Note: this is the stubbed version of module Xml. You should download the lhs version of this module and replace all parts marked undefined. Eventually, the complete version will be made available.

In class exercise: XML parsing

In today's exercise you will use the definitions from the Parsers lecture to build a simple parser for XML data.

> module Xml where

> import Control.Applicative (Alternative(..))
> import System.IO
> import Prelude hiding (filter)

This exercise is based on the following definitions from the Parsers lecture. Make sure that you have downloaded the solution.

> import Parsers (Parser, satisfy, char, string, doParse, filter)
> -- Note that this line imports these functions as well as the instance for Parser
> -- for the Functor, Applicative and Alternative classes.

Note, you can also use the files distributed with hw06 by replacing the above line with the following two:

import Parser (Parser, filter, doParse)
import ParserCombinators(satisfy, char, string)

Your goal: produce this structured data from a string

> -- | A simplified datatype for storing XML
> data SimpleXML =
>           PCDATA  String
>         | Element ElementName [SimpleXML]
>       deriving Show

> type ElementName = String

First: the characters /, <, and > are not allowed to appear in tags or PCDATA. Let's define a function that recognizes them.

> reserved :: Char -> Bool
> reserved c = c `elem` ['/', '<', '>']

Use this definition to parse a maximal nonempty sequence of nonreserved characters:

> text :: Parser String
> text = undefined

Xml> doParse text "skhdjf"
[("skhdjf","")]
Xml> doParse text "akj<skdfsdhf"
[("akj","<skdfsdhf")]
Xml> doParse text ""
[]

and then use this definition to parse nonreserved characters into XML.

> pcdata :: Parser SimpleXML
> pcdata = undefined

Xml> doParse pcdata "akj<skdfsdhf"
[(PCDATA "akj","<skdfsdhf")]

Parse an empty element, like "<br/>"

> emptyContainer :: Parser SimpleXML
> emptyContainer = undefined

Xml> doParse emptyContainer "<br/>sdfsdf"
[(Element "br" [],"sdfsdf")]

Parse a container element: this consists of an open tag, a (potentially empty) sequence of content parsed by p, and matching a closing tag. For example, container pcdata should recognize <br></br> or <title>A midsummer night's dream</title>. You do NOT need to make sure that the closing tag matches the open tag.

> container :: Parser SimpleXML -> Parser SimpleXML
> container p = undefined

Xml> doParse (container pcdata) "<br></br>"
[(Element "br" [],"")]
Xml> doParse (container pcdata) "<title>A midsummer night's dream</title>"
[(Element "title" [PCDATA "A midsummer night's dream"],"")]

-- This should also work, even though the tag is wrong
Xml> doParse (container pcdata) "<title>A midsummer night's dream</br>"
[(Element "title" [PCDATA "A midsummer night's dream"],"")]

Now put the above together to construct a parser for simple XML data:

> xml :: Parser SimpleXML
> xml = undefined

Xml> doParse xml "<body>a</body>"
[(Element "body" [PCDATA "a"],"")]
Xml> doParse xml "<body><h1>A Midsummer Night's Dream</h1><h2>Dramatis Personae</h2>THESEUS, Duke of Athens.<br/>EGEUS, father to Hermia.<br/></body>"
[(Element "body" [Element "h1" [PCDATA "A Midsummer Night's Dream"],Element "h2" [PCDATA "Dramatis Personae"],PCDATA "THESEUS, Duke of Athens.",Element "br" [],PCDATA "EGEUS, father to Hermia.",Element "br" []],"")]

Now let's try it on something a little bigger. How about sample.html from hw02?

> -- | Run a parser on a particular input file
> parseFromFile :: Parser a -> String -> IO [(a,String)]
> parseFromFile parser filename = do
>   handle <- openFile filename ReadMode
>   str    <- hGetContents handle
>   return $ doParse parser str

Xml> parseFromFile xml "sample.html"

Challenge: rewrite container so that it only succeeds when the closing tag matches the opening tag.

> container2 :: Parser SimpleXML -> Parser SimpleXML
> container2 p = undefined

Xml> doParse (container2 pcdata) "<title>A midsummer night's dream</title>"
[(Element "title" [PCDATA "A midsummer night's dream"],"")]
Xml> doParse (container2 pcdata) "<title>A midsummer night's dream</br>"
[]

CIS 552: Advanced Programming

Fall 2019

In class exercise: XML parsing