What is “Open Recursion”?

Re:What is “Open Recursion”? by Sean1708 on Wednesday July 19, 2017 @09:42AM (#14804192)

Do you mean like this?

  In [1]: def foo(f, a):
     ...:     return f(foo, a)
     ...: 

  In [2]: foo(lambda f, a: f(lambda f, a: a, a-1) if a > 0 else a, 10)
  Out[2]: 9

Obviously that's a completely useless and convoluted example, but I'm fairly sure the concept of a lambda calling the function it was passed to just works in most languages.

[ Parent | on HN ]

Re:What is “Open Recursion”? by jfoutz on Wednesday July 19, 2017 @02:01PM (#14806411)

it was a stupid joke about the y combinator. you can arrange a version with more function arguments that'll call all of the parts with their lambda friends. so they can all call each other.

so something vaguely like

   (define local-state)
   (super-y
      (lambda (inc get set v) 
          (set inc get set (+ 1(get inc get set v))))) ;this one is inc
      (lambda (inc get set v) local-state) ;this one is get
      (lambda (inc get set v) (set! local-state v)) ; this one is set

you don't really even need local state, just pass around an accumulator.

Once you have lambdas and a system that allows infinite types, mutual visibility is pretty easy to come by, mutual anonymous recursion is something i think is super nifty.

you can save yourself some pain by sticking the functions in a map, of course and passing that around instead.

[ Parent | on HN ]