = ifA (deep (hasName name)) (getChildren >>> getText >>> arr Just) (arr (const Nothing))

bankParser:：arrowXMLA=>XmlTree银行信息
bankParser=deep（isElem>>>hasLocalPart“accountBankingInfo”）>>>proc x->do
我刚刚发现了带default的
函数，并且能够让它像这样工作：

bankParser :: ArrowXml a => a XmlTree BankingInfo
bankParser = deep (isElem >>> hasLocalPart "accountBankingInfo") >>> proc x -> do
                   i <- getText <<< getChildren <<< deep (hasName "bankingId") -< x
                   d <- getText <<< getChildren <<< deep (hasName "description") -< x
                   b <- getText <<< getChildren <<< deep (hasName "bankCode") -< x
                   t <- getText <<< getChildren <<< deep (hasName "transit") -< x
                   a <- getText <<< getChildren <<< deep (hasName "accountNumber") -< x
                   g <- withDefault (arr Just <<< getText <<< getChildren <<< deep (hasName "defaultType")) Nothing -< x
                   returnA -< BankingInfo i d b t a g

bankParser:：arrowXMLA=>XmlTree银行信息
bankParser=deep（isElem>>>hasLocalPart“accountBankingInfo”）>>>proc x->do
我等一下，是不是应该是
nc“defaultType”
而不是
nc“accountNumber”
？你说得对@Cactus，我错了，我更新了。。。原来的问题还存在,；不过，我确实找到了答案，谢谢。
bankParser :: ArrowXml a => a XmlTree BankingInfo
bankParser = deep (isElem >>> hasLocalPart "accountBankingInfo") >>> proc x -> do
                   i <- getText <<< getChildren <<< deep (hasName "bankingId") -< x
                   d <- getText <<< getChildren <<< deep (hasName "description") -< x
                   b <- getText <<< getChildren <<< deep (hasName "bankCode") -< x
                   t <- getText <<< getChildren <<< deep (hasName "transit") -< x
                   a <- getText <<< getChildren <<< deep (hasName "accountNumber") -< x
                   g <- withDefault (arr Just <<< getText <<< getChildren <<< deep (hasName "defaultType")) Nothing -< x
                   returnA -< BankingInfo i d b t a g