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1 # Xleb
3 The `xleb` library defines a simple monadic language for easily parsing XML structures. It does not parse the XML itself, relying on the [`xml`]( library to define the underlying types and parser, but rather exposes a simple monad with helper functions to make defining XML-based structures quick and straightforward, of roughly equal complexity to defining a `fromJSON` instance for [`aeson`](
5 ## Basic Usage
8 The `Xleb` monad describes both parsing _and_ traversing a given XML structure: several of the functions to produce `Xleb` computations take other `Xleb` computations, which are run on various sub-parts of the XML tree. Consequently, instead of decomposing an XML structure and passing it around to various functions, the `Xleb` language treats "the current location in the tree" as an implicit piece of data in the `Xleb` monad.
10 You will generally want to identify your root note with the `elem` function to ensure that your root note has the tag you expect. Children of that node can be accessed using the `child` or `children` function to either unambiguously find a specific child element, or to find all child elements that match a given selector and apply a `Xleb` computation to each of them.
12 ~~~~.haskell
13 a <- X.child (X.byTag "a") parseA
14 b <- X.children (X.byTag "b") parseB
15 ~~~~
17 Leaf data tends to come in two forms in XML: attribute values (like `\<tag attr="value"\>`) or tag content (like `\<tag\>value\<\/tag\>`). In both cases, the `Xleb` functions allow you to parse that content however you'd like by providing an arbitrary function of type `String -> Either String a`. The `xleb` library provides several built-in functions of this type for common situations.
19 ~~~~.haskell
20 c <- X.attr "index" X.number
21 d <- X.contents X.string
22 ~~~~
24 Finally, the `Xleb` monad has `Alternative` instances which allow for concise expression of optional values or multiple possibilities.
26 ~~~~.haskell
27 e <- X.children X.any (parseA <|> parseB)
28 f <- optional (X.attr "total" X.number)
29 ~~~~
31 ## Simple Example
33 Say we want to parse a simple XML feed format that looks like the following, with the extra caveat that we'd like the `author` field to be optional:
35 ~~~~.xml
36 <feed>
37 <title>Feed Name</title>
38 <author>Pierre Menard</author>
39 <entry title="Entry 01">First Post</entry>
40 <entry title="Entry 02">Second Post Post</entry>
41 </feed>
42 ~~~~
44 We can write a `Xleb` computation which is capable of parsing this structure in a handful of lines, here written in a slightly unusual way in order to show off some features of the library:
46 ~~~~.haskell
47 import Control.Applicative (optional)
48 import qualified Text.XML.Xleb as X
50 feed :: X.Xleb (String, Maybe String, [(String, String)])
51 feed = X.elem "feed" $ do
52 feedTitle <- X.child (X.byTag "title") $
53 X.contents X.string
54 feedAuthor <- optional $ X.child (X.byTag "author") $
55 X.contents X.string
56 feedEntries <- X.children (X.byTag "entry") entry
57 return (feedTitle, feedAuthor, feedEntries)
59 entry :: X.Xleb (String, String)
60 entry = (,) <$> X.attr "title" X.string <*> X.contents X.string
61 ~~~~
63 For a larger example, look at the [Atom-parsing example](examples/atom/Main.hs), which is both more idiomatic and more complete.