Lisp-style lazy-lists in Lua

gistfile1.lua

function makeThunk(f, args)
    return { tag = "thunk", f = f, args = args }
end

function evalThunk(t)
    return t.f(unpack(t.args))
end

function cons(first, rest)
    return { first = first,
             rest  = rest }
end

function range(b, e, s)
    b = b or 0
    s = s or 1

    if e == nil or b <= e then
        return cons(b, makeThunk(range, {b+s, e, s}))
    end
end

function evalPart(t, p)
    if t == nil then
        return nil
    elseif type(t[p]) == "table" and t[p].tag == "thunk" then
        t[p] = evalThunk(t[p])
    end
    return t[p]
end

function first(t)
    return evalPart(t, "first")
end

function rest(t)
    return evalPart(t, "rest")
end

function nth(t, n)
    if n == 0 then
        return first(t)
    end
    return nth(rest(t), n-1)
end

function map(f, l)
    return cons(f(first(l)), makeThunk(map, {f, rest(l)}))
end

function filter(f, l)
    while not f(first(l)) do
        l = rest(l)
    end
    return cons(first(l), makeThunk(filter, {f, rest(l)}))
end

-- Examples --------------------------------------------------------------------

function fact(n, f)
    n = n or 1
    f = f or 1
    return cons(n, makeThunk(fact, {n*f, f+1}))
end

-- > a = fact()
-- > print(nth(a, 5))
-- 120
-- > print(nth(a, 250))
-- inf

function fib(a, b)
    a = a or 0
    b = b or 1
    return cons(a, makeThunk(fib, {b, a+b}))
end

-- > a = fib()
-- > print(nth(a, 0))
-- 0
-- > print(nth(a, 1))
-- 1
-- > print(nth(a, 2))
-- 1
-- > print(nth(a, 3))
-- 2
-- > print(nth(a, 13))
-- 233


-- Continuations ---------------------------------------------------------------

function sum(n, cont)
    if n <= 1 then
        return makeThunk(cont, {1})
    end
    local function newCont(v)
        return makeThunk(cont, {v+n})
    end

    return makeThunk(sum, {n-1, newCont})
end

function trampoline(thunk)
    while true do
        if type(thunk) ~= "table" then
            return thunk
        elseif thunk.tag == "thunk" then
            thunk = evalThunk(thunk)
        end
    end
end

-- > a = sum(123, function(n) print("result: ", n) end)
-- > trampoline(a)
-- result:         7626



-- Lazy list version
function sumList(sum, cur)
    sum = sum or 0
    cur = cur or 1
    return cons(sum, makeThunk(sumList, {sum+cur, cur+1}))
end

-- > a = sumList()
-- > print(nth(a, 10))
-- 55
-- > print(nth(a, 11))
-- 66
-- > print(nth(a, 123))
-- 7626