Lua: Sane Granular Defaults for User Supplied Data

kergoth (Chris Larson) — Mon, 05 Dec 2005 16:05:04 GMT

This code is an example of how to implement sane defaults for user supplied data in a generically useful fashion. It was written for use in World of Warcraft User Interface addons, though it would be useful elsewhere.

local mooDB



local defaults = {

    a = {

        foo = "bar"

    },

    b = 1,

    c = {

        bar = "foo"

    },

}



if not mooDB then

    -- NOTE: This would not be in the actual real world code.  This

    -- simulates user data, normally loaded from some sort

    -- of datastore, which is SavedVariables in the case of

    -- World of Warcraft.

    mooDB = {

        c = {

            bop = "whee"

        }

    }

end



function inheritdefaults(t, defaults)

    for k,v in pairs(defaults) do

        if t[k] then

            if type(v) == "table" then

                inheritdefaults(t[k], v)

            end

        end

    end



    local mt = getmetatable(t) or {}

    function mt:__index(k)

        return defaults[k]

    end

    return setmetatable(t, mt)

end



inheritdefaults(mooDB, defaults)



assert(mooDB ~= defaults)

assert(mooDB.a == defaults.a)

assert(mooDB.c ~= defaults.c)

assert(mooDB.a.foo == "bar")

assert(mooDB.b == 1)

assert(mooDB.c.bar == "foo")

assert(mooDB.c.bop == "whee")

DZone Snippets: metatables code

Lua: Sane Granular Defaults for User Supplied Data