Question

I needed to parse an XML file in Haskell, so I chose HXT. I like it so far, but I’m having trouble figuring out how to do one thing.

The file I’m parsing contains information as a config file. It has a structure similar to

<clients>
    <client>
        <name>SomeName</name>
        <info>MoreInfo</info>
        <table>
            <row>
                <name>rowname1</name>
                <value>rowvalue1</value>
            </row>
            <row>
                <name>rowname2</name>
                <value>rowvalue2</value>
            </row>
        </table>
    </client>
    ...
</clients>

This markup format makes me cringe, but it’s what I have to work with.

I have records for each of these in Haskell as follows

data Client = Client { name :: String, info :: String, table :: Table }
data Row = Row { name :: String, value :: String }
type Table = [Row]

And I want to get the data out of the file as a list of Clients. My current code looks like

data Client = Client { name :: String, info :: String, table :: Table }
data Row = Row { name :: String, value :: String }
type Table = [Row]

getClients = atTag "client" >>>
    proc client -> do
        name <- childText "name" -< client
        info <- childText "info" -< client
        table <- getTable <<< atTag "table" -< client

        returnA -< Client name info table
    where
        atTag tag = isElem >>> hasName tag
        atChildTag tag = getChildren >>> atTag tag
        text = getChildren >>> getText
        childText tag = atChildTag tag >>> text

        getTable = atChildTag "row" >>>
            proc row -> do
                name <- childText "name" -< row
                value <- childText "value" -< row
                returnA -< Row name value

But it doesn’t compile because it only gets a single Row back from getTable, instead of a list of Rows. Since this is my first time using HXT, I know I’m doing something wrong, but I don’t know how to fix it.

Any help would be great, thanks!

Answer 1

I ended up finding the answer in a related problem, I didn’t know of the existence of listA (I’m also really new to Arrows), and that fixed it!