Re: continuations (was Re: error() or nil, 'error msg')

[Tom N Harris <telliamed@...>]

On Tuesday, September 30, 2014 01:02:37 PM Coda Highland wrote:
> No, the goto B is skipping over the normal control flow to continue
> execution at the label. Think of it more like an exception than a
> coroutine:
> 
> function A()
>   local N=0
>   local C=function()
>     N=N+1
>     error("goto B")
>   end
>   pcall(function() N=1+C() end)
>   -- ::B:: goes here
>   return N
> end
> print(A())
> 

I could also rewrite it this way:

    function A()
      local N=0
      local B=function() return N end
      local C=function()
        N=N+1
        return B()
      end
      N=1+C()
      return B()
    end

A tail call replaces the current call with a different context. Goto also 
replaces the current context. Right now the limitation of goto is that the 
context must be within the same function and not introduce any new locals. A 
tail-call-goto relaxes the restriction to allow the function to change.

A continuation-goto on the other hand, is not only changing the context but 
unwinding the call stack. What happens, then, if A is not in the call chain?

    function setjmp(F)
      longjmp = F
      return true
    end
    function A()
      N=1
      if setjmp(function() goto two end) then
        return N
      end
      ::two::
      return N+1
    end
    function B()
      longjmp()
      return 0
    end
    print(A())
    print(B())

Does B() print 2, 0, or is it an error because A is not in the call stack?

-- 
tom <telliamed@whoopdedo.org>