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--- 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
|
