Re: continuations (was Re: error() or nil, 'error msg')
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- Subject: Re: continuations (was Re: error() or nil, 'error msg')
- From: Tom N Harris <telliamed@...>
- Date: 2014年9月30日 19:05:41 -0400
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>