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2013-07-24T09:41:33.750
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2017-05-23T12:23:07.490
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This is just an extension to the answer by <a href="https://stackoverflow.com/users/621719">EarlGray</a>. See the explanation of <code>>>.</code> and <code>>.</code>! After asking the question I recognized that I need to walk through the tree in a special and deterministic way. So this is the solution I’m using for my specific problem. For the case someone else tries to accomplish the same thing, I wanted to share the example code. Let’s say we want to extract the text of the first <code><a></code> and the second <code></code>. Not all <code><a></code> elements have at least two <code></code>s, so the code of <a href="https://stackoverflow.com/users/621719">EarlGray</a> would bail out, because you can’t use the <code>(!!)</code> function (empty list!). Have a look at the function <code>single</code> in <a href="http://hackage.haskell.org/packages/archive/hxt/9.3.1.1/doc/html/src/Control-Arrow-ArrowList.html#single" rel="nofollow noreferrer">Control.Arrow.ArrowList</a>, which is using only the first result of the list arrow: <pre><code>single :: ArrowList a => a b c -> a b c single f = f >>. take 1 </code></pre> We wanted to extract the n-th element: <pre><code>junction :: ArrowList a => a b c -> Int -> a b c junction a nth = a >>. (take 1 . drop (nth - 1)) </code></pre> Now we can use this new arrow to build up the selector. It’s necessary to use parentheses around the stuff we’re going to filter with <code>junction</code>, because <code>junction</code> modifies an existing arrow. <pre><code>selector :: ArrowXml a => a XmlTree String selector = getChildren -- There is only one root element. -- For each selected element: Get a list of all children and filter them out. -- The junction function now selects at most one element. >>> (getChildren >>> isElem >>> hasName "a") `junction` 1 -- selects first <a> -- The same thing to select the second for all the <a>s -- (But we had selected only one <a> in this case! -- Imagine commenting out the `junction` 1 above.) >>> (getChildren >>> isElem >>> hasName "b") `junction` 2 -- selects second -- Now get the text of the element. >>> getChildren >>> getText </code></pre> To extract the value and return a Maybe value: <pre><code>main :: IO () main = do let doc = readString [] testXml text <- listToMaybe <$> (runX $ doc >>> selector) print text </code></pre> This outputs <code>Just "second element"</code> with example XML file.
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