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Haskell包hxt的使用對我來說仍然有點奇怪。特別是箭頭符號和由此產生的類型是一種魔力。幾次處理一個XML文檔
我到目前爲止無法管理以下內容：我想處理一個主要由兩部分組成的XML文件。一個保留對象的定義，第二個對象的用法/目的。首先，我想寫一些hxt處理來獲取part1上的Haskell數據結構，在處理後的第2部分之後，最後將兩個數據結構組合在一起，以讀取程序的真實邏輯。
處理文件現在正常，感謝the arrows tutorial。但是我想現在有一個符號做三個步驟：讀取文檔（懶惰），用第一個處理器處理結果結構一次，然後再用第二個處理器處理相同的結構。我不想要的是像下面的例子那樣調用「readDocument」兩次。
import Text.XML.HXT.Core
import Data.Char(toUpper)
import Data.Tree.NTree.TypeDefs
play filename = do
results <- runX (getAllAddresses filename)
results2 <- runX (getAllAddressesUsages filename)
print results
print results2
getAllAddresses :: FilePath -> IOSArrow XmlTree [(String,NTree XNode)]
getAllAddresses filename =
readDocument [withValidate no] filename >>>
getChildren >>>
isElem >>> hasName "main" >>>
getChildren >>>
isElem >>> hasName "part1" >>>
getChildren >>>
isElem >>> hasName "address" >>>
listA(getAddress) -- create a list for each variable, so use listA
getAddress :: IOSArrow XmlTree (String,NTree XNode)
getAddress =
getChildren >>>
isElem >>>
(
neg (hasName "location") >>> -- all elements being no "location"
getName &&& (getChildren) -- get the name and the value for each element
)
<+>
(
hasName "location" >>> -- work on all nodes within the "location" subcontainer
getChildren >>>
isElem >>>
(getName &&& (getChildren)) -- get the name and the value for each element
)
getAllAddressesUsages :: FilePath -> IOSArrow XmlTree [(String,NTree XNode)]
getAllAddressesUsages filename =
readDocument [withValidate no] filename >>>
getChildren >>>
isElem >>> hasName "main" >>>
getChildren >>>
isElem >>> hasName "part2" >>>
getChildren >>>
listA(getAddressUsagePurpose2) -- create a list for each variable, so use listA
getAddressUsagePurpose2 :: IOSArrow XmlTree (String,NTree XNode)
getAddressUsagePurpose2 =
hasName "use_obj-names_for_purpose_2" >>> -- work on all nodes with usage 2
(getName &&& (getChildren)) -- get the name and the value for each element
示例數據：
<main>
<part1>
<address>
<obj-name>one</obj-name>
<name>peter 1</name>
<street>streetname 1</street>
<location>
<country>Germany</country>
<state>Baden Wuerttemberg</state>
</location>
</address>
<address>
<obj-name>two</obj-name>
<name>peter 2</name>
<street>streetname 2</street>
<location>
<country>Germany</country>
<state>Nordrhein Westfalen</state>
</location>
</address>
</part1>
<part2>
<use_obj-names_for_purpose_1>
<obj-name>two</obj-name>
</use_obj-names_for_purpose_1>
<use_obj-names_for_purpose_2>
<obj-name>two</obj-name>
</use_obj-names_for_purpose_2>
</part2>
</main>
所以正規的問題是：
怎樣的單子在比賽中的作用看，爲了得到這樣做，如：
readXmlDocument :: String -> IOSArrow XmlTree (NTree XNode)
readXmlDocument filename = readDocument [withValidate no] filename
play filename = do
document <- readXmlDocument filename
allAddresses <- getAllAddresses document
allPurposes <- getAllAddressesUsages document
result <- processLogics allAddresses allPurposes
print result
我如何從Monads到Arrows，回到Monads，再回到普通數據，然後回到Monads。
爲什麼我這麼做？