haskell - 用于图形库的 xml-tree 解析器 (Haskell)

Question

我正在编写一个用于处理图形的库。
主要任务 - 解析 xml-tree。
树看起来像

<graph nodes=4 arcs=5>
    <node id=1 />
    <node id=2 />
    <node id=3 />
    <node id=4 />
    <arc from=1 to=2 />
    <arc from=1 to=3 />
    <arc from=1 to=4 />
    <arc from=2 to=4 />
    <arc from=3 to=4 />
</graph>

存储结构:

type Id = Int

data Node = Node Id deriving (Show)
data Arc = Arc Id Id deriving (Show)

data Graph = Graph { nodes :: [Node],
             arcs  :: [Arc]}

如何将xml文件中的数据写入这个结构？
我无法为这种 xml 树编写解析器(HXT 库)

Answer 1

假设您将其转换为正确的 XML(用引号将所有属性值括起来)，以下代码将起作用(使用 xml-enumerator):

{-# LANGUAGE OverloadedStrings #-}
import Text.XML.Enumerator.Parse
import Control.Monad
import Data.Text (unpack)
import Control.Applicative

type Id = Int

data Node = Node Id deriving (Show)
data Arc = Arc Id Id deriving (Show)

data Graph = Graph { nodes :: [Node],
             arcs  :: [Arc]}
  deriving Show

main = parseFile_ "graph.xml" decodeEntities $ force "graph required" parseGraph

parseGraph = tagName "graph" getCounts $ \(nodeCount, arcCount) -> do
    nodes <- replicateM nodeCount parseNode
    arcs <- replicateM arcCount parseArc
    return $ Graph nodes arcs
  where
    requireNum name = do
        x <- requireAttr name
        case reads $ unpack x of
            (i, _):_ -> return i
            _ -> fail $ "Invalid integer: " ++ unpack x
    getCounts = do
        n <- requireNum "nodes"
        a <- requireNum "arcs"
        return (n, a)
    parseNode = force "node required" $ tagName "node"
        (Node <$> requireNum "id") return
    parseArc = force "arc required" $ tagName "arc"
        (Arc <$> requireNum "from" <*> requireNum "to") return

输出:

Graph {nodes = [Node 1,Node 2,Node 3,Node 4], arcs = [Arc 1 2,Arc 1 3,Arc 1 4,Arc 2 4,Arc 3 4]}