--- lam.eval

local base = require "base"
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

local function Env (inner, outer)
	local mt = {
		__type = "Environment",
		__index = outer,
		__newindex =
			function (self, key, value)
				if rawget(self, key) then
					-- Set the current environment's value
					rawset(self, key, value)
				else
					-- Set the outer value
					getmetatable(self).__index[key] = value
				end
			end,
	}
	return setmetatable(inner, mt)
end

local function 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
				local b = self.body
				local e = Env(inner, self.env)
				while not isNull(b.cdr) do
					eval(b.car, e)
					b = b.cdr
				end
				return eval(b.car, e)
			end,
	}
	return setmetatable(v, mt)
end

local specials = {
	quote =
		function (args, env)
			return args.car
		end,
	define =
		function (args, env)
			rawset(env, args.car, eval(args.cdr.car, env))
			return nil
		end,
	lambda =
		function (args, env)
			return Proc(args.car, args.cdr, env)
		end,
	["set!"] =
		function (args, env)
			env[args.car] = eval(args.cdr.car, env)
			return nil
		end,
	["if"] =
		function (args, env)
			local test, conseq, alt =
				args.car, args.cdr.car, args.cdr.cdr.car
			if eval(test)
			then return eval(conseq)
			else return eval(alt)
			end
		end,
}
-- Aliases
specials.lam = specials.lambda
specials.def = specials.define

local function eval (x, env)
	env = env or base.env
	if isa(x, "Symbol") then
		return env[x]
	elseif not isList(x) then
		return x
	else
		local op, args = x.car, x.cdr
		if specials[op] then
			return specials[op](args, env)
		else		-- procedure
			local proc = eval(op, env)
			local params = {}
			local a = args
			while a.cdr do
				table.insert(params, eval(a.car, env))
				a = a.cdr
			end
			return proc(List(params))
		end
	end
end

---
return {
	Env = Env,
	Proc = Proc,
	eval = eval,
}