From 4eaccb65390515fb833a58c92897a67b15803813 Mon Sep 17 00:00:00 2001
From: Case Duckworth
Date: Mon, 11 Mar 2024 00:40:05 -0500
Subject: Add IF, clean up definitions

---
 eval.lua | 55 +++++++++++++++++++++++++++++++++----------------------
 1 file changed, 33 insertions(+), 22 deletions(-)

diff --git a/eval.lua b/eval.lua
index 6179842..5e897e2 100644
--- a/eval.lua
+++ b/eval.lua
@@ -1,9 +1,9 @@
 --- lam.eval
 
 local eval = {}
-local read = require "read"
 local type = require "type"
-local util = require "util"
+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)
@@ -30,7 +30,6 @@ function eval.Proc (params, body, env)
 					inner[p.car] = a.car
 					p, a = p.cdr, a.cdr
 				end
-				-- pp.pp(self.body)
 				return eval.eval(
 					self.body,
 					eval.Env(inner, self.env))
@@ -41,47 +40,59 @@ end
 
 local global = {
 	begin =
-		function (args)
-			local a = args
-			while not type.isNull(a.cdr) do
-				a = a.cdr
+		function (r)
+			local r = r
+			while not isNull(r.cdr) do
+				r = r.cdr
 			end
-			return a.car
+			return r.car
 		end,
 	["+"] =
-		function (args)
-			local acc = 0
-			local car, cdr = args.car, args.cdr
-			while cdr do
-				acc = acc + car
-				car, cdr = cdr.car, cdr.cdr
+		function (r)
+			local r, a = r, 0
+			while r.cdr do
+				r, a = r.cdr, a + r.car
 			end
-			return acc
+			return a
 		end,
 	["-"] =
-		function (args)
-			return args.car - args.cdr.car
+		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 type.isa(x, "Symbol") then
+	if isa(x, "Symbol") then
 		return env[x]
-	elseif not type.isList(x) then
+	elseif not isList(x) then
 		return x
 	else
 		local op, args = x.car, x.cdr
 		if op == "quote" then
-			return args
+			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,
-				type.Cons("begin", args.cdr),
+				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 = {}
@@ -90,7 +101,7 @@ function eval.eval (x, env)
 				table.insert(params, eval.eval(a.car, env))
 				a = a.cdr
 			end
-			return proc(type.List(params))
+			return proc(List(params))
 		end
 	end
 end
-- 
cgit 1.4.1-21-gabe81