cps/trampoline

eopl 第五、六两章谈的就是这个问题。

我写了一个 python 版本的程序，先挖个坑，然后等彻底看完再补上。

 #!/usr/bin/env python
 # -*- coding: utf-8 -*-

 # the cps version
 def s(n, f):
     if n < 1:
         return f(n)
     return s(n-1,
              lambda x: f(x + n))

 # the trampoline version
 def t_s(n, f):
     if n < 1:
         return lambda :f(n)
     return lambda : t_s(n-1, lambda x: f(x + n))

 def trampoline(f):
     while hasattr(f, '__call__'):
         f = f()
     return f

 if __name__ == '__main__':
     print s(10, lambda x: x)
     print trampoline(t_s(10, lambda x: x))

下面是一个稍微复杂的例子，计算 fibonacci

 #!/usr/bin/env python
 # -*- coding: utf-8 -*-

 def trampoline(f):
     while hasattr(f, '__call__'):
         f = f()
     return f

 # the fibonacci
 def fib_cps(n, f):
     """

     Arguments:
     - `n`: the number of fibonacci
     - `f`: the contiunation
     """
     if n < 2:
         return f(1)
     return fib_cps((n-1),
                    lambda x: fib_cps((n-2),
                                      lambda y: f(x + y)))

 def fib_tramp(n, f):
     if n < 2:
         return lambda : f(1)
     return lambda : fib_tramp(n-1,
                               lambda x: lambda : fib_tramp(n-2,
                                                            lambda y: lambda: f(x + y)))

 if __name__ == '__main__':
     print fib_cps(10, lambda x: x)
     print trampoline(fib_tramp(10, lambda x: x))