Refactor eval

Localize stuff and also break out specials

1 files changed, 48 insertions, 27 deletions

@@ -1,13 +1,12 @@
 --- lam.eval
 
-local 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
 
-function eval.Env (inner, outer)
+local function Env (inner, outer)
         local mt = {
                 __type = "Environment",
                 __index = outer,
@@ -25,7 +24,7 @@ function eval.Env (inner, outer)
         return setmetatable(inner, mt)
 end
 
-function eval.Proc (params, body, env)
+local function Proc (params, body, env)
         local v = {
                 params = params,
                 body = body,
@@ -42,18 +41,51 @@ function eval.Proc (params, body, env)
                                         p, a = p.cdr, a.cdr
                                 end
                                 local b = self.body
-                                local e = eval.Env(inner, self.env)
+                                local e = Env(inner, self.env)
                                 while not isNull(b.cdr) do
-                                        eval.eval(b.car, e)
+                                        eval(b.car, e)
                                         b = b.cdr
                                 end
-                                return eval.eval(b.car, e)
+                                return eval(b.car, e)
                         end,
         }
         return setmetatable(v, mt)
 end
 
-function eval.eval (x, env)
+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]
@@ -61,29 +93,14 @@ function eval.eval (x, env)
                 return x
         else
                 local op, args = x.car, x.cdr
-                if op == "quote" then
-                        return args.car
-                elseif op == "define" or op == "def" then
-                        rawset(env, args.car, eval.eval(args.cdr.car, env))
-                        return nil
-                elseif op == "set!" then
-                        env[args.car] = eval.eval(args.cdr.car, env)
-                elseif op == "lambda" or op == "lam" then
-                        return eval.Proc(args.car, 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
+                if specials[op] then
+                        return specials[op](args, env)
                 else            -- procedure
-                        local proc = eval.eval(op, env)
+                        local proc = eval(op, env)
                         local params = {}
                         local a = args
                         while a.cdr do
-                                table.insert(params, eval.eval(a.car, env))
+                                table.insert(params, eval(a.car, env))
                                 a = a.cdr
                         end
                         return proc(List(params))
@@ -92,4 +109,8 @@ function eval.eval (x, env)
 end
 
 ---
-return eval
+return {
+        Env = Env,
+        Proc = Proc,
+        eval = eval,
+}