Xml in Lua

Our program demonstrates XML handling in Lua. We’ll use the lxp library for XML parsing and the luaexpat library for XML generation. Here’s the full source code:

local lxp = require("lxp")
local expat = require("luaexpat")

-- Plant will be mapped to XML. We'll use a table to represent the structure.
-- In Lua, we don't have built-in support for XML attributes, so we'll use a special
-- key '_attr' to represent attributes.
local Plant = {
    _attr = { id = 0 },
    name = "",
    origin = {}
}

function Plant:new(o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    return o
end

function Plant:__tostring()
    return string.format("Plant id=%d, name=%s, origin=%s",
        self._attr.id, self.name, table.concat(self.origin, ", "))
end

-- Function to create XML string
local function toXML(t)
    local result = {}
    for k, v in pairs(t) do
        if k == "_attr" then
            for attr, val in pairs(v) do
                table.insert(result, string.format('%s="%s"', attr, val))
            end
        elseif type(v) == "table" then
            for _, item in ipairs(v) do
                table.insert(result, string.format("<%s>%s</%s>", k, item, k))
            end
        else
            table.insert(result, string.format("<%s>%s</%s>", k, v, k))
        end
    end
    return string.format("<plant %s>%s</plant>", table.concat(result, " "), table.concat(result, ""))
end

-- Main function
local function main()
    local coffee = Plant:new({_attr = {id = 27}, name = "Coffee", origin = {"Ethiopia", "Brazil"}})

    -- Generate XML representing our plant
    local xml = toXML(coffee)
    print(xml)

    -- To add a generic XML header to the output, prepend it explicitly
    print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" .. xml)

    -- Parse XML back into a table
    local p = {}
    local parser = lxp.new({
        StartElement = function(parser, name, attrs)
            if name == "plant" then
                p._attr = attrs
            end
        end,
        CharacterData = function(parser, data)
            if parser:getcurrentbytecount() > 0 then
                table.insert(p, data)
            end
        end
    })
    parser:parse(xml)
    parser:close()

    print(Plant:new(p))

    local tomato = Plant:new({_attr = {id = 81}, name = "Tomato", origin = {"Mexico", "California"}})

    -- Create a nested structure
    local nesting = {
        plant = {coffee, tomato}
    }

    -- Generate nested XML
    local function nestedToXML(t)
        local result = {}
        for k, v in pairs(t) do
            if type(v) == "table" then
                if #v > 0 then
                    for _, item in ipairs(v) do
                        table.insert(result, toXML(item))
                    end
                else
                    table.insert(result, nestedToXML(v))
                end
            else
                table.insert(result, string.format("<%s>%s</%s>", k, v, k))
            end
        end
        return table.concat(result, "\n")
    end

    print("<nesting>\n  <parent>\n    <child>")
    print(nestedToXML(nesting))
    print("    </child>\n  </parent>\n</nesting>")
end

main()

To run the program, save it as xml_example.lua and use the Lua interpreter:

$ lua xml_example.lua
<plant id="27" name="Coffee"><origin>Ethiopia</origin><origin>Brazil</origin></plant>
<?xml version="1.0" encoding="UTF-8"?>
<plant id="27" name="Coffee"><origin>Ethiopia</origin><origin>Brazil</origin></plant>
Plant id=27, name=Coffee, origin=Ethiopia, Brazil
<nesting>
  <parent>
    <child>
<plant id="27" name="Coffee"><origin>Ethiopia</origin><origin>Brazil</origin></plant>
<plant id="81" name="Tomato"><origin>Mexico</origin><origin>California</origin></plant>
    </child>
  </parent>
</nesting>

This Lua code demonstrates XML handling, including creating XML structures, generating XML strings, parsing XML, and working with nested XML structures. Note that Lua doesn’t have built-in XML support, so we’re using third-party libraries and custom functions to handle XML operations.

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