Functional Programming: A PragPub Anthology

Building with Functional Primitives

We had to define our own map() earlier, which we’ll use again in a minute. Let’s define a few more functional primitives that we can arrange into bigger and better things:

function each (things, fn)
    for i, thing in ipairs(things) do
        fn(thing)
    end
end

function filter (things, fn)
    local filtered = {}
    each(things, function(thing)
        if fn(thing) then
            table.insert(filtered, thing)
        end
    end)
    return filtered
end

function cons (things, ...)
    local all = {}
    each({...}, function(t)
        table.insert(all, t)
    end)
    each(things, function(t)
        table.insert(all, t)
    end)
    return all
end

I should point out that all of these are relatively slow, because they copy elements between Lua tables. We ...