local function eval (prog, env)
    local env = env or {}
    local last
    for _, fn in ipairs (prog) do
        last = fn (env)
    end
    return last
end

local function force (env)
    return function (x)
        if type (x) == "function" then
            x = x (env)
        end
        return x
    end
end

local function keywords_for (apply)
    return function (word_list)
        local keywords = {}
        for _, word in ipairs (word_list) do
            keywords[word] = apply
        end
        return setmetatable (keywords, {__call = function (t, ...)
            return apply(...)
        end})
    end
end

local function default (apply)
    return function (operations)
        return setmetatable (operations, {__call = function (t, ...)
            return apply (...)
        end})
    end
end

local function let (var)
    local _let = function (val)
        return function (env)
            env[force (env) (var)] = force (env) (val)
        end
    end
    return keywords_for (_let) { "equal", "be" }
end

local function get (var)
    return function (env)
        return env[force (env) (var)]
    end
end

local function add (x)
    local _add = function (y)
        return function (env)
            return force (env) (x) + force (env) (y)
        end
    end
    return keywords_for (_add) { "to", "with" }
end

local function subtract (x)
    local _sub = function (y)
        return function (env)
            return force (env) (x) - force (env) (y)
        end
    end

    return default (_sub) {
        from = function (y)
            return function (env)
                return force (env) (y) - force (env) (x)
            end
        end,

        with = _sub
    }
end

local function show (val)
    local _show = function (env)
        print (force (env) (val))
    end

    return _show
end

local function is (a)
    local _equals = function (b)
        return function (env)
            return force (env) (a) == force (env) (b)
        end
    end

    return default (_equals) {
        greater_than = function (b)
            return function (env)
                return force (env) (a) > force (env) (b)
            end
        end,
        less_than = function (b)
            return function (env)
                return force (env) (a) < force (env) (b)
            end
        end,
    }
end

local function when (cond)
    local _when_true = function (when_true)
        local _when_false = function (when_false)
            return function (env)
                if force (env) (cond) then
                    return eval (when_true, env)
                else
                    return eval (when_false, env)
                end
            end
        end
        return { otherwise = _when_false }
    end
    return _when_true
end

local function seed_rand ()
    math.randomseed (os.time ())
end

local function rand ()
    return math.random ()
end

local function return_with (value)
    return function (env)
        return value
    end
end

local function define (name)
    return function (bindings)
        return function (body)
            return function (env)
                env[force (env) (name)] = { bindings, body }
            end
        end
    end
end

local function call (name)
    return function (args)
        return function (env)
            local call_env = setmetatable({}, {__index = env})
            local bindings, body = env[name][1], env[name][2]
            for i, binding in ipairs (bindings) do
                call_env[binding] = force (env) (args[i])
            end
            return eval (body, call_env)
        end
    end
end

print
eval {
    define "fib" { "n" } {
        when (is (get "n") .less_than (1)) {
            return_with (0) ;
        } .otherwise {
            when (is (get "n") (1)) {
                return_with (1)
            } .otherwise {
                let "n1" .equal (subtract (1) .from (get "n")) ;
                let "n2" .equal (subtract (2) .from (get "n")) ;
                add (call "fib" {get "n1"}) .with (call "fib" {get "n2"}) ;
            }
        }
    } ;
    show (call "fib" { 10 }) ;
}