--- lam.eval

local eval = {}
local type = require "type"
local isNull, isList, isa, List, Cons =
	type.isNull, type.isList, type.isa, type.List, type.Cons
local unpack = table.unpack or unpack

function eval.Env (inner, outer)
	local mt = {
		__type = "Environment",
		__index = outer,
	}
	return setmetatable(inner, mt)
end

function eval.Proc (params, body, env)
	local v = {
		params = params,
		body = body,
		env = env,
	}
	local mt = {
		__type = "Procedure",
		__call =
			function (self, args)
				local inner = {}
				local p, a = self.params, args
				while p.cdr and a.cdr do
					inner[p.car] = a.car
					p, a = p.cdr, a.cdr
				end
				return eval.eval(
					self.body,
					eval.Env(inner, self.env))
			end,
	}
	return setmetatable(v, mt)
end

local global = {
	begin =
		function (r)
			local r = r
			while not isNull(r.cdr) do
				r = r.cdr
			end
			return r.car
		end,
	["+"] =
		function (r)
			local r, a = r, 0
			while r.cdr do
				r, a = r.cdr, a + r.car
			end
			return a
		end,
	["-"] =
		function (r)
			if isNull(r) then return -1 end
			if isNull(r.cdr) then return (- r.car) end
			local r, a = r.cdr, r.car
			while r.cdr do
				r, a = r.cdr, a - r.car
			end
			return a
		end,
}

function eval.eval (x, env)
	env = env or global
	if isa(x, "Symbol") then
		return env[x]
	elseif not isList(x) then
		return x
	else
		local op, args = x.car, x.cdr
		if op == "quote" then
			return args.car
		elseif op == "define" then
			env[args.car] = eval.eval(args.cdr.car, env)
			return nil
		elseif op == "lambda" then
			return eval.Proc(
				args.car,
				Cons("begin", args.cdr),
				env)
		elseif op == "if" then
			assert(not isNull(args.cdr), "Malformed 'if'")
			local test, conseq, alt =
				args.car, args.cdr.car, args.cdr.cdr.car
			if eval.eval(test)
			then return eval.eval(conseq)
			else return eval.eval(alt)
			end
		else		-- procedure
			local proc = eval.eval(op, env)
			local params = {}
			local a = args
			while a.cdr do
				table.insert(params, eval.eval(a.car, env))
				a = a.cdr
			end
			return proc(List(params))
		end
	end
end

---
return eval