下面是Lisp的程序员手册第13页的Lisp解释了Ruby代码:
# Kernel Extensions to support Lisp
class Object
def lisp_string
to_s
end
end
class NilClass
def lisp_string
"nil"
end
end
class Array
# Convert an Array into an S-expression (i.e. linked list).
# Subarrays are converted as well.
def sexp
result = nil
reverse.each do |item|
item = item.sexp if item.respond_to?(:sexp)
result = cons(item, result)
end
result
end
end
# The Basic Lisp Cons cell data structures. Cons cells consist of a
# head and a tail.
class Cons
attr_reader :head, :tail
def initialize(head, tail)
@head, @tail = head, tail
end
def ==(other)
return false unless other.class == Cons
return true if self.object_id == other.object_id
return car(self) == car(other) && cdr(self) == cdr(other)
end
# Convert the lisp expression to a string.
def lisp_string
e = self
result = "("
while e
if e.class != Cons
result << ". " << e.lisp_string
e = nil
else
result << car(e).lisp_string
e = cdr(e)
result << " " if e
end
end
result << ")"
result
end
end
# Lisp Primitive Functions.
# It is an atom if it is not a cons cell.
def atom?(a)
a.class != Cons
end
# Get the head of a list.
def car(e)
e.head
end
# Get the tail of a list.
def cdr(e)
e.tail
end
# Construct a new list from a head and a tail.
def cons(h,t)
Cons.new(h,t)
end
# Here is the guts of the Lisp interpreter. Apply and eval work
# together to interpret the S-expression. These definitions are taken
# directly from page 13 of the Lisp 1.5 Programmer's Manual.
def apply(fn, x, a)
if atom?(fn)
case fn
when :car then caar(x)
when :cdr then cdar(x)
when :cons then cons(car(x), cadr(x))
when :atom then atom?(car(x))
when :eq then car(x) == cadr(x)
else
apply(eval(fn,a), x, a)
end
elsif car(fn) == :lambda
eval(caddr(fn), pairlis(cadr(fn), x, a))
elsif car(fn) == :label
apply(caddr(fn), x, cons(cons(cadr(fn), caddr(fn)), a))
end
end
def eval(e,a)
if atom?(e)
cdr(assoc(e,a))
elsif atom?(car(e))
if car(e) == :quote
cadr(e)
elsif car(e) == :cond
evcon(cdr(e),a)
else
apply(car(e), evlis(cdr(e), a), a)
end
else
apply(car(e), evlis(cdr(e), a), a)
end
end
# And now some utility functions used by apply and eval. These are
# also given in the Lisp 1.5 Programmer's Manual.
def evcon(c,a)
if eval(caar(c), a)
eval(cadar(c), a)
else
evcon(cdr(c), a)
end
end
def evlis(m, a)
if m.nil?
nil
else
cons(eval(car(m),a), evlis(cdr(m), a))
end
end
def assoc(a, e)
if e.nil?
fail "#{a.inspect} not bound"
elsif a == caar(e)
car(e)
else
assoc(a, cdr(e))
end
end
def pairlis(vars, vals, a)
while vars && vals
a = cons(cons(car(vars), car(vals)), a)
vars = cdr(vars)
vals = cdr(vals)
end
a
end
# Handy lisp utility functions built on car and cdr.
def caar(e)
car(car(e))
end
def cadr(e)
car(cdr(e))
end
def caddr(e)
car(cdr(cdr(e)))
end
def cdar(e)
cdr(car(e))
end
def cadar(e)
car(cdr(car(e)))
end
所以我们说你有以下Lisp代码:
(defun reverse (list)
(rev-shift list nil))
(defun rev-shift (list result)
(cond ((null list) result)
(t (rev-shift (cdr list) (cons (car list) result)))))
你可以在DSL中呈现为:
require 'lisp'
# Create an environment where the reverse, rev_shift and null
# functions are bound to an appropriate identifier.
env = [
cons(:rev_shift,
[:lambda, [:list, :result],
[:cond,
[[:null, :list], :result],
[:t, [:rev_shift, [:cdr, :list],
[:cons, [:car, :list], :result]]]]].sexp),
cons(:reverse,
[:lambda, [:list], [:rev_shift, :list, nil]].sexp),
cons(:null, [:lambda, [:e], [:eq, :e, nil]].sexp),
cons(:t, true),
cons(nil, nil)
].sexp
# Evaluate an S-Expression and print the result
exp = [:reverse, [:quote, [:a, :b, :c, :d, :e]]].sexp
puts "EVAL: #{exp.lisp_string}"
puts " => #{eval(exp,env).lisp_string}"
(解释器和示例的原始源can be found here。)
更新:刚才意识到你在你的问题中提到了这个解决方案。
Prolog的东西看起来更像'草图',而不是实际的实现。你为什么要在Ruby中使用Lisp? Ruby可能是实现其他语言的最差语言之一。有些情况下Ruby比典型的Lisp慢百倍:http://shootout.alioth.debian.org/u32/benchmark.php?test=all&lang = yarv&lang2 = sbcl - 现在想象一下运行在Ruby之上的Lisp的缓慢。 Lisp也不是DSL,而是一系列完全一般的编程语言。 – 2010-08-09 21:05:45
这与“外部DSL”有何不同? “Prolog作为Ruby DSL”稍微改变了Prolog的语法以在Ruby中工作。 “在Ruby中编写的Lisp解释器”还允许你在Ruby中编写具有稍微不同语法的Lisp,例如'[]'而不是'()'和':lambda'而不是'lambda'。你还想要什么? – Ken 2010-08-09 21:10:15
这纯粹是作为一种学习练习。我发现Lisp作为一种语言引人入胜,但是很痛苦。我知道可以在Lisp里面实现Lisp(我在20多年前就已经完成了!)。推测在Ruby中实现Lisp同样很容易,只需要简单得多的语法 - 这会让我更容易理解发生了什么。 – 2010-08-09 21:38:42